Re: [Haskell-cafe] Stack ADT?
What's going on is that data structures in Haskell are immutable. Thus, when you call push on a stack, you get a new stack with the new element pushed onto it, and the original stack is left un-touched. On Fri, Feb 5, 2010 at 10:56 AM, michael rice nowg...@yahoo.com wrote: Not using Stack for anything, just trying to understand how things can be done in Haskell. To that end... What's going on here? I'm not even calling function POP. Michael == module Data.Stack (Stack, emptyStack, isEmptyStack, push, pop, top) where newtype Stack a = Stack [a] emptyStack = Stack [] isEmptyStack (Stack xs) = null xs push x (Stack xs) = Stack (x:xs) pop (Stack (_:xs)) = Stack xs top (Stack (x:_)) = x == [mich...@localhost ~]$ ghci Stack.hs GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Data.Stack ( Stack.hs, interpreted ) Ok, modules loaded: Data.Stack. *Data.Stack let s1 = emptyStack *Data.Stack top (push 1 s1) 1 *Data.Stack top (push 2 s1) 2 *Data.Stack top (push 3 s1) 3 *Data.Stack let s2 = pop s1 *Data.Stack top s2 *** Exception: Stack.hs:8:0-28: Non-exhaustive patterns in function pop *Data.Stack --- On *Fri, 2/5/10, Casey Hawthorne cas...@istar.ca* wrote: From: Casey Hawthorne cas...@istar.ca Subject: Re: [Haskell-cafe] Stack ADT? To: haskell-cafe@haskell.org Date: Friday, February 5, 2010, 10:36 AM You could also implement stacks with mutable data structures, e.g. STArray, etc. What do you want to use a stack ADT for? Usually stacks are discussed for pedagogical purposes but usually recursion is used if you need a stack like operation. -- Regards, Casey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mc/compose?to=haskell-c...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pattern matching does not work like this?
Err, technically, aren't functions and constructors mutually exclusive? So if something is a function, it's, by definition, not a constructor? On Wed, Jul 15, 2009 at 6:25 AM, Eugene Kirpichov ekirpic...@gmail.comwrote: Technically, the reason is not that (++) is a function, but that it is not a constructor of the [] type. And, not only is it not a constructor, but it also *can't* be one, because the main characteristic of pattern matching in Haskell is that it is (contrary to Prolog's unification) unambiguous (unambiguity of constructors is guaranteed by the semantics of Haskell's algebraic datatypes). If ++ could be pattern matched, what should have been the result of let (x++y)=[1,2,3] in (x,y)? 2009/7/15 minh thu not...@gmail.com: 2009/7/15 Magicloud Magiclouds magicloud.magiclo...@gmail.com: Hi, I do not notice this before. fun ([0, 1] ++ xs) = .. in my code could not be compiled, parse error. ++ is a function; you can't pattern-match on that. Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Eugene Kirpichov Web IR developer, market.yandex.ru ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What to say about Haskell?
http://www.haskell.org/haskellwiki/Tying_the_Knot would seem to be relevant. On Tue, Jul 14, 2009 at 9:55 AM, Cristiano Paris fr...@theshire.org wrote: I would like to know them. I'm looking for small snippets of code that are elegant when written in Haskell (which is lazy by default) but not in other languages supporting some form of laziness (like Python+Iterators). Can you post some? Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combine to List to a new List
Try c = zip a b On Tue, Jun 9, 2009 at 9:05 AM, ptrash ptr...@web.de wrote: Hi, I have the following two lists: a = [1,2,3] b = [A,B,C] I want a combination of the to lists: c = [(1,A), (2, B), (3, C)] How can I do this? I have tried c = [(x,y) | x - a, y - b] But this just returns me a list with every possible combination of the 2 lists. Thanks... -- View this message in context: http://www.nabble.com/Combine-to-List-to-a-new-List-tp23942440p23942440.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problem w/YAHT code example
I'm not sure what you're using at the end of the identifier search', but it needs to be the single quote that's on the same key as the double quotes. I suspect that's where it's blowing up. On Thu, May 28, 2009 at 9:56 PM, michael rice nowg...@yahoo.com wrote: This code, from YAHT (Section 8.4.2, PDF pg. 119 of 192) seems to have a problem. Code below. Michael = [mich...@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude :l graph [1 of 1] Compiling Main ( graph.hs, interpreted ) graph.hs:6:24: lexical error at character '\8217' Failed, modules loaded: none. == Prelude data Graph v e = Graph [(Int,v)] [(Int,Int,e)] search :: Graph v e - Int - Int - Maybe [Int] search g@(Graph vl el) src dst | src == dst = Just [src] | otherwise = search’ el where search’ [] = Nothing search’ ((u,v,_):es) | src == u = case search g v dst of Just p - Just (u:p) Nothing - search’ es | otherwise = search’ es ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell] Re: [Haskell-cafe] ANN: Haskell Hackathon in Philadelphia
Is there a list of projects that will be worked on during this, or how will that work? On Thu, May 21, 2009 at 5:39 PM, Brent Yorgey byor...@seas.upenn.eduwrote: Hi all! We are in the early stages of planning a Haskell hackathon/get together, Hac φ, to be held this summer at the University of Pennsylvania, in Philadelphia. Right now we're looking at two possible dates: June 19-21or July 24-26 If you might be interested in attending, please add your name on the wiki page: http://www.haskell.org/haskellwiki/Hac_%CF%86 You can also note whether either of those dates absolutely doesn't work for you. (If you don't have an account on the wiki, you can email Ashley Yakeley for an account [1], or feel free to just respond to this email.) Expect more details (such as a nailed-down date) soon, once we have gauged the level of interest. Hope to see you in Philadelphia! Brent (byorgey) Daniel Wagner (dmwit) [1] http://haskell.org/haskellwiki/HaskellWiki:New_accounts ___ Haskell-Cafe mailing list haskell-c...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell-cafe] ANN: Haskell Hackathon in Philadelphia
Is there a list of projects that will be worked on during this, or how will that work? On Thu, May 21, 2009 at 5:39 PM, Brent Yorgey byor...@seas.upenn.eduwrote: Hi all! We are in the early stages of planning a Haskell hackathon/get together, Hac φ, to be held this summer at the University of Pennsylvania, in Philadelphia. Right now we're looking at two possible dates: June 19-21or July 24-26 If you might be interested in attending, please add your name on the wiki page: http://www.haskell.org/haskellwiki/Hac_%CF%86 You can also note whether either of those dates absolutely doesn't work for you. (If you don't have an account on the wiki, you can email Ashley Yakeley for an account [1], or feel free to just respond to this email.) Expect more details (such as a nailed-down date) soon, once we have gauged the level of interest. Hope to see you in Philadelphia! Brent (byorgey) Daniel Wagner (dmwit) [1] http://haskell.org/haskellwiki/HaskellWiki:New_accounts ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell in 3 Slides
Don't forget to include higher-order functions in one of those important points. On Mon, May 18, 2009 at 12:36 PM, John Van Enk vane...@gmail.com wrote: Thanks Joe, Your assumption is correct--the whole presentation will be longer. I wanted to use 3 or 4 slides to introduce the language and the balance for the interesting stuff. :P /jve PS to Joe: I will not forget to reply to all. I will not forget to reply to all. I will not forget to reply to all. There, hopefully I won't forget any more. :) On Mon, May 18, 2009 at 12:29 PM, Joe Fredette jfred...@gmail.com wrote: While an incredibly small font is a clever option, a more serious suggestion may be as follows. 3-4 slides imply 3-4 topics, so the question is what are the 3-4 biggest topics in haskell? I would think they would be: * Purity/Referential Transparency * Lazy Evaluation * Strong Typing + Type Classes * Monads Assuming you have, say, 10-15 minutes for the talk, and the people there are versed with imperative programming and maybe have some experience in functional programming, you can probably jump over each of those slides in about a minute, just enough to touch the subject. I also assume that you don't need to fit the whole presentation in 3-4 slides, if you do, then yah. /Joe David Leimbach wrote: Use an incredibly small font. On Mon, May 18, 2009 at 8:16 AM, John Van Enk vane...@gmail.commailto: vane...@gmail.com wrote: Hi all, I'm giving a presentation to an IEEE group on Embedded DSL's and Haskell at the end of June. I need a 3 to 4 slide introduction to Haskell. What suggestions does the community have? Is such a short intro possible? It just needs to introduce the basics so I can show some code without alienating the audience. I'm hoping some one else has attempted this before, but if not, some boiler plate slides could be useful for every one! --/jve ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- /jve ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pretty printing a tree
Perhaps drawTree on http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Tree.html 2009/5/14 José Romildo Malaquias j.romi...@gmail.com Hello. I would like to pretty print a tree in a way that its structure is easily perceived. For instance, consider the declarations: data Node a = Node a [Node a] type Tree a = [ Node a ] t = [ Node a [ Node b [] , Node c [ Node c1 [] , Node c2 [] ] , Node d [ Node d1 [ Node d1a [] ] , Node d2 [] ] ] ] Then the resulting of pretty printing the given tree would be something like the following: a | +-+ ||| bcd || +---++---+ | || | c1 c2 d1 d2 | d1a There is the module Text.PrettyPrint.HughesPJ, but it lacks examples on how to use the pretty print combinators, and it is not well docomented. I would like to see solutions for this problem, or clues on how to solve it. Regards, José Romildo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Structural sharing in haskell data structures?
So far as I know, all immutable languages do it. Like Don, I think the reason it's emphasized so much in Clojure is because of trying to convert the Java crowd and introducing them to immutability. With mutable languages you have to clone _everything_ for fear of later changes affecting earlier copies. Because of this, folks used to mutable languages often erroneously suppose that immutable languages do the same thing. There's a lot of FUD in the mainstream about declarative/immutable/functional languages and their performance behavior. The idea is simple and you can do it in mutable languages perfectly well (clone the changed part of the spine, point to old substructure) so long as you're careful not to do mutation, e.g. by marking everything final. You don't see it as much in mutable languages because people get squeamish about the first step: clone the changed spine; they'd much rather mutate it. In heavily GCed languages like Haskell allocation and collection is cheap, so we don't mind too much; but in Java and the like, both allocation and collection are expensive so the idea of cheap throwaway objects is foreign. Are you guys saying that the Clojure target audience is current Java programmers, whereas Haskell's is people who already do functional, immutable programming? Because how long the technique has been used, or how widespread it's used, is irrelevant when trying to convince someone who doesn't have experience with immutable data structures that it's not completely insane. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Structural sharing in haskell data structures?
So I've been reading a lot about a (relatively) new language called Clojure. One of its goals is to make concurrency easier via a built-in home-grown STM. Anyway, one of the ways it tries to do this is to have completely immutable data structures. Every time I read a tutorial about this in Clojure, it says ...yes, it sounds awful to think that your whole data structure gets copied every time you want to make a change to it, but it's sane because of a technique called structural sharing. Yet every time I hear immutability talked about in Haskell, what I hear is ...yes, it sounds awful to think that your whole data structure gets copied every time you want to make a change to it, but it's sane because of laziness...unless you need the whole data structure So I'm just curious, does GHC use structural sharing or something similar? Do other implementations? Does it matter? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List of exports of a module - are there alternatives?
On Tue, May 12, 2009 at 10:05 AM, Maurício briqueabra...@yahoo.com wrote: snip (I think something like that could be nice when we have modules with 200 declarations and just a few are (not) going to be exported.) Thanks, Maurício Uh, show me such a module, and I'll show you a module that's quite bloated and desperately needs to be refactored. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Structural sharing in haskell data structures?
Purity allows our data structures to have a lot of sharing. This is separate to laziness. Ah, so haskell does do it. Interesting that it so rarely comes up, whereas it's frequently mentioned in clojure. Laziness lets us build up interesting structures that have unusual sharing. Actually, what kind of persistant structures does Clojure have at this stage? I was under the impression they were reusing Java data structures. E.g. some of the nicer ones on hackage are zippers, patricia tries, finger trees, which I can't imaging have been ported. It has some built-in persistent data structures: lists, vectors (arrays), maps, and sets. It also has strong interoperability with Java, so that any existing Java library can easily be used in Clojure code. In some ways, that makes it a VERY mature language already. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Poking fun at haskell and other programming languages
http://james-iry.blogspot.com/2009/05/brief-incomplete-and-mostly-wrong.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
Err, I'm not seeing the danger of this (+) :: forall a. (Num a) = a - a - a Doesn't this require the two parameters to be the same instance of Num? On Fri, May 8, 2009 at 10:51 AM, Sittampalam, Ganesh ganesh.sittampa...@credit-suisse.com wrote: Stephan Friedrichs wrote: When looking for an xor function, I found one in Data.Bits but couldn't use it for Bool, because Bool is no instance of Bits and of Num (which would be necessary, because it's class (Num b) = Bits b). My question is: Why not? [...] quite trivial... Why is this not part of base? Or am I missing something? One reason would be that we don't want 1 + True to typecheck, even if it does have a sensible interpretation. Ganesh === Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html === ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Bool no instance of Num and Bits?
Hmm, I never knew that. Is that a GHC thing? Is it strictly necessary? Seems like it could be done in the Num instance for Integers, Ints, etc. On Fri, May 8, 2009 at 11:51 AM, Neil Mitchell ndmitch...@gmail.com wrote: Err, I'm not seeing the danger of this (+) :: forall a. (Num a) = a - a - a Doesn't this require the two parameters to be the same instance of Num? I didn't at first, then I remembered: 1 + True = fromInteger 1 + True And if we have Num for Bool, it type checks. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How difficult would creating a collaborative multi-user online virtual world application be in Haskell?
