Re: [Haskell-cafe] Postdoctoral Fellowship in Functional Programming
On 9/26/07, Graham Hutton [EMAIL PROTECTED] wrote: Salary will be within the range 25,134 - 32,796 pounds per year, depending on qualifications and experience. The post is available immediately, and will be offered on a fixed-term contract for 3 years. I don't mean to diminish the seriousness of your message, but why is the salary range so exact? Couldn't you have just rounded the upper bound to 32,768 for the sake of readability? Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Basic question....
On 8/17/07, rodrigo.bonifacio [EMAIL PROTECTED] wrote: Hi all. I want to create the following polymorphic type (EnvItem) that we can apply two functions (envKey and envValue). I tried the following: type Key = String data EnvItem a = EnvItem (Key, a) envKey :: EnvItem (Key, a) - String envKey EnvItem (key, value) = key envValue :: EnvValue(Key, a) - a envValue EnvItem (key, value) = value But this is resulting in the error: [Constructor EnvItem must have exactly 1 argument in pattern] I think this is a very basic problem, but I don't know what is wrong. Regards, Rodrigo. In addition to what others have already said, I'd like to point out that you do not really need a tuple in your data item. data EnvItem a = EI Key a envKey :: EnvItem a - Key envKey (EI key _) = key envValue :: EnvValue a - a envValue (EI _ value) = value Also, you made a distinction between 'Key' and 'String', which is good. But then in 'envKey', you used 'String' in the signature instead of 'Key'.That's a little confusing, and also should you ever want to change the representation of 'Key', you would then have to change the signature of envKey. Just my two cents, Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Question about arrows
On 8/3/07, Lewis-Sandy, Darrell [EMAIL PROTECTED] wrote: Is there a class property of the Control.Arrow class that represents the evaluatation of an arrow: eval :: (Arrow a)=a b c-b-c You could always use the ArrowEval class. Granted, it doesn't exist, so you'd have to write it, but it should serve your purposes. class ArrowEval a where eval :: a b c - b - c instance ArrowEval (-) where eval = ... instance ArrowEval YourPartial where eval = ... (That's off the cuff, so I probably didn't get the syntax correct.) Does that fit your problem? But like Mr. Morris said, eval isn't valid for every arrow, so you can only define instances of ArrowEval where it is valid. Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining the community
On 7/18/07, Martin Coxall [EMAIL PROTECTED] wrote: On 7/18/07, Jon Harrop [EMAIL PROTECTED] wrote: On Tuesday 17 July 2007 23:26:08 Hugh Perkins wrote: Am I the only person who finds it interesting/worrying that there are few to no people in the group who are ex-C# programmers. I mean, you could argue that C# programmers are simply too stupid to do Haskell, but ... you know, there is another explanation ;-) To understand this, I think you must look at the number of technical users for each language. There are a huge number of technical C++ and Java programmers but a tiny number of technical C# programmers in comparison. The few technical C# programmers are migrating to F# because it is next door and F# programmers are better looking. Most C# programmers are (a) GUI programmers and (b) former VB programmers. This means they are *guaranteed* to be less attractive that the average C++ developer. I have proof. But it's too big to be contained in this margin. Martin I heard that Fermat didn't even actually have a proof. You wouldn't be trying to hoodwink us in the same way, would you? :) I haven't been paying attention to the subject, but I suppose I should pipe in now. I really enjoy Haskell. I'm probably like most people here in that I like learning new languages: I've given Scheme a fair shot; F# captured my interest for a while, and right now I'm toying with Erlang. I've tried Python, used Perl for a job, determined after an hour that PHP wasn't for me, and even looked at Ruby. The list goes on. (Always, of course, I keep GHC on my computer.) But for work, I use C#. And I, for one, am looking forward to C#3.0, because it will be easier to apply my FP experience to problems when FP is the better way to solve a problem. (You've heard the maxim that when all you have is a hammer, everything looks like a nail; the flip side of it is that when you've got a whole tool set including a screwdriver and you see a screw, but your company only lets you use your hammer, it can be frustrating to beat on the screw with the hammer.) And since I'm fresh out of college with no experience, I'm neither in a position to even suggest a language change in my company, nor do I have the experience to move to the occassional Scheme or Erlang job opening I see (I don't know if I've ever seen a Haskell job opening, and I'm guessing if I did it would get snatched up by a more qualified programmer quite quickly). I guess the point being made is that there are a smaller percentage of attractive programmers in C#; but it looked to me that people were implying that there are /no/ knowledgeable programmers in C#; and I'd just like to assert that maybe there are some that don't really have a choice right now. :) Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help with IO and randomR
On 7/16/07, Niko Korhonen [EMAIL PROTECTED] wrote: I'm writing some code to generate a dither (=noise) signal. I'm trying to generate an infinite series of noise with triangular distribution but my code hangs into an infinite loop. The problem is that I'm not very good with Haskell IO yet and I can't figure out how to write this piece of IO code without it looping infinitely. So, in short, how do I do this without getting into an infinite loop: tpdfs :: (Int, Int) - IO [Int] tpdfs (low, high) = do first - getStdRandom (randomR (low, high)) second - getStdRandom (randomR (low, high)) let r = (first + second) `div` 2 rest - tpdfs (low, high) return (r : rest) Caller site: do nums - tpdfs (2, 12) let ns = take 7 nums Niko I did not look at it long enough to tell you why there is an infinite loop. However, think about it on a high level with me. You want a stream of these random numbers (I'm not sure what a triangular distribution is, but that's okay). To get one of these, you take two random numbers and perform a combination function (\x y - (x + y) `div` 2 ) on them. So you can lift this from one random numbers to a stream of random numbers if you have have two streams of random numbers instead of just two random numbers. zipWith is the function that brings us from one number to a stream of numbers. tpdfs range = do g - newStdGen -- get a random generator (g1, g2) - return $ split g -- make two random generators out of it return $ zipWith combine (randomRs range g1) (randomRs range g2) -- get two streams of random numbers, and combine them elementwise. combine x y = (x + y) `div` 2 Uh, I know that's a very poor explanation, but hopefully it gives you an alternate way to look at the problem. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Messing around with types [newbie]
On 6/21/07, Cristiano Paris [EMAIL PROTECTED] wrote:
Hi,
I'm making my way through Haskell which seems to me one of the languages
with steepest learning curve around.