This reminds me of a server app I saw recently in a language called Clojure. Clojure is a relatively new lisp variant targeting the JVM, and has a home-grown STM layer built into the language. Anyway, the app I saw was a (admittedly didactic-focused) multi-threaded MUD server (google clojure mire), which could easy serve as the foundation for a project like this. Thus, I would say that STM is up for the challenge. The question in my mind would be whether or not Haskell's graphics/video libraries are mature enough. On May 7, 2009, at 6:28 AM, Benjamin L.Russell dekudekup...@yahoo.com wrote: One question that has been coming up at the back of my mind for the past several weeks has been how difficult would it be to create a collaborative multi-user online virtual world application in Haskell. More specifically, last August, I came across a very interesting application called Croquet (see http://www.opencroquet.org/index.php/Main_Page), which happens to be based on Squeak (see http://www.squeak.org/), a dialect of Smalltalk. Croquet, in turn, provides the basis for Cobalt (see http://www.duke.edu/~julian/Cobalt/Home.html), a virtual workspace browser and construction toolkit for accessing, creating, and publishing hyperlinked multi-user virtual environments (according to the home page for that project). What struck me as especially interesting was how Croquet allows multiple users to collaborate together in a multi-user online virtual world in software development and other collaborative projects. As one application, the video clip on the upper-right-hand corner of the above-mentioned Croquet home page illustrates how a user can, by writing code from inside the application, create on-the-fly additional virtual environments, which can then be entered by either the programmer or other programmers. Other applications (shown in other video clips on the Screenshots/Videos page (see http://www.opencroquet.org/index.php/Screenshots/Videos) show alternative applications that include text-based annotations, a 3D spreadsheet, and writing a conventional blog from within a virtual world. Unfortunately, Smalltalk is an object-oriented language. If possible, I would like to see something similar in a functional programming language such as Haskell. Does anybody know whether duplicating this project in Haskell would be feasible? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 Furuike ya, kawazu tobikomu mizu no oto. -- Matsuo Basho^ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Interesting Thread on OO Usefulness (scala mailing list)
This sounds like a really interesting question. To save some people weeding through the thread and Jon Harrop's usual trolling garbage, here's a description of the problem: [quote] Here's [a]language to to interpret (where postfix * means tupling): Variables: x Integer literals: i Terms: t = Lambda x*. t | Apply t t* | Var(x) | Num(i) We assume usual operational semantics of lambda calculus (i.e. static scoping). The task is to write two interpreters, one with variables x being DeBruijn indices and one with them being names. You should go for maximal sharing, i.e. factor out commonalities into a common class/typeclass/functor/whatever, so that there remains no duplication of code in the two solutions. [/quote] On Mon, May 4, 2009 at 6:05 AM, Paolo Losi paolo.l...@gmail.com wrote: Hi all, I'm following an interesting thread on the scala mailing list: http://www.nabble.com/-scala--usefulness-of-OOP-td23268250.html Martin Odersky advocates the OO features of the scala language proposing an interesting problem where the OO approach seams valuable. I would be very much interested in seeing an Haskell solution to that problem. Any haskell guru want to take a stub at it or give an opinion from a pure FP point of view? Thanks Paolo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANN] Hack: a sexy Haskell Webserver Interface ^^
Err, is there some reason you don't have simpler interfaces like:plaintext
:: String - Application
plaintext text = \env - return $ Response
{ status = 200
, headers = [ (Content-Type, text/plain) ]
, body= text
}
On Mon, Apr 20, 2009 at 12:30 PM, Joe Fredette jfred...@gmail.com wrote:
We need to start referring to more haskell packages as sexy
/Joe
Jinjing Wang wrote:
Simplest app should look like this
module Main where
import Hack
import Hack.Handler.Kibro
hello :: Application
hello = \env - return $ Response
{ status = 200
, headers = [ (Content-Type, text/plain) ]
, body= Hello World
}
main = run hello
Hack is a brainless port of the brilliant Ruby Rack framework. Rack is
very modular and that's
very important.
Rack utilities and middlewares should be able to be ported with
minimal effort, I hope.
link / source: http://github.com/nfjinjing/hack/tree/master
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Looking for the fastest Haskell primes algorithm
Some other ideas for things to put in this package possibly: is_prime :: Int - Bool nth_prime :: Int - Int -- or Int - Integer prime_factors :: Int - [Int] I'm assuming there are faster ways of doing the first 2 than by just simply looking through all of primes. Someone should also look through Euler - I'm sure that will generate other ideas of things that could be useful in playing with primes. On Tue, Apr 14, 2009 at 8:40 AM, Niemeijer, R.A. r.a.niemei...@tue.nlwrote: Today I happened to need a large list of prime numbers. Obviously this is a well-known problem, so I figured there would be something on Hackage that I could use. Surprisingly, there isn’t, or if there is it’s not easy to find. Searching for prime or primes on Hackage reveals nothing. Searching for primes on Hayoo gives Codec.Encryption.RSA.NumberTheory, but that uses the inefficient one-liner implementation. The HaskellWiki article on primes (http://www.haskell.org/haskellwiki/Prime_numbers) has a number of implementations, but the faster they get, the longer and uglier they become. Since it’s such a common problem I’d say it would be a good idea to add a package to Hackage that exports primes :: [Integer] and hides the ugly implementation details. Data.Numbers.Primes seems a logical choice for the namespace, but I’m open to suggestions. The trick then is to find the most efficient implementation of primes. The Haskell wiki article mentions ONeillPrimes.hs as one of the fastest ones, but maybe there’s a faster version. So my question is: does anybody know what the fastest Haskell algorithm for generating primes is? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Here's another way of looking at what others have already said. The only way you can do that is within the scope of another IO action. For example: outputXmlTrees :: IO () outputXmlTrees = do trees - inputXmlTrees; let newTrees = transform trees; print . show $ newTrees Notice a few things: - First, the line trees - inputXmlTrees effectively takes an IO [XmlTree] and turns it into a [XmlTrees]. That is, it runs the IO action inputXmlTrees, and gives you the resulting list of XmlTrees to work with. - You can then pass these off to a pure function which will monkey around with them in a pure (non-IO) context - You must do this in an IO action, however, and any monadic action gets its type from the last line. Thus, the last line must be of type IO something. In this case, it is simply the action that would print out the trees. - Thus, this gives you a way to glue together different IO actions and pure actions and combine them into larger IO actions Hope this clarifies On Tue, Apr 14, 2009 at 10:54 AM, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? Regards, Rodrigo. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] community.haskell.org unreachable?
Works for me. The admin address referenced there is support [AT] community.haskell.org On Mon, Apr 6, 2009 at 1:32 PM, Claus Reinke claus.rei...@talk21.comwrote: I seem to be having problems reaching anything on that server. Does anyone know what is going on, or who to contact? It would help if the haskellwiki page http://www.haskell.org/haskellwiki/Haskell.org_domain would mention the admin address directly, instead of referring to http://community.haskell.org/admin/ which is also unreachable. Claus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] happstack example code wanted
Copied there. On Sat, Apr 4, 2009 at 1:30 PM, Thomas Hartman tphya...@gmail.com wrote: Johannes, You'll have a better response if you post to the happs list. Have you had a look at tutorial.happstack.com? Thomas. 2009/4/4 Johannes Waldmann waldm...@imn.htwk-leipzig.de: Hi, I'm thinking of using happstack for a simple online registration system. [ Customer gives Name and Address and registers for certain workshops. Then the app server should just produce a bill. (Payment is handled through a different channel.) The store owner needs to get some overview of registrations. ] According to happs(tack) marketing department, this should be an easy exercise? Any code that I could use as a starting point? ( Or, if you do the coding, I can link to you from the workshop site. But I don't have a serious budget :-) Thanks - J.W. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combining sequences
Here's a pretty straightforward approach: combine [] ys = ys combine xs [] = xs combine (x:xs) (y:ys) | x = y = x : combine xs (y:ys) | otherwise = y : combine (x:xs) ys On Sat, Apr 4, 2009 at 8:40 PM, michael rice nowg...@yahoo.com wrote: Is there a simple way to combine two sequences that are in ascending order into a single sequence that's also in ascending order? An example would be the squares [1,4,9,16,25..] combined with the cubes [1,8,27,64,125..] to form [1,1,4,8,9,16,25,27..]. Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad transformer, liftIO
You haven't really said what happens when you try this, but I would bet that things would be clarified greatly if you put type signatures on your two definitions. On Fri, Apr 3, 2009 at 12:49 PM, Michael Roth mr...@nessie.de wrote: Hello list, maybe I'm just stupid, I'm trying to do something like this: import Control.Monad import Control.Monad.Trans import Control.Monad.List foobar = do a - [1,2,3] b - [4,5,6] liftIO $ putStrLn $ (show a) ++ ++ (show b) return (a+b) main = do sums - foobar print sums But this apparently doesn't work... I'm total clueless how to achieve the correct solution. Maybe my mental image on the monad transformer thing is totally wrong? Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The votes are in!
Ooh, let me pop some popcorn! 2009/3/24 John Van Enk vane...@gmail.com Is this the part where all the pundits come out and talk about how Jeff isn't a citizen, eats babies, and wants to turn Haskell into an imperative language? /jve 2009/3/24 Eelco Lempsink ee...@lempsink.nl The results of the Haskell logo competition are in! You can view them at http://www.cs.cornell.edu/w8/~andru/cgi-perl/civs/http://www.cs.cornell.edu/w8/%7Eandru/cgi-perl/civs/ results.pl?num_winners=1id=E_d21b0256a4fd5ed7algorithm=beatpath Congratulations Jeff Wheeler! I'll set up a page with the results visibile. -- Regards, Eelco Lempsink ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The votes are in!
He ONCE used unsafeInterleaveIO, but he never evaluated it, and never tried it again! 2009/3/24 Brandon S. Allbery KF8NH allb...@ece.cmu.edu On 2009 Mar 24, at 8:29, John Van Enk wrote: Is this the part where all the pundits come out and talk about how Jeff isn't a citizen, eats babies, and wants to turn Haskell into an imperative language? He uses unsafeInterleaveIO! -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANN] random-stream package
Maybe the feature has been added, but not released, because somebody is strict about their code... It should be possible to do: cabal install http://haskell.mperillo.ath.cx/random-stream-0.0.1.tar.gz It's unfortunate that this is not supported. Thanks Manlio ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Abuse of the monad [was: monadic logo]
Can you expand on this a bit? I'm curious why you think this. On Thu, Mar 12, 2009 at 10:13 AM, Thomas Davie tom.da...@gmail.com wrote: On 12 Mar 2009, at 15:04, Gregg Reynolds wrote: At risk of becoming the most hated man in all Haskelldom, I'd like to suggest that the Haskell logo not use lambda symbols. Or at least not as the central element. Sorry, I know I'm late to the party, but the thing is there is nothing distinctive about lambda; it's common to all FPLs. Besides, Lisp/Scheme already have that franchise. What is distinctive about Haskell it's use of the monad. The Pythagorean monad symbol is wonderfully simple: No, what's distinctive about Haskell is usually the abuse of the monad. Encouraging people to think Haskell is all about monadic programming even more is a recipe for disaster. Just my 2¢ Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Abuse of the monad [was: monadic logo]
Hmm, perhaps what we need is another monad tutorial that.err...never mind. Actually, I'd like to see more written about Applicatives. I think they just finally clicked with me when reading the Typeclassopedia, and seeing the intended way to use them. Before it was always like ok...so, if I've got a function already in my functor, I could use this, but...why would I have that? On Thu, Mar 12, 2009 at 10:28 AM, Thomas Davie tom.da...@gmail.com wrote: On 12 Mar 2009, at 15:16, Andrew Wagner wrote: Can you expand on this a bit? I'm curious why you think this. For two reasons: Firstly, I often find that people use the Monadic interface when one of the less powerful ones is both powerful enough and more convenient, parsec is a wonderful example of this. When the applicative instance is used instead of the monadic one, programs rapidly become more readable, because they stop describing the order in which things should be parsed, and start describing the grammar of the language being parsed instead. Secondly, It seems relatively common now for beginners to be told about the IO monad, and start writing imperative code in it, and thinking that this is what Haskell programming is. I have no problem with people writing imperative code in Haskell, it's an excellent imperative language. However, beginners seeing this, and picking it up is usually counter productive – they never learn how to write things in a functional way, and miss out on most of the benefits of doing so. Hope that clarifies what I meant :) Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Abuse of the monad [was: monadic logo]
Conal,Do you think imperative Haskell can be a sort of gateway drug to real haskell? On Thu, Mar 12, 2009 at 11:25 AM, Conal Elliott co...@conal.net wrote: Thank you Bob! I'll throw in another 2 cents: Yes, *one* aspect of Haskell is that it's a power tool for imperative programming -- a clever way to keep plugging away at the old sequential von Neumann paradigm. C. I'd rather we strongly encourage Haskell-newbies toward shifting out of the imperative paradigm to thinking and programming *functionally*. It's a big shift, to make, and imperative-Haskell is a relatively easy substitute. - Conal On Thu, Mar 12, 2009 at 7:28 AM, Thomas Davie tom.da...@gmail.com wrote: On 12 Mar 2009, at 15:16, Andrew Wagner wrote: Can you expand on this a bit? I'm curious why you think this. For two reasons: Firstly, I often find that people use the Monadic interface when one of the less powerful ones is both powerful enough and more convenient, parsec is a wonderful example of this. When the applicative instance is used instead of the monadic one, programs rapidly become more readable, because they stop describing the order in which things should be parsed, and start describing the grammar of the language being parsed instead. Secondly, It seems relatively common now for beginners to be told about the IO monad, and start writing imperative code in it, and thinking that this is what Haskell programming is. I have no problem with people writing imperative code in Haskell, it's an excellent imperative language. However, beginners seeing this, and picking it up is usually counter productive – they never learn how to write things in a functional way, and miss out on most of the benefits of doing so. Hope that clarifies what I meant :) Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Abuse of the monad [was: monadic logo]
I was thinking the same thing. If I remember correctly, RWH does a parser in an applicative style, but I'll have to look when I get home to be sure. If so, then maybe we could try doing the same thing in a monadic style, for comparison and contrast purposes. On Thu, Mar 12, 2009 at 11:29 AM, Jeff Heard jefferson.r.he...@gmail.comwrote: Come to think of it, I've never seen an applicative tutorial of Parsec, only a monadic one. Does such a beast exist, and if so, maybe we could merge the two together, work the same example in both, and thus help the programmer make the shift from monadic to applicative, from order of parsing to describing the grammar. -- Jeff 2009/3/12 Conal Elliott co...@conal.net: Thank you Bob! I'll throw in another 2 cents: Yes, *one* aspect of Haskell is that it's a power tool for imperative programming -- a clever way to keep plugging away at the old sequential von Neumann paradigm. C. I'd rather we strongly encourage Haskell-newbies toward shifting out of the imperative paradigm to thinking and programming *functionally*. It's a big shift, to make, and imperative-Haskell is a relatively easy substitute. - Conal On Thu, Mar 12, 2009 at 7:28 AM, Thomas Davie tom.da...@gmail.com wrote: On 12 Mar 2009, at 15:16, Andrew Wagner wrote: Can you expand on this a bit? I'm curious why you think this. For two reasons: Firstly, I often find that people use the Monadic interface when one of the less powerful ones is both powerful enough and more convenient, parsec is a wonderful example of this. When the applicative instance is used instead of the monadic one, programs rapidly become more readable, because they stop describing the order in which things should be parsed, and start describing the grammar of the language being parsed instead. Secondly, It seems relatively common now for beginners to be told about the IO monad, and start writing imperative code in it, and thinking that this is what Haskell programming is. I have no problem with people writing imperative code in Haskell, it's an excellent imperative language. However, beginners seeing this, and picking it up is usually counter productive – they never learn how to write things in a functional way, and miss out on most of the benefits of doing so. Hope that clarifies what I meant :) Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Partial pattern matching
The question is, is there some very important reason you can't do this? firstCoord (P x _) = x 2009/3/9 Peter Verswyvelen bugf...@gmail.com In Haskell, a data constructor can be used partially applied: data Pair a b = P a b f = P 1 however, I cannot do partial pattern matching, e.g firstCoord (P x) = x does not work. I guess a very important reason must exist why this is the case? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Purely Functional Data Structures
So...is there some reason this is in the hApps package? On Sat, Mar 7, 2009 at 9:04 PM, Jeremy Shaw jer...@n-heptane.com wrote: At Sun, 08 Mar 2009 00:13:14 +0100, G?uenther Schmidt wrote: In SQL I would have the data indexed by several different columns, if I use maps I'd only have one key, so if I need to lookup data in the map by a value that is not the key the lookups will become quite expensive. happstack-ixset offers a data-type similar to Map except that you can have multiple keys. You can even have keys that are calculated from the data but don't actually appear in the data itself. For, example, if your ixset just contains Strings, one of the keys could be the length of the String. happstack-ixset (and its dependencies) also offers compact serialization/deserialization of the ixset to disk, data migration options, and a smattering of other features that may or may not be useful to you. While happstack-ixset is built to work with happstack, it is does not depend on the happstack http server or persistent store layer, so it should be useful even if you are not being an application server. - jeremy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Finding the Convex Hull (Problem 12 of Real World Haskell)
Whenever I'm looking for a bug in Haskell code, I find it helpful to start by seeing if I can simplify the code any first. In this case, there are a couple of things I notice: - validPointsOf is just a filter. It would be easier to write valid :: MyDirection - Bool and then validPointsOf = filter (valid . snd) - Similarly, there's no need to write your own minimum-finder and call it lowestY. Instead, write (or derive!) an Ord instance, and then use the standard prelude function minimum - a small simplification of sortByCoTan: sortByCoTan pivot = sortBy (comparing (coTan pivot)) Hope this helps! 2009/3/5 Rob Crowther weila...@gmail.com I wrote a solution to this problem, but it appears to return incorrect results. There's a pastebin of the code at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2121 and a picture of the inputs, outputs, and expected results graphed at http://img510.imageshack.us/img510/9971/resultsg.jpg I'm wondering if this is a flaw in my code, my understanding of the problem, or both. Any ideas on how to track this one down would be very much appreciated. Thank you! -- ヽ(^o^)ノ -rob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] DSLs with {in,}equalities
Not to hijack the thread, but I thought I was the only one that used unix
notation for statements like {in,}equalities. I like it!
On Mon, Mar 2, 2009 at 11:13 PM, Andrew Hunter ahun...@cs.hmc.edu wrote:
Several times now I've had to define an EDSL for working with
(vaguely) numeric expressions. For stuff like 2*X+Y, this is easy,
looking pretty much like:
data Expr = Const Integer | Plus Expr Expr | Times Expr Expr
instance Num Expr where
fromInterger = Const
(+) = Plus
(*) = Times
c. This lets me get a perfectly nice AST, which is what I want.
When I want to be able to express and work with inequalities and
equalities, this breaks. Suppose I want to write 2*X + Y 3. I
either have to:
a) Hide Prelude.() and define a simple that builds the AST term I want.
b) Come up with a new symbol for it that doesn't look totally awful.
Neither of these work decently well. Hiding Eq and Ord operators,
which is what I effectively have to do for a), is pretty much a
nonstarter--we'll have to use them too much for that to be practical.
On the other hand, b) works...but is about as ugly as it gets. We
have lots and lots of symbols that are already taken for important
purposes that are syntactically near ,=,==, and the like: and
and = for monads, for arrows, etc. There...are not good
choices that I know of for the symbols that don't defeat the purpose
of making a nice clean EDSL for expressions; I might as well use 3*X +
Y `lessthan` 3, which is just not cool.
Does anyone know of a good solution, here? Are there good
substitutions for all the six operators that are important
(,,=,=,==,/=), that are close enough to be pretty-looking but not
used for other important modules?
Better yet, though a little harder, is there a nice type trick I'm not
thinking of? This works for Num methods but not for Ord methods
because:
(+) :: (Num a) = a - a - a
() :: (Ord a) = a - a - Bool
i.e. the return type of comparisons is totally fixed. I don't suppose
there's a good way to...well, I don't know what the *right* answer is,
but maybe define a new typeclass with a more flexible type for that
lets both standard types return Bool and my expressions return Expr?
Any good solution would be appreciated.
Thanks,
AHH
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] DSLs with {in,}equalities
Err, I was actually talking about the thread subject, where he actually has
the word {in,}equalities, short for inequalities and equalities (more or
less). AFAIK, that's unix notation.
On Tue, Mar 3, 2009 at 12:36 PM, Brandon S. Allbery KF8NH
allb...@ece.cmu.edu wrote:
On 2009 Mar 3, at 12:25, Andrew Wagner wrote:
Not to hijack the thread, but I thought I was the only one that used unix
notation for statements like {in,}equalities. I like it!
It's actually closer to Windows notation with the bracket on both sides
(and I actually considered making it %% but to me that looks more
cluttered, plus the S-curve in $ can be a mnemonic for symbolic for those
who don't live their lives on Unix).
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon universityKF8NH
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Difficulties in accessing inner elements of data types
Seems like Conal's semantic editor combinators could be of interest too, if I'm understanding correctly: http://conal.net/blog/posts/semantic-editor-combinators/ On Tue, Mar 3, 2009 at 8:00 PM, Tim Docker t...@dockerz.net wrote: While writing an OrgFile is fairly easy, reading (and accessing inner parts) of an org file is very tedious, and modifying them is horrendous. Have you looked at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-accessor It's something I've used successfully when wanting to manipulate the internals of complex types. Tim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Stacking State on State.....
I know very little about profiling, but your comment about spending a lot of time garbage collecting rang a bell with me - the example on RWH talks about that exact issue. Thus, I would recommend walking through the profiling techniques described on http://book.realworldhaskell.org/read/profiling-and-optimization.html . On Sun, Mar 1, 2009 at 3:03 PM, Phil pbeadl...@mail2web.com wrote: Hi, Thanks for the replies - I haven't had a chance to try out everything suggested yet - but your explanations of transformers nailed it for me. However, in terms of performance when stacking, I've come across something I'm struggling to explain - I was wondering if anyone could offer up and explanation. I've rewritten my code twice - one with 3 stacked monads, and one with 2 stacked monads and a load of maps. Heuristically I would have thought the 3 stacked monads would have performed as well, or even better than the 2 stacked solution, but the 2 stacked solution is MUCH faster and MUCH less memory is used. They are both using 90% of the same code and both chain together the same number of computations using replicateM. From profiling I can see that the pure function 'reflect' takes up most of the umph in both cases - which I'd expect. But in the triple stacked version the garbage collector is using up 90% of the time. I've tried using BangPatterns to reduce memory usage in the Triple Stack version - doing this I can half the time it takes, but it is still running at over twice the time of the two stack version. The BangPatterns were also put in Common Code in the reflect function - so I'd expect both solutions to need them? Even though both pieces of code are a bit untidy, the triple stacked monad 'feels' nicer to me - everything is encapsulated away and one evaluation in main yields the result. From purely a design perspective I prefer it - but obviously not if it runs like a dog! Any ideas why the triple stack runs so slow? Thanks again! Phil * Triple Stack Specific Impl: type MonteCarloStateT = StateT Double mc :: MonteCarloStateT BoxMullerQuasiState () mc = StateT $ \s - do nextNormal - generateNormal let stochastic = 0.2*1*nextNormal let drift = 0.05 - (0.5*(0.2*0.2))*1 let newStockSum = payOff 100 ( 100 * exp ( drift + stochastic ) ) + s return ((),newStockSum) iterations = 100 main :: IO() main = do let sumOfPayOffs = evalState ( evalStateT ( execStateT (do replicateM_ iterations mc) $ 0 ) $ (Nothing,nextHalton) ) $ (1,[3,5]) let averagePO = sumOfPayOffs / fromIntegral iterations let discountPO = averagePO * exp (-0.05) print discountPO * Double Stack and Map Specific Impl: iterations = 100 main :: IO() main = do let normals = evalState ( evalStateT (do replicateM iterations generateNormal) $ (Nothing,nextHalton) ) $ (1,[3,5]) let stochastic = map (0.2*1*) normals let sde = map ((( 0.05 - (0.5*(0.2*0.2)) )*1)+) stochastic let expiryMult = map exp sde let expiry = map (100*) expiryMult let payoff = map (payOff 100) expiry let averagePO = (foldr (+) 0 payoff) / fromIntegral iterations let discountPO = averagePO * exp (-0.05) print discountPO * Common Code Used By Both Methods: import Control.Monad.State import Debug.Trace -- State Monad for QRNGs - stores current iteration and list of -- bases to compute type QuasiRandomState = State (Int,[Int]) nextHalton :: QuasiRandomState [Double] nextHalton = do (n,bases) - get let !nextN = n+1 put (nextN,bases) return $ map (reflect (n,1,0)) bases type ReflectionThreadState = (Int,Double,Double) reflect :: ReflectionThreadState - Int - Double reflect (k,f,h) base | k = 0 = h | otherwise = reflect (newK,newF,newH) base where newK = k `div` base newF = f / fromIntegral base newH = h + fromIntegral(k `mod` base) * newF -- So we are defining a state transform which has state of 'maybe double' and an -- operating function for the inner monad of type QuasiRandomMonad returning a [Double] -- We then say that it wraps an QuasiRandomMonad (State Monad) - it must of course -- if we pass it a function that operates on these Monads we must wrap the same -- type of Monad. And finally it returns a double type BoxMullerStateT = StateT (Maybe Double, QuasiRandomState [Double]) type BoxMullerQuasiState = BoxMullerStateT QuasiRandomState generateNormal :: BoxMullerQuasiState Double generateNormal = StateT $ \s - case s of (Just d,qrnFunc) - return (d,(Nothing,qrnFunc)) (Nothing,qrnFunc) - do qrnBaseList - qrnFunc let (norm1,norm2) = boxMuller (head qrnBaseList) (head $ tail qrnBaseList)
Re: [Haskell-cafe] Stacking State on State.....
Ok, so this question of stacking state on top of state has come up several times lately. So I decided to whip up a small example. So here's a goofy little example of an abstract representation of a computer that can compute a value of type 'a'. The two states here are a value of type 'a', and a stack of functions of type (a-a) which can be applied to that value. Disclaimer: this code is only type-checked, not tested! import Control.Monad.State -- first, we'll rename the type, for convenience type Programmable a = StateT [a-a] (State a) -- add a function to the stack of functions that can be applied -- notice that we just use the normal State functions when dealing -- with the first type of state add :: (a - a) - Programmable a () add f = modify (f:) -- add a bunch of functions to the stack -- this time, notice that Programmable a is just a monad addAll :: [a - a] - Programmable a () addAll = mapM_ add -- this applies a function directly to the stored state, bypassing the function stack -- notice that, to use State functions on the second type of state, we must use -- lift to get to that layer modify' :: (a - a) - Programmable a () modify' f = lift (modify f) -- pop one function off the stack and apply it -- notice again the difference between modify' and modify. we use modify' to modify the value -- and modify to modify the function stack. This is again because of the order in which we wrapped -- the two states. If we were dealing with StateT a (State [a-a]), it would be the opposite. step :: Programmable a () step = do fs - get let f = if (null fs) then id else (head fs) modify' f modify $ if (null fs) then id else (const (tail fs)) -- run the whole 'program' runAll :: Programmable a () runAll = do fs - get if (null fs) then (return ()) else (step runAll) On Sat, Feb 28, 2009 at 8:31 AM, Daniel Fischer daniel.is.fisc...@web.dewrote: Am Samstag, 28. Februar 2009 13:23 schrieb Phil: Hi, The problem is HOW DO I WRAP ANOTHER INDEPENDENT STATE AROUND THIS? After some googling it looked like the answer may be Monad Transformers. Specifically we could add a StateT transform for our Box Muller state to our VanDerCorput State Monad. Google didn¹t yield a direct answer here so I¹m not even sure if my thinking is correct, people describe the process of using a transform as Œwrapping one monad in another¹ or Œthreading one monad into another¹. What we want to do is have some internal state controlled by an independent outer state - this sounds about right to me? If you absolutely don't want to have a state describing both, yes. So I started playing around with the code, and got the below to compile. test :: StateT (Bool,Double) (State Int) Double test = do (isStored,normal) - get let (retNorm,storeNorm) = if isStored then (normal,0) else (n1,n2) where n1 = 2 n2 = 3 put (not isStored, storeNorm) return retNorm Now this is incomplete and may be even wrong! I¹ll Explain my thinking: (Bool,Double) is equivalent to myState and storedNormal in the C example The last Double is the return value of the BoxMuller Monad The (State Int) is supposed to represent the VanDerCorput monad but the compiler (GHC 6.10) will only let me specify one parameter with it so I¹ve put the state and left the return type to the gods!! As I said this isn¹t quite right any ideas how to specify the type? You can't, the second argument to StateT must be a Monad, hence a type constructor you can pass an arbitrary type which then produces a new type from that. Fortunately, you don't need to. Say you have type VDCMonad = State Int nextVDC :: VDCMonad Double nextVDC = do n - get put $! (n+1) return $ calculateVDCFromInt n Then you could have boxMullerVDC :: StateT (Maybe Double) VDCMonad Double boxMullerVDC = StateT $ \s - case s of Just d - return (d,Nothing) Nothing - do d1 - nextVDC d2 - nextVDC let (b1,b2) = boxMullerTransform d1 d2 return (b1,Just b2) (I find a state of Maybe a more natural to indicate that *maybe* I have one a in store to use directly, than using (Bool,a)). However, I suspect that you would get better code if you abstracted over the sequence of pseudorandom Doubles and had simply calculation :: Sate [Double] whatever calculation = ??? result = evalState calculation bmVDC bmVDC = boxMuller $ map vanDerCorput [1 .. ] where boxMuller (k:n:more) = u:v:boxMuller more where
Re: [Haskell-cafe] Stacking State on State.....
Thanks for helping clean up my dirty little hacking. This could actually be made nicer by defining the following, and rewriting the original code in terms of it: type State2 a b = StateT a (State b) type Programmable a = State2 a (a-a) I'll leave the rewrite as an exercise for the reader, since I'm standing in the store writing this on my iPhone :) On Feb 28, 2009, at 10:08 AM, Daniel Fischer daniel.is.fisc...@web.de wrote: Am Samstag, 28. Februar 2009 15:36 schrieb Andrew Wagner: Ok, so this question of stacking state on top of state has come up several times lately. So I decided to whip up a small example. So here's a goofy little example of an abstract representation of a computer that can compute a value of type 'a'. The two states here are a value of type 'a', and a stack of functions of type (a-a) which can be applied to that value. Nice. Disclaimer: this code is only type-checked, not tested! import Control.Monad.State import Control.Moand (unless) -- first, we'll rename the type, for convenience type Programmable a = StateT [a-a] (State a) -- add a function to the stack of functions that can be applied -- notice that we just use the normal State functions when dealing -- with the first type of state add :: (a - a) - Programmable a () add f = modify (f:) -- add a bunch of functions to the stack -- this time, notice that Programmable a is just a monad addAll :: [a - a] - Programmable a () addAll = mapM_ add Be aware that this adds the functions in reverse order, an alternative is addAll = modify . (++) (addAll fs = modify (fs ++)) -- this applies a function directly to the stored state, bypassing the function stack -- notice that, to use State functions on the second type of state, we must use -- lift to get to that layer modify' :: (a - a) - Programmable a () modify' f = lift (modify f) -- pop one function off the stack and apply it -- notice again the difference between modify' and modify. we use modify' to modify the value -- and modify to modify the function stack. This is again because of the order in which we wrapped -- the two states. If we were dealing with StateT a (State [a-a]), it would be the opposite. step :: Programmable a () step = do fs - get let f = if (null fs) then id else (head fs) modify' f modify $ if (null fs) then id else (const (tail fs)) Last line could be modify (drop 1) -- run the whole 'program' runAll :: Programmable a () runAll = do fs - get if (null fs) then (return ()) else (step runAll) runAll = do stop - gets null unless stop (step runAll) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Stacking State on State.....
Heh. Actually, there is no more rewrite. But I muffed the second definition: it should, of course be: type Programmable a = State2 [a-a] a On Sat, Feb 28, 2009 at 10:35 AM, Andrew Wagner wagner.and...@gmail.comwrote: Thanks for helping clean up my dirty little hacking. This could actually be made nicer by defining the following, and rewriting the original code in terms of it: type State2 a b = StateT a (State b) type Programmable a = State2 a (a-a) I'll leave the rewrite as an exercise for the reader, since I'm standing in the store writing this on my iPhone :) On Feb 28, 2009, at 10:08 AM, Daniel Fischer daniel.is.fisc...@web.de wrote: Am Samstag, 28. Februar 2009 15:36 schrieb Andrew Wagner: Ok, so this question of stacking state on top of state has come up several times lately. So I decided to whip up a small example. So here's a goofy little example of an abstract representation of a computer that can compute a value of type 'a'. The two states here are a value of type 'a', and a stack of functions of type (a-a) which can be applied to that value. Nice. Disclaimer: this code is only type-checked, not tested! import Control.Monad.State import Control.Moand (unless) -- first, we'll rename the type, for convenience type Programmable a = StateT [a-a] (State a) -- add a function to the stack of functions that can be applied -- notice that we just use the normal State functions when dealing -- with the first type of state add :: (a - a) - Programmable a () add f = modify (f:) -- add a bunch of functions to the stack -- this time, notice that Programmable a is just a monad addAll :: [a - a] - Programmable a () addAll = mapM_ add Be aware that this adds the functions in reverse order, an alternative is addAll = modify . (++) (addAll fs = modify (fs ++)) -- this applies a function directly to the stored state, bypassing the function stack -- notice that, to use State functions on the second type of state, we must use -- lift to get to that layer modify' :: (a - a) - Programmable a () modify' f = lift (modify f) -- pop one function off the stack and apply it -- notice again the difference between modify' and modify. we use modify' to modify the value -- and modify to modify the function stack. This is again because of the order in which we wrapped -- the two states. If we were dealing with StateT a (State [a-a]), it would be the opposite. step :: Programmable a () step = do fs - get let f = if (null fs) then id else (head fs) modify' f modify $ if (null fs) then id else (const (tail fs)) Last line could be modify (drop 1) -- run the whole 'program' runAll :: Programmable a () runAll = do fs - get if (null fs) then (return ()) else (step runAll) runAll = do stop - gets null unless stop (step runAll) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Left fold enumerator - a real pearl overlooked?
Can someone point us to some resources about these? I for one have not heard of them before On Sat, Feb 28, 2009 at 12:59 PM, Johan Tibell johan.tib...@gmail.comwrote: On Sat, Feb 28, 2009 at 5:08 PM, Gü?nther Schmidt gue.schm...@web.de wrote: Hi all, in the last few months I was looking for haskell database library, eventually settling for HDBC (thanks John btw). Takusen also caught my eye although I even failed installing it. Nevertheless a particular property of takusen, left-fold-enumerator, also sparked my interest and I tried to follow it up. I had the impression this is something of relevance but something that is not mentioned or made use of often. There are actually only a few references on the net and most of those by one person only, Oleg. There was one post from John Goerzen about Haskell HTTP libraries in which he hinted using left-fold enumerators. Anyway what I'm saying is that the whole topic is somewhat off the radar of the larger haskell community, how come, or am I merely overestimating its relevance and usefulness? A couple of people (me included) have been discussing different possible designs outside this mailing list. I think we're getting a better understanding of the design space but I feel like there's more to be discovered which is probably the reason none of us have release a library that makes use of left fold enumerators. Cheers, Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskellers on Twitter!
Modified. Is this how you want it to show? 2009/2/23 Brandon S. Allbery KF8NH allb...@ece.cmu.edu On 2009 Feb 21, at 16:14, Daniel Peebles wrote: http://haskell.org/haskellwiki/Twitter. Please update with yourself or any other Haskellers we may have missed. Since I can't create an account to fix my entry: (1) my name is misspelled (2) partly to synchronize my accounts and partly to try to figure out why searches for me on http://search.twitter.com fail, I changed my account name. If you're looking to follow me on Twitter, I'm now geekosaur. (I still don't show up in searches though. One of many; the folks at Twitter seem to have trouble with getting their indexes to work right) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to create an online poll
Just wrap it in a ReallySafeWePromiseMonad 2009/2/18 gregg reynolds d...@mobileink.com I.e. war, plague.famine. etc. gregg reynolds d...@mobileink.com wrote: But what about the side effects? Rick R rick.richard...@gmail.com wrote: I'm sure Premier Election Solutions (formerly Diebold) can provide us with an online voting solution. Their value-add services allows us to set the outcome beforehand, so, in effect, the the voting process will be determinate. Which is certainly of interest to Haskell coders. On Wed, Feb 18, 2009 at 4:05 PM, Max Rabkin max.rab...@gmail.com wrote: On Wed, Feb 18, 2009 at 10:40 PM, Anton van Straaten an...@appsolutions.com wrote: There's also the Condorcet Internet Voting Service: http://www.cs.cornell.edu/andru/civs.html This looks like exactly what we need! Any objections? --Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- We can't solve problems by using the same kind of thinking we used when we created them. - A. Einstein ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to create an online poll
Sure it will. Don't worry, it uses unsafePerformIO under the hood, so it's pretty much indestructible. On Wed, Feb 18, 2009 at 4:47 PM, gregg reynolds d...@mobileink.com wrote: Ok, that might protect us against the 7 plagues, so I'm not so worried about locusts. But what about Java, PHP etc. Is the monad sufficient protection against ultra-supernatural evil? Andrew Wagner wagner.and...@gmail.com wrote: Just wrap it in a ReallySafeWePromiseMonad 2009/2/18 gregg reynolds d...@mobileink.com I.e. war, plague.famine. etc. gregg reynolds d...@mobileink.com wrote: But what about the side effects? Rick R rick.richard...@gmail.com wrote: I'm sure Premier Election Solutions (formerly Diebold) can provide us with an online voting solution. Their value-add services allows us to set the outcome beforehand, so, in effect, the the voting process will be determinate. Which is certainly of interest to Haskell coders. On Wed, Feb 18, 2009 at 4:05 PM, Max Rabkin max.rab...@gmail.com wrote: On Wed, Feb 18, 2009 at 10:40 PM, Anton van Straaten an...@appsolutions.com wrote: There's also the Condorcet Internet Voting Service: http://www.cs.cornell.edu/andru/civs.html This looks like exactly what we need! Any objections? --Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- We can't solve problems by using the same kind of thinking we used when we created them. - A. Einstein ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to create an online poll
Help! Help! I'm being suppressed!! On Feb 18, 2009, at 5:05 PM, Rick R rick.richard...@gmail.com wrote: I can't comment on any quantitative side effects, but the some intangible side effects include Distrustful Populace and MaliciousDissenters. However, if ghc is run with the - XUnscrupulousPolitics flag, those can be suppressed. On Wed, Feb 18, 2009 at 4:33 PM, gregg reynolds d...@mobileink.com wrote: I.e. war, plague.famine. etc. gregg reynolds d...@mobileink.com wrote: But what about the side effects? Rick R rick.richard...@gmail.com wrote: I'm sure Premier Election Solutions (formerly Diebold) can provide us with an online voting solution. Their value-add services allows us to set the outcome beforehand, so, in effect, the the voting process will be determinate. Which is certainly of interest to Haskell coders. On Wed, Feb 18, 2009 at 4:05 PM, Max Rabkin max.rab...@gmail.com wrote: On Wed, Feb 18, 2009 at 10:40 PM, Anton van Straaten an...@appsolutions.com wrote: There's also the Condorcet Internet Voting Service: http://www.cs.cornell.edu/andru/civs.html This looks like exactly what we need! Any objections? --Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- We can't solve problems by using the same kind of thinking we used when we created them. - A. Einstein ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- We can't solve problems by using the same kind of thinking we used when we created them. - A. Einstein ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell] Re: [Haskell-cafe] ANN: The Typeclassopedia, and request for feedback
Positively brilliant. What else can be said? Time for Brent to sign a Haskell recipes deal with O'Reilly (or whatever the next normal book should be). On Mon, Feb 16, 2009 at 4:29 AM, Brent Yorgey byor...@seas.upenn.eduwrote: Hi all, If you've noticed the lack of a HWN this week, that's because I've been doggedly finishing my article entitled 'The Typeclassopedia', which I have just submitted for publication in the Monad.Reader. Here's the abstract: The standard Haskell libraries feature a number of type classes with algebraic or categorical underpinnings. Becoming a fluent Haskell hacker requires intimate familiarity with them all, yet acquiring this familiarity often involves combing through a mountain of tutorials, blog posts, mailing list archives, and IRC logs. The goal of this article is to serve as a starting point for the student of Haskell wishing to gain a firm grasp of its standard type classes. The essentials of each type class are introduced, with examples, commentary, and extensive references for further reading. My hope is that this will be a valuable resource to the Haskell community, especially those who are learning. Any feedback would be greatly appreciated, especially if it helps improve the article before publication. A draft can be found here: http://www.cis.upenn.edu/~byorgey/papers/typeclassopedia-draft-090216.pdf Also see my blog post for a bit more info: http://byorgey.wordpress.com/2009/02/16/the-typeclassopedia-request-for-feedback/ happy haskelling! -Brent ___ Haskell-Cafe mailing list haskell-c...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell-cafe] ANN: The Typeclassopedia, and request for feedback
Positively brilliant. What else can be said? Time for Brent to sign a Haskell recipes deal with O'Reilly (or whatever the next normal book should be). On Mon, Feb 16, 2009 at 4:29 AM, Brent Yorgey byor...@seas.upenn.eduwrote: Hi all, If you've noticed the lack of a HWN this week, that's because I've been doggedly finishing my article entitled 'The Typeclassopedia', which I have just submitted for publication in the Monad.Reader. Here's the abstract: The standard Haskell libraries feature a number of type classes with algebraic or categorical underpinnings. Becoming a fluent Haskell hacker requires intimate familiarity with them all, yet acquiring this familiarity often involves combing through a mountain of tutorials, blog posts, mailing list archives, and IRC logs. The goal of this article is to serve as a starting point for the student of Haskell wishing to gain a firm grasp of its standard type classes. The essentials of each type class are introduced, with examples, commentary, and extensive references for further reading. My hope is that this will be a valuable resource to the Haskell community, especially those who are learning. Any feedback would be greatly appreciated, especially if it helps improve the article before publication. A draft can be found here: http://www.cis.upenn.edu/~byorgey/papers/typeclassopedia-draft-090216.pdf Also see my blog post for a bit more info: http://byorgey.wordpress.com/2009/02/16/the-typeclassopedia-request-for-feedback/ happy haskelling! -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Another point-free question (=, join, ap)
Check out liftM2. It's almost what you want. On Thu, Feb 12, 2009 at 6:36 PM, Edsko de Vries devri...@cs.tcd.ie wrote: Hi, I can desugar do x' - x f x' as x = \x - f x' which is clearly the same as x = f However, now consider do x' - x y' - y f x' y' desugared, this is x = \x - y = \y' - f x' y' I can simplify the second half to x = \x - y = f x' but now we are stuck. I feel it should be possible to write something like x ... y ... f or perhaps f ... x ... y the best I could come up with was join $ return f `ap` x `ap` y which is not terrible but quite as easy as I feel this should be. Any hints? Edsko ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Is this related to continuations somehow?
So, I was reading a bit about continuations the other day, and, since I've been thinking about good ways of expressing chess strategies in Haskell, I thought that I'd play around a bit with something like continuations for game-playing strategies. The idea is that you have combinators that allow you full access to the strategies which remain to be applied. In this way, strategies can activate and de-activate other strategies. Here's a simple little toy app for Tic-Tac-Toe (Naughts and Crosses): http://codepad.org/nN9JsxFK You can run main on 'example', and see that it searches every line and fails. And, as you can see, it aborts after finding a win in example2. This would be easily extensible to say things like if you've seen a blocking move, and you don't have a win, then play the blocking move, and of course the other deep intricacies of the game. My question is, is this, in fact, related to continuations somehow? Could continuations simplify it? Or am I doing something completely different? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Control.Arrow being icky
It's spelled 'Kleisli'. You spelled it 'Kliesli'
2009/2/9 Louis Wasserman wasserman.lo...@gmail.com
In GHCi, I import Control.Arrow, and Kliesli doesn't appear:
Prelude Control.Arrow :t Kliesli
interactive:1:0: Not in scope: data constructor `Kliesli'
Prelude Control.Arrow :browse
(^) :: (Arrow a) = a c d - (b - c) - a b d
(^) :: (Arrow a) = a b c - (c - d) - a b d
class (Control.Category.Category a) = Arrow a where
arr :: (b - c) - a b c
first :: a b c - a (b, d) (c, d)
second :: a b c - a (d, b) (d, c)
(***) :: a b c - a b' c' - a (b, b') (c, c')
() :: a b c - a b c' - a b (c, c')
class (Arrow a) = ArrowApply a where app :: a (a b c, b) c
class (Arrow a) = ArrowChoice a where
left :: a b c - a (Either b d) (Either c d)
right :: a b c - a (Either d b) (Either d c)
(+++) :: a b c - a b' c' - a (Either b b') (Either c c')
(|||) :: a b d - a c d - a (Either b c) d
class (Arrow a) = ArrowLoop a where
loop :: a (b, d) (c, d) - a b c
newtype (ArrowApply a) = ArrowMonad a b = ArrowMonad (a () b)
class (ArrowZero a) = ArrowPlus a where
(+) :: a b c - a b c - a b c
class (Arrow a) = ArrowZero a where zeroArrow :: a b c
newtype Kleisli m a b = Kleisli {runKleisli :: a - m b}
(^) :: (Arrow a) = (c - d) - a b c - a b d
(^) :: (Arrow a) = (b - c) - a c d - a b d
leftApp :: (ArrowApply a) = a b c - a (Either b d) (Either c d)
returnA :: (Arrow a) = a b b
() ::
(Control.Category.Category cat) = cat b c - cat a b - cat a c
() ::
(Control.Category.Category cat) = cat a b - cat b c - cat a c
Does anybody know what's going on?
Louis Wasserman
wasserman.lo...@gmail.com
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell Fest
We believe in Haskell! 2009/2/9 Lyle Kopnicky li...@qseep.net Looks like a lot of fun! http://www.haskellchamber.com/page6.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The Haskell re-branding exercise
We need a voting site set up. There was some progress prior to the end of the year. Updates welcome! -- Don Can't we just use the haskell proposal reddit for this? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The Haskell re-branding exercise
Um, ok. Glad we could discuss it On Sat, Feb 7, 2009 at 1:12 PM, Don Stewart d...@galois.com wrote: wagner.andrew: We need a voting site set up. There was some progress prior to the end of the year. Updates welcome! -- Don Can't we just use the haskell proposal reddit for this? Hmm... not ideal. Would make a backup should all else fail. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: evaluation semantics of bind
Are you saying that using equations to add a level of indirection prevents optimization? I still don't see it - discarding x doesn't change the semantics, so a good compiler /should/ do this. How is this different from optimizing out application of a constant function? No, no compiler should change getChar = \x - getChar to just getChar, because it's just wrong. The first will try to read in two values of type Char, the second will only try to read one. Side effects aren't THAT hated! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad explanation
I think the point of the Monad is that it works as a container of stuff, that still allows mathematically pure things to happen, while possibly having some opaque other stuff going on. This at least sounds, very wrong, even if it's not. Monads are not impure. IO is, but it's only _one_ instance of Monad. All others, as far as I know, are pure. It's just that the bind operation allows you to hide the stuff you don't want to have to worry about, that should happen every time you compose two monadic actions. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad explanation
On Thu, Feb 5, 2009 at 3:21 PM, David Leimbach leim...@gmail.com wrote: Well all I can tell you is that I can have (IO Int) in a function as a return, and the function is not idempotent in terms of the stuff inside IO being the same. Sounds impure to me. Right, thus IO is impure. but as long as the function that returns the IO Int returns the same IO Int every time you provide the same input, it's pure. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Bind as a sequencing operator
bind is no more a sequencing operator than function composition is. Just as the order in which you pass two functions (ignoring the type issue for the moment) matters, the order in which you pass things to bind matters. The sense in which the order _doesn't_ matter is that of the order in which what you pass gets evaluated. On Thu, Feb 5, 2009 at 3:32 PM, Jake McArthur j...@pikewerks.com wrote: -BEGIN PGP SIGNED MESSAGE- Hash: SHA1 m...@justinbogner.com wrote: | Jake McArthur j...@pikewerks.com writes: | The problem with your description is that you said and then. The | result will be generated lazily. There is no sequencing here. Consider: | | ~do x - [0..] | ~ y - [0..9] | ~ return (x, y) | | Which list is generated first? | | | This is an implementation detail, since y has no dependency on x the | compiler's free to reorder instructions, as it would be an imperative | language. Semantically this means do x and then y That the order does not matter is definitely not an implementation detail. Order not mattering is exactly what declarative programming is about. The semantics of the above expression are the same as this: ~($ [0..9]) . (,) = [0..] There is no do X and then do Y, only this is what I want. Declarative semantics. As I said, just because do notation makes it look imperative doesn't mean it actually is. | Just because the compiler is allowed (and even encouraged) to change the | sequence where that won't change the results, considering this a | sequence is still valid and meaningful. It can be helpful sometimes, but I don't think it should be the standard way to think of bind. There are too many cases when it makes little sense as a sequencing operator. - - Jake -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAkmLTPEACgkQye5hVyvIUKkoCACgz/IYvxa6PoEvuqgxljGAwZ8+ TXQAn30MyLDwhLyZV3+dRuJvttx93ZNh =P9em -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Just how unsafe is unsafe
So we all know the age-old rule of thumb, that unsafeXXX is simply evil and anybody that uses it should be shot (except when it's ok). I understand that unsafeXXX allows impurity, which defiles our ability to reason logically about haskell programs like we would like to. My question is, to what extent is this true? Suppose we had a module, UnsafeRandoms, which had a function that would allow you to generate a different random number every time you call it. The semantics are relatively well-defined, impurity is safely sectioned off in its own impure module, which is clearly labeled as such. How much damage does this do? Can we push the lines elsewhere? Is sectioning unsafeXXX into Unsafe modules a useful idiom that we can use for other things as well? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad explanation
This is nice and simple. My only concern is I'm not sure there's enough of a distinction between Monad and State Monad. That is, I'm not sure it's clear enough that the way you're binding the small programs together in the initial example is only one way you could bind them together, and thus it's only one instance of Monad. On Wed, Feb 4, 2009 at 12:41 PM, Tim Newsham news...@lava.net wrote: I put up a small Monad explanation (I wouldn't quite call it a tutorial): http://www.thenewsh.com/~newsham/haskell/monad.html The intent here is to is to have a short description of what a Monad is that is approachable by Haskell beginners or non- Haskell programmers who are looking for an intuitive definition but may not yet have the background or the inclination to jump into full tutorial to tackle the subject. Tim Newsham http://www.thenewsh.com/~newsham/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell] Re: [Haskell-cafe] ANN: #haskell-in-depth IRC channel
Well-done! I've said for many months that we need a channel like this! On Tue, Feb 3, 2009 at 7:15 PM, Philippa Cowderoy fli...@flippac.orgwrote: Hi folks. As I've been daft enough to get a few things rolling, it looks like it's fallen on me to announce the new IRC channel, #haskell-in-depth. #haskell has been a roaring success over the last few years, as Don has repeatedly pointed out. Unfortunately that roar is starting to make it hard for some to participate - there's so much traffic that it can be difficult to fit an additional conversation in. We're starting to hit the limits as to how far one channel can take us, and that means we need to explore ways to divide up traffic. Actually, this isn't entirely new - a number of years ago now, #haskell-blah was formed as a space for off-topic (and sometimes less-than-worksafe) conversation among #haskell regulars. We also created #haskell-overflow, but it doesn't see much use except among some of the cabal implementors because it's hard to know when to take the conversation you're currently in to -overflow. We need channels that people start their conversations in, not ones to send people to! So we're trying a space for in-depth discussion. The new channel is open to everyone, just like #haskell. But just as we're hoping for certain kinds of discussion, there're others we want to avoid. If you need to know how to use monads so you can do IO, #haskell-in-depth isn't the place. On the other hand, if you want to discuss how Haskell's monads compare to the category theory or what the category theory can tell us about how individual monads relate to the language as a whole, -in-depth is a good place! In particular, we're hoping that the kind of category theory discussions that give the mistaken impression you actually need to know CT will increasingly live in #haskell-in-depth. We're not after a theory channel though - architectural discussion, compiler implementation, possible type system extensions, library design, all are good subjects. Anyway, I shouldn't ramble on for too long here - #haskell-in-depth is open for business and we look forward to seeing you there! -- Philippa Cowderoy fli...@flippac.org ___ Haskell-Cafe mailing list haskell-c...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell-cafe] Converting Lists to Sets
Actually, a list of list of literals is needed, since it's a Set Clause, and
a Clause is a [Literal].
On Tue, Feb 3, 2009 at 5:24 PM, Robin Green gree...@greenrd.org wrote:
On Tue, 3 Feb 2009 19:58:51 -0200
rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote:
Hi all,
I'm trying to use the Funsat library. One of its data types is CNF:
data CNF = CNF {
numVars :: Int
numClauses :: Int
clauses :: Set Clause
}
I have a list of clauses, but I'm getting an error when converting
such a list to a Set. Using the fromList function, the ghc compiler
reports that the fromList function is not applicable to literals.
type Clause = [Lit]
newtype Lit = L {
unLit :: Int
}
You don't provide your code or the exact error message, which makes
it harder to diagnose - but I'd guess you are mistakenly trying to apply
fromList to a *single* literal, rather than a *list* of literals.
--
Robin
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: #haskell-in-depth IRC channel
Well-done! I've said for many months that we need a channel like this! On Tue, Feb 3, 2009 at 7:15 PM, Philippa Cowderoy fli...@flippac.orgwrote: Hi folks. As I've been daft enough to get a few things rolling, it looks like it's fallen on me to announce the new IRC channel, #haskell-in-depth. #haskell has been a roaring success over the last few years, as Don has repeatedly pointed out. Unfortunately that roar is starting to make it hard for some to participate - there's so much traffic that it can be difficult to fit an additional conversation in. We're starting to hit the limits as to how far one channel can take us, and that means we need to explore ways to divide up traffic. Actually, this isn't entirely new - a number of years ago now, #haskell-blah was formed as a space for off-topic (and sometimes less-than-worksafe) conversation among #haskell regulars. We also created #haskell-overflow, but it doesn't see much use except among some of the cabal implementors because it's hard to know when to take the conversation you're currently in to -overflow. We need channels that people start their conversations in, not ones to send people to! So we're trying a space for in-depth discussion. The new channel is open to everyone, just like #haskell. But just as we're hoping for certain kinds of discussion, there're others we want to avoid. If you need to know how to use monads so you can do IO, #haskell-in-depth isn't the place. On the other hand, if you want to discuss how Haskell's monads compare to the category theory or what the category theory can tell us about how individual monads relate to the language as a whole, -in-depth is a good place! In particular, we're hoping that the kind of category theory discussions that give the mistaken impression you actually need to know CT will increasingly live in #haskell-in-depth. We're not after a theory channel though - architectural discussion, compiler implementation, possible type system extensions, library design, all are good subjects. Anyway, I shouldn't ramble on for too long here - #haskell-in-depth is open for business and we look forward to seeing you there! -- Philippa Cowderoy fli...@flippac.org ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Fold that quits early?
This is almost a fold, but seemingly not quite? I know I've seem some talk of folds that potentially quit early. but not sure where I saw that, or if it fits. f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (nub (y:xs)) (ys' ++ ys) where ys' = g y xs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fold that quits early?
Actually, I think I can rewrite it this way: f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (f (y:xs) ys') ys where ys' = g y ys This should help, I think. On Sat, Jan 24, 2009 at 12:11 PM, Dan Doel dan.d...@gmail.com wrote: On Saturday 24 January 2009 11:39:13 am Andrew Wagner wrote: This is almost a fold, but seemingly not quite? I know I've seem some talk of folds that potentially quit early. but not sure where I saw that, or if it fits. f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (nub (y:xs)) (ys' ++ ys) where ys' = g y xs Quitting early isn't the problem. For instance, any is defined in terms of foldr, and it works fine on infinite lists, quitting as soon as it finds a True. As long as you don't use the result of the recursive call (which is the second argument in the function you pass to foldr), it will exit early. The problem is that your function doesn't look structurally recursive, and that's what folds (the usual ones, anyway) are: a combinator for structural recursion. The problem is in your inductive case, where you don't just recurse on ys, you instead recurse on ys' ++ ys, where ys' is the result of an arbitrary function. folds simply don't work that way, and only give you access to the recursive call over the tail, but in a language like Agda, the termination checker would flag even this definition, and you'd have to, for instance, write a proof that this actually does terminate, and do induction on the structure of the proof, or use some other such technique for writing non- structurally recursive functions. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fold that quits early?
Ahem. That's what happens when you don't type-check your brainstorming before barfing it out, kids. Sorry about that. Let's try this again: f _ [] = [] f y x | c y = [] | otherwise = foldr f (y:x) (g y x) I've got a fold on the inside now, but I'm pretty sure the whole thing is just a fold. Not quite there yet, though... On Sat, Jan 24, 2009 at 1:42 PM, Andrew Wagner wagner.and...@gmail.comwrote: Actually, I think I can rewrite it this way: f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (f (y:xs) ys') ys where ys' = g y ys This should help, I think. On Sat, Jan 24, 2009 at 12:11 PM, Dan Doel dan.d...@gmail.com wrote: On Saturday 24 January 2009 11:39:13 am Andrew Wagner wrote: This is almost a fold, but seemingly not quite? I know I've seem some talk of folds that potentially quit early. but not sure where I saw that, or if it fits. f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (nub (y:xs)) (ys' ++ ys) where ys' = g y xs Quitting early isn't the problem. For instance, any is defined in terms of foldr, and it works fine on infinite lists, quitting as soon as it finds a True. As long as you don't use the result of the recursive call (which is the second argument in the function you pass to foldr), it will exit early. The problem is that your function doesn't look structurally recursive, and that's what folds (the usual ones, anyway) are: a combinator for structural recursion. The problem is in your inductive case, where you don't just recurse on ys, you instead recurse on ys' ++ ys, where ys' is the result of an arbitrary function. folds simply don't work that way, and only give you access to the recursive call over the tail, but in a language like Agda, the termination checker would flag even this definition, and you'd have to, for instance, write a proof that this actually does terminate, and do induction on the structure of the proof, or use some other such technique for writing non- structurally recursive functions. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fold that quits early?
There's at least one thing; I won't call it a flaw in your logic, but it's not true of my usage. Your definition always produces a non-null list. The particular g in my mind will eventually produce a null list, somewhere down the line. I think if that's true, we can guarantee termination. On Sat, Jan 24, 2009 at 10:16 PM, Ryan Ingram ryani.s...@gmail.com wrote: foldr f z xs = x0 `f` (x1 `f` (x2 `f` ... (xN `f` z) ...)) In particular, note that if f is a total function, xs is finite and totally defined, and z is totally defined, then the result of this fold is totally defined. Now consider your f (rewritten slightly) f x xs | null xs = [] | p x = [] | otherwise = foldr f (x:xs) (g x xs) with these definitions: c _ = False g = (:) c and g are definitely total functions, so what happens when we evaluate f 0 [1]? f 0 [1] = foldr f [0,1] [0,1] = let t1 = foldr f [0,1] [1] in f 0 $ t1 = let t1 = foldr f [0,1] [1] in foldr f (0 : t1) (0 : t1) = let t1 = foldr f [0,1] [1] t2 = foldr f (0:t1) t1 in f 0 t2 etc.; this is clearly non-terminating. Therefore there is no way to make f into a fold. Any errors in my logic? -- ryan 2009/1/24 Andrew Wagner wagner.and...@gmail.com: Ahem. That's what happens when you don't type-check your brainstorming before barfing it out, kids. Sorry about that. Let's try this again: f _ [] = [] f y x | c y = [] | otherwise = foldr f (y:x) (g y x) I've got a fold on the inside now, but I'm pretty sure the whole thing is just a fold. Not quite there yet, though... On Sat, Jan 24, 2009 at 1:42 PM, Andrew Wagner wagner.and...@gmail.com wrote: Actually, I think I can rewrite it this way: f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (f (y:xs) ys') ys where ys' = g y ys This should help, I think. On Sat, Jan 24, 2009 at 12:11 PM, Dan Doel dan.d...@gmail.com wrote: On Saturday 24 January 2009 11:39:13 am Andrew Wagner wrote: This is almost a fold, but seemingly not quite? I know I've seem some talk of folds that potentially quit early. but not sure where I saw that, or if it fits. f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (nub (y:xs)) (ys' ++ ys) where ys' = g y xs Quitting early isn't the problem. For instance, any is defined in terms of foldr, and it works fine on infinite lists, quitting as soon as it finds a True. As long as you don't use the result of the recursive call (which is the second argument in the function you pass to foldr), it will exit early. The problem is that your function doesn't look structurally recursive, and that's what folds (the usual ones, anyway) are: a combinator for structural recursion. The problem is in your inductive case, where you don't just recurse on ys, you instead recurse on ys' ++ ys, where ys' is the result of an arbitrary function. folds simply don't work that way, and only give you access to the recursive call over the tail, but in a language like Agda, the termination checker would flag even this definition, and you'd have to, for instance, write a proof that this actually does terminate, and do induction on the structure of the proof, or use some other such technique for writing non- structurally recursive functions. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fold that quits early?
Ok, maybe let's make this a little more concrete. I'm trying to write forward-chaining logical inference as a fold. I actually don't mind this code much as it is (about half or 2/3 the size it was an hour ago), but I'm wondering if I can do better. -- a sentence is a conjunction of clauses type Sentence a = [Clause a] -- each clause is a disjunction of terms type Clause a = [Term a] -- one sentence entails a second sentence if negating the second -- and inserting its clauses, and the clauses that can be deduced, into -- the first, results in a contradiction (which we'll represent as an -- empty list of clauses) entails :: Eq a = Sentence a - Sentence a - Bool entails s1 s2 = null $ foldr (flip insertInto) s1 (neg s2) insertInto :: Eq a = [Clause a] - Clause a - [Clause a] insertInto [] _ = [] -- inserting into a contradictory set of clauses is still contradictory insertInto x y | contradiction y = [] -- inserting a clause which is contradictory is also a contradiction | otherwise = foldr (flip insertInto) (y:x) (concatMap (resolve y) x) contradiction :: Clause a - Bool contradiction = null -- a list of all the ways the resolution law can be applied: -- http://en.wikipedia.org/wiki/Resolution_(logic)#Resolution_in_propositional_logic resolve :: Eq a = Clause a - Clause a - [Clause a] resolve xs ys = [combine x (xs ++ ys) | x - xs, (neg_term x) `elem` ys] combine :: Eq a = Term a - Clause a - Clause a combine x = nub . delete x . delete (neg_term x) On Sat, Jan 24, 2009 at 11:25 PM, Ryan Ingram ryani.s...@gmail.com wrote: My point is that without further knowledge of this function, it's not possible to simplify into a fold. For f to be a fold, the result would have to terminate for *every* total function g; given that there are some total functions for which f does not terminate, f cannot be a fold. It's possible that the particular combination of f and g that you have in mind *does* simplify to a fold, but the current function cannot, unless I've made a mistake. -- ryan On Sat, Jan 24, 2009 at 7:26 PM, Andrew Wagner wagner.and...@gmail.com wrote: There's at least one thing; I won't call it a flaw in your logic, but it's not true of my usage. Your definition always produces a non-null list. The particular g in my mind will eventually produce a null list, somewhere down the line. I think if that's true, we can guarantee termination. On Sat, Jan 24, 2009 at 10:16 PM, Ryan Ingram ryani.s...@gmail.com wrote: foldr f z xs = x0 `f` (x1 `f` (x2 `f` ... (xN `f` z) ...)) In particular, note that if f is a total function, xs is finite and totally defined, and z is totally defined, then the result of this fold is totally defined. Now consider your f (rewritten slightly) f x xs | null xs = [] | p x = [] | otherwise = foldr f (x:xs) (g x xs) with these definitions: c _ = False g = (:) c and g are definitely total functions, so what happens when we evaluate f 0 [1]? f 0 [1] = foldr f [0,1] [0,1] = let t1 = foldr f [0,1] [1] in f 0 $ t1 = let t1 = foldr f [0,1] [1] in foldr f (0 : t1) (0 : t1) = let t1 = foldr f [0,1] [1] t2 = foldr f (0:t1) t1 in f 0 t2 etc.; this is clearly non-terminating. Therefore there is no way to make f into a fold. Any errors in my logic? -- ryan 2009/1/24 Andrew Wagner wagner.and...@gmail.com: Ahem. That's what happens when you don't type-check your brainstorming before barfing it out, kids. Sorry about that. Let's try this again: f _ [] = [] f y x | c y = [] | otherwise = foldr f (y:x) (g y x) I've got a fold on the inside now, but I'm pretty sure the whole thing is just a fold. Not quite there yet, though... On Sat, Jan 24, 2009 at 1:42 PM, Andrew Wagner wagner.and...@gmail.com wrote: Actually, I think I can rewrite it this way: f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (f (y:xs) ys') ys where ys' = g y ys This should help, I think. On Sat, Jan 24, 2009 at 12:11 PM, Dan Doel dan.d...@gmail.com wrote: On Saturday 24 January 2009 11:39:13 am Andrew Wagner wrote: This is almost a fold, but seemingly not quite? I know I've seem some talk of folds that potentially quit early. but not sure where I saw that, or if it fits. f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (nub (y:xs)) (ys' ++ ys) where ys' = g y xs Quitting early isn't the problem. For instance, any is defined in terms of foldr, and it works fine on infinite lists, quitting as soon as it finds a True. As long as you don't use the result of the recursive call (which is the second argument in the function you pass to foldr), it will exit early. The problem is that your function doesn't
Re: [Haskell-cafe] Why monoids will abide...
See, that's the kind of name we need! StructureWithAssociativeOperationAndIdentity -- make both the mathematicians AND the non-mathematicians mad! On Thu, Jan 22, 2009 at 9:53 AM, Dan Piponi dpip...@gmail.com wrote: On Wed, Jan 21, 2009 at 11:12 PM, Eugene Kirpichov ekirpic...@gmail.com wrote: To my mind, in the map-reduce case you generally need a commutative monoid. Or, you need an extra infrastructure that mappend's only results from adjacent machines, or something like that. This is a good paper on the stuff I'm talking about: http://citeseer.ist.psu.edu/blelloch90prefix.html It doesn't explicitly mention monoids but it's all about associative operations with identity. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell mode for Emacs question
Interesting. I have a similar, but worse problem. For me, ':load'ing main.hs would fail to find the imported files. The only thing I appear to be able to :load is files that don't import from local directories. 2009/1/22 Peter Verswyvelen bugf...@gmail.com I have a silly problem. I'm using Emacs with the Haskell mode extension on Windows I have a source file in say c:/foo/src/main.hs main.hs is importing some other modules in that same src directory When I invoke GHCi from within Emacs, the first thing it does is :cd c:/foo and then :load src/main.hs But of course GHCi won't find the imported modules now, since the current directory is wrong. If I type in GHCi :cd src :load main.hs then it compiles fine. Does anyone have an idea why Emacs or the Haskell mode is switching to the parent directory of src instead of src itself, and how to fix this? Thanks a lot, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell mode for Emacs question
Hmm, strange. This doesn't appear to fix my problem. Perhaps I have something bigger broken locally :( On Thu, Jan 22, 2009 at 12:44 PM, Peter Verswyvelen bugf...@gmail.comwrote: indeed, that works! the comment above those lines is: ;; Not sure if it's useful/needed and if it actually works. :-) On Thu, Jan 22, 2009 at 6:41 PM, Miguel Mitrofanov miguelim...@yandex.ruwrote: On my Mac I've had the same problem. Commenting out lines (unless (equal default-directory root) (setq default-directory root) (inferior-haskell-send-command proc (concat :cd default-directory))) solved it for me. On 22 Jan 2009, at 20:23, Andrew Wagner wrote: Interesting. I have a similar, but worse problem. For me, ':load'ing main.hs would fail to find the imported files. The only thing I appear to be able to :load is files that don't import from local directories. 2009/1/22 Peter Verswyvelen bugf...@gmail.com I have a silly problem. I'm using Emacs with the Haskell mode extension on Windows I have a source file in say c:/foo/src/main.hs main.hs is importing some other modules in that same src directory When I invoke GHCi from within Emacs, the first thing it does is :cd c:/foo and then :load src/main.hs But of course GHCi won't find the imported modules now, since the current directory is wrong. If I type in GHCi :cd src :load main.hs then it compiles fine. Does anyone have an idea why Emacs or the Haskell mode is switching to the parent directory of src instead of src itself, and how to fix this? Thanks a lot, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
That's..evil On Thu, Jan 22, 2009 at 7:56 PM, Jonathan Cast jonathancc...@fastmail.fmwrote: On Tue, 2009-01-20 at 21:01 +, Andrew Coppin wrote: Paul Johnson wrote: So next time I hear the you can't get the programmers line I'm going to respond with something like this: If you post an advert for a Haskell developer you will get 20 applicants. All of those people will be the kind of developer who learns new programming languages to improve their own abilities and stretch themselves, because nobody yet learns Haskell just to get a job. If you post an advert for a Java developer you will get 200 applicants. Most of them will be the kind of developer who learned Java because there are lots of Java jobs out there, and as long as they know enough to hold down a job then they see no reason to learn anything. If this isn't an elevator pitch for using Haskell then I don't know what the hell *is*! Seriously, this is a pretty damned persuasive argument... Not really. My company *advertises* for Haskell developers, and then when they come in to interview, informs them that the code base is actually written in Perl. Works, too --- we have several Haskellers working here. If all you care about is the quality of the developers, and not their productivity once you've got them, you don't actually need to let them use Haskell after the interview is over... jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Quite confused by simple transformations on this code not working
Strange little bit of code: http://moonpatio.com:8080/fastcgi/hpaste.fcgi/view?id=829#a829 If I do any of the following, all of which seem natural to me, it fails to typecheck: 1. move f out of the 'where' clause (with or without a type signature) 2. put the same type signature on f as is on (/\) 3. replace f with (/\) completely What's going on here? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Quite confused by simple transformations on this code not working
So...there's just no good way to avoid the duplication? On Tue, Jan 20, 2009 at 11:10 PM, wren ng thornton w...@freegeek.orgwrote: Andrew Wagner wrote: Strange little bit of code: http://moonpatio.com:8080/fastcgi/hpaste.fcgi/view?id=829#a829 If I do any of the following, all of which seem natural to me, it fails to typecheck: 1. move f out of the 'where' clause (with or without a type signature) 2. put the same type signature on f as is on (/\) 3. replace f with (/\) completely What's going on here? :t (nub .) . (++) (nub .) . (++) :: (Eq a) = [a] - [a] - [a] :t foldr (map . (nub .) . (++)) foldr (map . (nub .) . (++)) :: (Eq a) = [[a]] - [[a]] - [[a]] The type you give to (/\) is more restrictive than the type of the expression, and f uses the generality of the expression. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
http://www.haskell.org/haskellwiki/Haskell_in_industry could be of interest to you On Mon, Jan 19, 2009 at 2:34 PM, Andrew Coppin andrewcop...@btinternet.comwrote: Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
Such a database would help me counter by boss's argument that it's impossible to find and hire Haskell programmers. Err, people actually say such things? And they say _we're_ out of touch with the real world? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Simplification of this function?
I've been playing around with this, but haven't been able to come up with
anything.
myFunc f (a,b) (c,d) = (f a c, f b d)
It feels as if there should be a nice simple version of this using some
combination of {un,}curry, on, , ***, or something else.
Any thoughts?
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comments from OCaml Hacker Brian Hurt
I think perhaps the correct question here is not how many instances of Monoid are there?, but how many functions are written that can use an arbitrary Monoid. E.g., the fact that there are a lot of instances of Monad doesn't make it useful. There are a lot of instances of Monad because it's useful to have instances of Monad. Why? Because of http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html ! Look at all the cool stuff you can automagically do with your type just because it's an instance of Monad! I think that's the point. What can you do with arbitrary Monoids? Not much, as evidenced by http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Monoid.html On Thu, Jan 15, 2009 at 3:51 PM, Don Stewart d...@galois.com wrote: duncan.coutts: On Thu, 2009-01-15 at 19:46 +, Andrew Coppin wrote: PS. As a small aside... Is the Monoid class actually used *anywhere* in all of Haskell? Yes. They're used quite a lot in Cabal. Package databases are monoids. Configuration files are monoids. Command line flags and sets of command line flags are monoids. Package build information is a monoid. It is also used in the Foldable class which is a nice interface for traversing/visiting structures. Binary serialisation is also a monoid. Also, xmonad configuration hooks are monoidal. So all those xmonad users gluing together keybindings are using the Monoid class. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Propositional logic implementation
All, Is there some better way to do this? It seems like a lot of boilerplate. Thanks! http://hpaste.org/13807#a1 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Propositional logic implementation
Nice Idea, though I don't know that I want something that extensive. I was more looking for whether there was a better way I could define the algebraic data type. On Sat, Jan 10, 2009 at 6:04 PM, Miguel Mitrofanov miguelim...@yandex.ruwrote: Look at SYB (Data.Data, Data.Generics.*). For example, your symbols function can be rewritten as symbols :: Sentence - [Symbol] symbols s = nub $ listify (const True) s true is not that simple, because this code is NOT boilerplate - each alternative is valuable by itself. On 10 Jan 2009, at 23:56, Andrew Wagner wrote: All, Is there some better way to do this? It seems like a lot of boilerplate. Thanks! http://hpaste.org/13807#a1 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Monads aren't evil
Holy concatenated operators, Batman! Is that an operator or Batman? (yes, I know, 3 operators) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Reading group for Programming Collective Intelligence
I think this sounds like a great idea! I'll have to get ahold of the book first though. On Tue, Dec 30, 2008 at 8:10 PM, Creighton Hogg wch...@gmail.com wrote: Hello Haskellers, For those of your who aren't nose-deep in Real World Haskell at the moment, I'd like to start a small group for the O'Reilly book Programming Collective Intelligence*. I think it might be very instructive to convert the examples exercises to Haskell. I think it would be good experience for doing neat applications in Haskell may provide opportunities for adding to Hackage if the book relies on Python libraries that we don't have an equivalent for. If anyone is interested, just e-mail me I can work on the organization. Cheers, Creighton * http://tinyurl.com/7x4rga ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] instance Enum [Char] where ...
Err, is this just for academic purposes, or is there some deeper reason you want an Enum instance of String? That will dictate how to define the functions. For example, you could just as easily imagine other definitions of fromEnum, such as: fromEnum = read . concatMap (show . ord) Why? *shrug*...the types match. So...you need to figure out why you're doing what you're doing before you can really figure out what you're doing. Hope this helps! On Mon, Dec 29, 2008 at 10:25 PM, JustinGoguen adek...@hamiltonshells.cawrote: I am having difficulty making [Char] an instance of Enum. fromEnum is easy enough: map fromEnum to each char in the string and take the sum. However, toEnum has no way of knowing what the original string was. For example, running fromEnum on the string d will result in 100. But when we pass 100 to toEnum, it does not know if it should treat 100 as d or 22 (fromEnum '2' == 50). Source so far: instance Enum [Char] where succ = undefined pred = undefined toEnum n = undefined -- what to do? fromEnum xs = sum $ map fromEnum xs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Typeclass question
I'm sure there's a way to do this, but it's escaping me at present. I want to do something like this: data Foo = Bar a = Foo a Bool ... That is, I want to create a new type, Foo, whose constructor takes both a Boolean and a value of a type of class Bar. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Typeclass question
Hmm, I actually simplified my problem too much. What I actually want is:
data Foo a = forall a. Bar a = Foo a Bool
...except I want the 'a' on the left to match the 'a' on the right, so
that you can only construct values out of values of the parameterized
type, which also must be of the Bar class.
On Dec 27, 2008, at 1:44 PM, David Menendez d...@zednenem.com wrote:
On Sat, Dec 27, 2008 at 2:24 PM, Andrew Wagner wagner.and...@gmail.com
wrote:
I'm sure there's a way to do this, but it's escaping me at present.
I want
to do something like this:
data Foo = Bar a = Foo a Bool ...
That is, I want to create a new type, Foo, whose constructor takes
both a
Boolean and a value of a type of class Bar.
Try this:
{-# LANGUAGE ExistentialQuantification #-}
data Foo = forall a. Bar a = Foo a Bool
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monades - I've got it! (hopefully)
I wouldn't call it a programming model so much as a library. A programming model sounds to me like an idiom, whereas there's an actual typeclass in the standard library called Monad. Yes, there's special sugar built into GHC (and, likely, any haskell implementation) for it, but it really is at its heart just a typeclass. On Wed, Dec 24, 2008 at 5:28 AM, Tobias Kräntzer i...@tobias-kraentzer.dewrote: Am 24.12.2008 um 11:56 schrieb Luke Palmer: It is only a concept of the language insofar as it is needed to do IO (because of the IO monad). You are correct that it is really more of a programming model. [...] About the prestress, that's one of the motivations behind renaming them (warm fuzzy thing is the current tongue-in-cheek alternative). I think it would help a lot, if this would be mentioned in all the explanations. Maybe I over read it, but the information that monads are a data structure, which are used to do for example IO and no special datatypes would help. But enough programming for these days. . . . Tobias -- Tobias Kräntzer i...@tobias-kraentzer.de ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining a containing function on polymorphic list
The problem here is even slightly deeper than you might realize. For example, what if you have a list of functions. How do you compare two functions to each other to see if they're equal? There is no good way really to do it! So, not only is == not completely polymorphic, but it CAN'T be. There is a nice solution for this, however, and it's very simple: contain :: Eq a - [a] - Bool contain x [] = False contain x (y:ys) = if x == y then True else contain x ys The Eq a in the type signature says that 'a' must be a member of the 'Eq' typeclass. That says, in turn, that 'a' must have == defined for it. Fortunately, most types have, or can easily derive that definition. Here is the definition of the typeclass: class Eqhttp://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#t%3AEqa where(==)http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v%3A%3D%3D:: a - a - Boolhttp://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t%3ABool (/=)http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v%3A%2F%3D:: a - a - Boolhttp://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t%3ABool That is, for 'a' to be a member of 'Eq', it must have a == operator which can take 2 values of that type and return a Boolean, saying whether or not they're equal, and it must also have a definition for the /= operator, which is not equal. These two are also defined in terms of each other, so if you define ==, you get /= for free, and vice versa. That's probably more information than you needed to know, but I hope it helps. 2008/12/22 Raeck Zhao ra...@msn.com I am trying to define a containing function to see if a value is one of the elements within a list which is polymorphic, but failed with the following codes: contain :: a - [a] - Bool contain x [] = False contain x (y:ys) = if x == y then True else contain x ys it seems that the problem is the 'operator' == does not support a polymorphic check? Any way can solve the problem? or any alternative solution to achieve the purpose? Thanks! Raeck -- It's the same Hotmail(R). If by same you mean up to 70% faster. Get your account now.http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_broad1_122008 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining a containing function on polymorphic list
Yes, of course, sorry for the typo. On Mon, Dec 22, 2008 at 9:17 AM, Denis Bueno dbu...@gmail.com wrote: 2008/12/22 Andrew Wagner wagner.and...@gmail.com: The problem here is even slightly deeper than you might realize. For example, what if you have a list of functions. How do you compare two functions to each other to see if they're equal? There is no good way really to do it! So, not only is == not completely polymorphic, but it CAN'T be. There is a nice solution for this, however, and it's very simple: contain :: Eq a - [a] - Bool Please note that the syntax here should be: contain :: Eq a = a - [a] - Bool Denis ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Defining a containing function on polymorphic list
There are two ways to fix this. Let me see if I can get my syntax right this time :) 1.) Let GHC work out the Eq instance: data Shape = Square | Triangle | Circle deriving Eq 2.) Tell GHC how to do it explicitly: data Shape = Square | Triangle | Circle instance Eq Shape where Square == Square = True Triangle == Triangle = True Circle == Circle = True _ == _ = False Note that the last line here means that any other comparisons are false. On Mon, Dec 22, 2008 at 9:35 AM, Raeck Zhao ra...@msn.com wrote: Thank you very much for your reply! It is really helpful! But I just found another 'problem', I just realize that the list does not support the user-defined data type? the list is also depending on the Eq function? For example, data Shape = Square | Triangle | Circle when I type either [Square, Triangle, Circle] or Square == Square there are errors! So there is no way to construct a truly polymorphic List? any way to extend the list to support some user-defined data type? Or... I define the Shape in a wrong way actually? Thanks Raeck -- Date: Mon, 22 Dec 2008 09:02:53 -0500 From: wagner.and...@gmail.com To: ra...@msn.com Subject: Re: [Haskell-cafe] Defining a containing function on polymorphic list CC: haskell-cafe@haskell.org; beginn...@haskell.org The problem here is even slightly deeper than you might realize. For example, what if you have a list of functions. How do you compare two functions to each other to see if they're equal? There is no good way really to do it! So, not only is == not completely polymorphic, but it CAN'T be. There is a nice solution for this, however, and it's very simple: contain :: Eq a - [a] - Bool contain x [] = False contain x (y:ys) = if x == y then True else contain x ys The Eq a in the type signature says that 'a' must be a member of the 'Eq' typeclass. That says, in turn, that 'a' must have == defined for it. Fortunately, most types have, or can easily derive that definition. Here is the definition of the typeclass: class Eqhttp://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#t:Eqa where (==)http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:%3D%3D:: a - a - Boolhttp://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t:Bool (/=)http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:/%3D:: a - a - Boolhttp://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t:Bool That is, for 'a' to be a member of 'Eq', it must have a == operator which can take 2 values of that type and return a Boolean, saying whether or not they're equal, and it must also have a definition for the /= operator, which is not equal. These two are also defined in terms of each other, so if you define ==, you get /= for free, and vice versa. That's probably more information than you needed to know, but I hope it helps. 2008/12/22 Raeck Zhao ra...@msn.com I am trying to define a containing function to see if a value is one of the elements within a list which is polymorphic, but failed with the following codes: contain :: a - [a] - Bool contain x [] = False contain x (y:ys) = if x == y then True else contain x ys it seems that the problem is the 'operator' == does not support a polymorphic check? Any way can solve the problem? or any alternative solution to achieve the purpose? Thanks! Raeck -- It's the same Hotmail(R). If by same you mean up to 70% faster. Get your account now.http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_broad1_122008 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Life on your PC is safer, easier, and more enjoyable with Windows Vista(R). See how http://clk.atdmt.com/MRT/go/127032870/direct/01/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pattern combinators
I'd love to see a copy of this go up on hackage for experimentation. Would
you care to upload your code, or send it to me so I can upload it?
On Sun, Dec 21, 2008 at 12:37 AM, David Menendez d...@zednenem.com wrote:
On Sat, Dec 20, 2008 at 9:34 PM, Jacques Carette care...@mcmaster.ca
wrote:
Andrew Wagner wrote:
Wadler posted a blog entry the other day about a paper on
pattern-matching
in Haskell (http://wadler.blogspot.com/). I've taken a first stab at
turning
it into actual code for hackage (http://hpaste.org/13215). There are
two
commented-out definitions that don't type-check, though, and the types
are
too wild for me to grok. Anybody have any suggestions for 1.) How to fix
it
and/or 2.) How to use data/type/newtype to simplify the types and make
it
more manageable? Thanks!
Both errors are because you are using any instead of any'; you might
wish to put
import Prelude hiding any
at the top of your code, just to avoid such confusion.
Example 14 also uses (.-.) where it should use (..), and it either
needs some more parentheses or some precedence declarations for the
operators.
To make the types more readable (but not necessarily more manageable), I
have made some changes to my version of this code.
One oddity in the paper is the type of the failure continuations, ()
- ans. I'm guessing that's left over from an earlier phase of
development. In my own transcription of the library, I eliminated the
() parameter without apparent loss of functionality.
I think I've managed to work out the structure of the types, which can
mostly be expressed in modern Haskell.
The matching part of the patterns have this general form:
type PMatch vec vec' ans = (vec - ans) - (() - ans) - vec' - ans
where vec and vec' are the list of argument types before and after the
subpattern match, and ans is the final answer. (In my code, I just use
ans instead of () - ans for the failure continuation.)
This gets us:
nil :: PMatch vec vec ans
one :: a - PMatch (a,vec) vec ans
(#) :: PMatch vec vec' ans - PMatch vec' vec'' ans - PMatch vec vec''
ans
fail :: PMatch vec vec' ans
catch :: PMatch vec vec' ans - PMatch vec vec' ans - PMatch vec vec' ans
These types are less general than the ones Haskell would infer, but
they do not appear to lose any functionality.
The currying part of the pattern is less easy to analyze. I've been
able to use type families to relate the curried and uncurried form of
the function types, but I'm working with GHC 6.8, so it's possible
this won't work with the more modern implementations.
Given the list of argument types and the answer type, generate a
curried function type:
type family Curry vec ans
type instance Curry () ans = ans
type instance Curry (a,vec) ans = a - Curry vec ans
zero, succ zero, and so forth take a function in curried form and
transform it into a function that takes a nested tuple:
type CurryDigit vec ans = Curry vec ans - vec - ans
zero :: CurryDigit () ans
succ zero :: CurryDigit (a,()) ans
succ :: CurryDigit vec ans - CurryDigit (a,vec) ans
succ . succ :: CurryDigit vec ans - CurryDigit (a,(b,vec)) ans
So the currying part of the pattern will have the form:
type PCurry vec vec' ans = CurryDigit vec' ans - CurryDigit vec ans
So a pattern has the type,
type Pattern a vec vec' ans = (PCurry vec vec' ans, a - PMatch
vec vec' ans)
where a is the value being examined, vec and vec' are the list of
unbound argument types before and after the match, and ans is the
result.
var :: Pattern a (a,vec) vec ans
cst :: (Eq a) = a - Pattern a vec vec ans
pair :: Pattern a vec vec' ans - Pattern b vec' vec'' ans -
Pattern (a,b) vec vec'' ans
Coming from the other side, match takes a value and a case statement
and produces a result:
type Case a ans = a - (() - ans) - ans -- or just a - ans -
ans in my code
match :: a - Case a ans - ans
(|||) combines case statements:
(|||) :: Case a ans - Case a ans - Case a ans
and (-) creates them from a pattern and a curried function,
(-) :: Pattern a vec () ans - Curry vec ans - Case a ans
Note that (-) requires the pattern to leave no unbound variables
after matching.
Given the way everything is polymorphic in ans, it may be possible to
hide it, but I haven't tried yet.
The principal weakness of these pattern-matching combinators is that
there
is no support for algebraic types, i.e. things like
data Tree a = Leaf | Branch (Tree a) (Tree a)
I can see how to use Typeable to deal with that, but is there a simpler
way?
You can define the patterns manually:
leaf = (id, \v - case v of { Leaf - nil; _ - fail })
branch p q = (curry_p . curry_q, \v - case v of { Branch l r -
match_p l # match_q r; _ - fail})
where
(curry_p, match_p) = p
(curry_q, match_q) = q
I assume generating these would be pretty straightforward to automate
with Template
[Haskell-cafe] Pattern combinators
All, Wadler posted a blog entry the other day about a paper on pattern-matching in Haskell (http://wadler.blogspot.com/). I've taken a first stab at turning it into actual code for hackage (http://hpaste.org/13215). There are two commented-out definitions that don't type-check, though, and the types are too wild for me to grok. Anybody have any suggestions for 1.) How to fix it and/or 2.) How to use data/type/newtype to simplify the types and make it more manageable? Thanks! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Functional version of this OO snippet
Hi all,
public interface IEngine {
void foo();
void bar(string bah);
...
}
public class Program {
public void Run(IEngine engine){
while(true){
string command = GetLine();
if (command.startsWith(foo)){
engine.foo();
} else if (command.startsWith(bar)){
engine.bar(command);
...
else break;
}
In other words, I want to provide the same UI across multiple
implementations of the engine that actually processes the commands.
Thanks in advance.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Binary Trees missing on hackage
The reasons I've always heard for this is that 1.) It's so easy to define a tree and 2.) There are tons of different variations of trees and what you can do with them. Not that I 100% agree, just what I've always heard. On Mon, Dec 1, 2008 at 6:09 AM, Christian Maeder [EMAIL PROTECTED]wrote: Hi, I was surprised that I could not find a classical binary tree data structure on hackage, mainly for teaching purposes, like: data BinTree a = Nil | Node (BinTree a) a (BinTree a) with a couple of utility functions and instances (i.e. in-order traversal). Surely, one may argue, that such a tree can always be defined on the fly when needed, but nobody would argue so for lists or tuples. (Although I've rarely seen someone redefining lists, it is quite common to introduce user-defined products or enumerations.) There are rose trees in Data.Tree and many other variants of trees are conceivable, ie. data Boom a = Leaf a | Node (Boom a) (Boom a) or a kind of combination: data BinLeafTree a b = Leaf b | Node (BinLeafTree a b) a (BinLeafTree a b) I don't know, why I find the above BinTree most important. I'm not even sure if such a tree could be re-used for Search- or AVL-trees that need strict fields with redundant size or height counters for efficiency reasons. In any case I would find such a data type at least as useful as http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OneTuple Who would supply and maintain such a package? Or did I simply not search hard enough? Cheers Christian P.S. I took the (non-empty) Boom from the Boom-Hierarchy described in An Exploration of the Bird-Meertens Formalism by Roland Backhouse 1988, Groningen ___ Libraries mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/libraries ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Binary Trees missing on hackage
Hm, I've been thinking about this this morning as I've gone about my commute. I could indeed imagine a class like the following that had multiple implementations like you're talking about: class Tree t where drawTree :: t String - String flatten :: t a - [a] etc. (see http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Tree.html)http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Tree.html I think that for this to be valuable, though, we would need to show that there are functions which can take a generic Tree t and do something useful with it. Still, I suspect there's something there. Maybe I'll take a stab at it this week sometime. On Mon, Dec 1, 2008 at 6:24 AM, Andrew Wagner [EMAIL PROTECTED]wrote: The reasons I've always heard for this is that 1.) It's so easy to define a tree and 2.) There are tons of different variations of trees and what you can do with them. Not that I 100% agree, just what I've always heard. On Mon, Dec 1, 2008 at 6:09 AM, Christian Maeder [EMAIL PROTECTED] wrote: Hi, I was surprised that I could not find a classical binary tree data structure on hackage, mainly for teaching purposes, like: data BinTree a = Nil | Node (BinTree a) a (BinTree a) with a couple of utility functions and instances (i.e. in-order traversal). Surely, one may argue, that such a tree can always be defined on the fly when needed, but nobody would argue so for lists or tuples. (Although I've rarely seen someone redefining lists, it is quite common to introduce user-defined products or enumerations.) There are rose trees in Data.Tree and many other variants of trees are conceivable, ie. data Boom a = Leaf a | Node (Boom a) (Boom a) or a kind of combination: data BinLeafTree a b = Leaf b | Node (BinLeafTree a b) a (BinLeafTree a b) I don't know, why I find the above BinTree most important. I'm not even sure if such a tree could be re-used for Search- or AVL-trees that need strict fields with redundant size or height counters for efficiency reasons. In any case I would find such a data type at least as useful as http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OneTuple Who would supply and maintain such a package? Or did I simply not search hard enough? Cheers Christian P.S. I took the (non-empty) Boom from the Boom-Hierarchy described in An Exploration of the Bird-Meertens Formalism by Roland Backhouse 1988, Groningen ___ Libraries mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/libraries ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (A little humour)
Brilliant. This made my day. I must admit, I looked briefly at the paper after I saw the link, yawned, and closed it. Then I saw Andrew's comment, skimmed the paper, becoming more and more convinced that it was a joke, saw the last line, and then had to go back and read the whole thing again. Just awesome. On Fri, Sep 26, 2008 at 3:22 PM, Andrew Coppin [EMAIL PROTECTED] wrote: Miguel Mitrofanov wrote: I think you might be interested in http://www.research.att.com/~bs/whitespace98.pdf Thankyou. That is the most insane thing I've read all day. And I've been reading a C++ tutorial all day! o_O ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trees (Rose Trees?)
There are a few different kinds of trees, but if you only need to store data on the leaves, that representation will work. As for your sumTree function, you are indeed missing a case...consider sumTree (Leaf 3)! Once you deal with that case, the other two can actually be combined (since sum [] = 0). 2008/7/21 Ryan Bloor [EMAIL PROTECTED]: hi I was curious as to whether my implementation of a Rose Tree and a sumTree function was correct. The aumTree adds up the elements of a tree. data Tree a = Leaf a | Node [Tree a] sumTree :: Tree Int - Int sumTree (Node []) = 0 sumTree (Node xs) = sum (map sumTree xs) The problem with this is I get a pattern matching error. Am I representing trees right... see below. Also, would an empty tree be represented by ... Node [] with this implementation? How would I represent a tree of the form... Tree (Node 2(Node 6 Empty Empty) Empty) taken from a binary one. Like this? Node [ [Leaf 2], Node [ Leaf 6,Node[],Node[] ], Node[] ] Ryan Find out how to make Messenger your very own TV! Try it Now! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Having trouble with zip12..
Wow. Where did you come up with the stack trace? That's...impressive. On Sun, Jul 6, 2008 at 5:07 PM, Don Stewart [EMAIL PROTECTED] wrote: I win, almost ... 13:13:18 dons dolio: yeah, it was ... almost ... an April 1 style post :) And yes, this was truly shocking on a number of levels. However, we have people doing a lot of weird things with Haskell these days, so its not as absurd that someone would be hacking up a zip12 for an air traffic control system on some MS platform, with SQL in the backend, as it might have been a few years ago :) mfeathers: Sorry guys. I was just bored on a Sunday afternoon so I thought I'd type up a little joke. I thought to myself Gee, how outrageous can I make it? 1) Using and debugging a zip12 function. 2) That fails only on 'take 5' (Brubeck fans take note) 3) Has some absurd arguments like (nub . nub) 4) Is embedded in an air traffic control system 5) Is written in a Microsoft variant of Haskell called H# 6) Silently makes SQL calls when evaluating a pure function 7) Yields an mile long stack trace Sorry all. Boredom made me do it, Michael Paul Visschers wrote: You're zipping 12 lists here, so how about testing each list individually? This will narrow down the problem considerably. Michael Feathers wrote: I have some code that looks like this and I'm having trouble with it: zip12 ((tails . nub) flightPaths) wayPoints etopsPackets (hd geoCaches) groundSpeeds headings (map windShift headings) (regulations !! 2) (foldr (\|/) (tail pathDistances)) [ghy x | x - [1..], full x] (nub . nub) arrivalSchedule The domain is air traffic control and I need to generate 12-tuples for aircraft that are within a particular radius of the tower. When I evaluate that expression with 'take 4' it works fine. When I evaluate it with 'take 6' it works as well. But, when I evaluate it with 'take 5' I get the following runtime error from H# in Visual Studio (it runs fine on the command line). This is particularly odd because I'm not using Sql. The type initializer for 'System.Data.SqlClient.SqlConnection' threw an exception. Exception (TypeInitializationException): Source=System.Data; Target=null; Tag=null; TypeName=System.Data.SqlClient.SqlConnection; Message = The type initializer for 'System.Data.SqlClient.SqlConnection' threw an exception. InnerException (TypeInitializationException): Source=System.Data; Target=null; Tag=null; Message = The type initializer for 'System.Data.SqlClient.SqlConnectionFactory' threw an exception. StackTrace = at System.Data.SqlClient.SqlConnection..cctor() InnerException (TypeInitializationException): Source=System.Data; Target=null; Tag=null; Message = The type initializer for 'System.Data.SqlClient.SqlPerformanceCounters' threw an exception. StackTrace = at System.Data.SqlClient.SqlConnectionFactory..ctor() at System.Data.SqlClient.SqlConnectionFactory..cctor() InnerException (ConfigurationErrorsException): Source=System.Configuration; Target=null; Tag=null; Line=21; Message = The value of the property 'traceOutputOptions' cannot be parsed. The error is: The enumeration value must be one of the following: None, LogicalOperationStack, DateTime, Timestamp, ProcessId, ThreadId, Callstack. (C:\Documents and Settings\Paey\Desktop\Projects\RPMC\bin\Debug\RPMC.vshost.exe.config line 21) StackTrace = at System.Configuration.BaseConfigurationRecord.EvaluateOne(String[] keys, SectionInput input, Boolean isTrusted, FactoryRecord factoryRecord, SectionRecord sectionRecord, Object parentResult) at System.Configuration.BaseConfigurationRecord.Evaluate(FactoryRecord factoryRecord, SectionRecord sectionRecord, Object parentResult, Boolean getLkg, Boolean getRuntimeObject, Object result, Object resultRuntimeObject) at System.Configuration.BaseConfigurationRecord.GetSectionRecursive(String configKey, Boolean getLkg, Boolean checkPermission, Boolean getRuntimeObject, Boolean requestIsHere, Object result, Object resultRuntimeObject) at System.Configuration.BaseConfigurationRecord.GetSectionRecursive(String configKey, Boolean getLkg, Boolean checkPermission, Boolean getRuntimeObject, Boolean requestIsHere, Object result, Object resultRuntimeObject) at System.Configuration.BaseConfigurationRecord.GetSectionRecursive(String configKey, Boolean getLkg, Boolean ch... (truncated) ...olean checkPermission) at System.Configuration.BaseConfigurationRecord.GetSection(String configKey) at System.Configuration.ClientConfigurationSystem.System.Configuration.Internal.IInternalConfigSystem.GetSection(String sectionName) at System.Configuration.ConfigurationManager.GetSection(String sectionName) at System.Configuration.PrivilegedConfigurationManager.GetSection(String sectionName) at System.Diagnostics.DiagnosticsConfiguration.GetConfigSection() at System.Diagnostics.DiagnosticsConfiguration.Initialize() at
Re: [Haskell] on starting Haskell-Edu, a new education-related Haskell-related mailing list
I would certainly join such a list. My only concer would be that moderating such a list could be tricky. How do you judge when a discussion has become too technical? And what do you do about it? Force the members to take it to -cafe? Anyway, I like the idea of having a more beginner-friendly list, and have suggested something similar for the IRC channel multiple times. On Wed, Jul 2, 2008 at 3:39 AM, Benjamin L. Russell [EMAIL PROTECTED] wrote: So far, I have received three positive responses on starting the new Haskell-Edu mailing list, and no negative responses. In the latest response, the respondent suggested that I post another message to this mailing list advising readers on how to react. Basically, the Haskell.org mailing list administrator, Simon Marlow, had originally suggested that I ask for feedback on my idea from this mailing list, and wait for the discussion to proceed to Haskell-Cafe, so for those interested in this idea, please respond either in this thread or, after a few rounds, in Haskell-Cafe on whether you agree, disagree, feel neutral, or have mixed feelings regarding this idea. In any case, as the above-mentioned respondent suggested, rapid responses to questions on the new mailing list will probably prove vital to keeping it alive. Participation by educators using Haskell, once Haskell-Edu is started, would be most welcome. Please post your responses initially in this thread. After a few rounds, this discussion will probably move to Haskell-Cafe. -- Benjamin L. Russell --- On Tue, 7/1/08, Benjamin L. Russell [EMAIL PROTECTED] wrote: From: Benjamin L. Russell [EMAIL PROTECTED] Subject: [Haskell] on starting Haskell-Edu, a new education-related Haskell-related mailing list To: The Haskell Mailing List haskell@haskell.org Date: Tuesday, July 1, 2008, 8:37 PM I am interested in starting a new mailing list on Haskell.org, aimed mainly at liberal arts teachers and elementary-level learners of Haskell, called Haskell-Edu: The Haskell Educational Mailing List. This new mailing list would be guided by the principle that Haskell is useful not just in research, but also in teaching programming as part of a liberal arts education, on a par with Scheme. When I suggested the idea of this mailing list to Simon Marlow, the Haskell.org mailing list administrator, he suggested that I post this idea on The Haskell Mailing List, so I am posting it here to ask for feedback. The main purposes of this new (proposed) mailing list would be as follows: 1) To provide a primarily non-research-oriented discussion forum to serve the needs of users wishing to focus on the uses of Haskell in education, such as in high school and in introductory computer science college courses, as opposed to in research. 2) To provide a primarily non-research-oriented discussion forum to serve the needs of non-computer-science students of Haskell who wish to focus on Haskell as a language for learning programming as part of a well-rounded a liberal arts education, as opposed to an engineering/mathematics/science-oriented education. Currently, there are two main Haskell mailing lists: a) The Haskell Mailing List, currently used mainly for announcements and for non-beginner discussions b) The Haskell-Cafe, currently ostensibly used for everything else, but in fact used primarily for serious academic computer-science research-oriented discussion of the language Haskell. Neither mailing list addresses Haskell as a tool for teaching functional programming as part of a liberal arts education, and while The Haskell Cafe is ostensibly responsible for addressing beginner questions, I have witnessed several instances in which new users who were not familiar with the academic culture of The Haskell Cafe have been frowned upon for either posting messages that did not assume enough mathematical background, or for posting messages that were written in a tongue-in-cheek style, and that therefore did not fit into the serious tone of the mailing list. (For example, a few months ago, one poster received a private e-mail message from another poster asking the former not to pollute The Haskell-Cafe Mailing List for assuming that screen pixel resolution was somehow related to the precision of an algorithm that picked points randomly from a square in approximating pi. Avoiding this question required the knowledge that screen resolution could be considered independently from the precision of the algorithm itself, but while this point may be elementary to mathematicians and researchers, the poster was not familiar enough with the issue to grasp this immediately, and received the above-mentioned response.) This new mailing list is intended to cover both the issue of teaching Haskell as part of a liberal arts curriculum, and of answering beginner questions about Haskell from students who may not have a sophisticated mathematics background.
Re: [Haskell-cafe] Haskell, Microsoft, and interview questions
Did they score you on coding or on geometry? It was definitely more on coding and my ability to think about the problem. For what it's worth, a 3-dimensional kd tree really flew on this problem. I did some reading up on this, and it seems interesting. It would be need to implement something like this in Haskell, but I can't seem to find any detailed specs on the data-structure. Got any recommendations? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