Now, please consider this snippet:
{-# OPTIONS_GHC -fglasgow-exts #-}
module Main where
class FooOp a b where
foo :: a - b - IO ()
instance FooOp Int Double where
foo x y = putStrLn $ (show x) ++ Double ++ (show y)
partialFoo = foo (10::Int)
bar = partialFoo (5.0::Double)
I hope the indentation looks ok in your email client. I'm experimenting with
currying and typeclasses at the moment.
If I try to import this in ghci, it works flawlessy. Now, if I remove the
type signature from 10 and 5.0, ghci complaints saying:
example.hs:12:6:
Ambiguous type variable `t' in the constraint:
`Num t' arising from use of `partialFoo' at example.hs:12:6-19
Probable fix: add a type signature that fixes these type variable(s)
example.hs:12:6:
Ambiguous type variables `t', `t1' in the constraint:
`FooOp t t1' arising from use of `partialFoo' at example.hs:12:6-19
Probable fix: add a type signature that fixes these type variable(s)
example.hs:12:17:
Ambiguous type variable `t1' in the constraint:
`Fractional t1'
arising from the literal `5.0' at example.hs:12:17-19
Probable fix: add a type signature that fixes these type variable(s)
I switched off the monomorphism restriction (btw, is this bad? No flame war
please :D) otherwise it'd have complained louder.
Can you explain how to fix the code (if possible) and give some explanation?
Here's a quick transcript of a GHCi session:
Prelude :t 10
10 :: (Num t) = t
Prelude :t 5.0
5.0 :: (Fractional t) = t
From this you can see that 10 is not necessarily an Int, and 5.0 is
not necessarily a Double. So the typechecker does not know, given just
10 and 5.0, which instance of 'foo' to use. But when you explicitly
told the typechecker that 10 is an Int and 5.0 is a Double, then the
type checker was able to choose which instance of 'foo' it should use.
Does that make sense? (I hope it makes sense, and I also hope it is correct!)
And I do not really know how to fix it, maybe somebody else can write
about that.
Bryan Burgers
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] To yi or not to yi, is this really the question? A plea for a cooperative, ubiquitous, distributed integrated development system.
On 6/18/07, Pasqualino 'Titto' Assini [EMAIL PROTECTED] wrote: Having just presented a case for the possible rationality of the irrational decision of creating an Emacs-like IDE in Haskell, I wonder if we should not be even more irrational and contemplate the possibility of using Haskell to create a radically different kind of IDE. [...] Up to you now, what is your dream? I just did a quick read through of your dream and I'm not going to say either way with it. But I would like to point out, just to make sure you've considered it, that my dream--or maybe my reality--involves being able to code without the requirement of a network connection. Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell] Who pays for *.haskell.org machines?
is its funding will be reliable? for example, if we don't get money from Google in 2008 year? in irc some time ago i brought up the topic of something like the freebsd or wikimedia foundations, but for haskell. if you can give me a secure and trustworthy method of payment, and as a bonus, a tax receipt (what is known as 501-c-3 status in the US), i will gladly start writing checks on a yearly basis. i am sure others would join me. Similarly, the Perl community has a foundation, and I believe giving to it is tax-deductible. You could look in to how they do it. http://www.perlfoundation.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Mathematics in Haskell Re: Why the Prelude must die
Jacques Carette wrote: perhaps i was mistaken in thinking that there is a group of math-interested haskellers out there discussing, developing, and documenting the area? or perhaps that group needs introductory tutorials presenting its work? My guess is that there are a number of people waiting in the wings, waiting for a critical mass of features to show up before really diving in. See http://www.cas.mcmaster.ca/plmms07/ for my reasons for being both interested and wary). Probably the simplest test case is the difficulties that people are (still) encountering doing matrix/vector algebra in Haskell. One either quickly encounters efficiency issues (although PArr might help), or typing issues (though many tricks are known, but not necessarily simple). Blitz++ and the STL contributed heavily to C++ being taken seriously by people in the scientific computation community. Haskell has even more _potential_, but it is definitely unrealised potential. I am one of those mathematicians waiting in the wings. Haskell looked very appealing at first, and the type system seems perfect, especially for things like multilinear algebra where currying and duality is fundamental. I too was put off by the Num issues though--strange mixture of sophisticated category theory and lack of a sensible hierarchy of algebraic objects. However, I've decided I'm more interested in helping to fix it than wait; so count me in on an effort to make Haskell more mathematical. For me that probably starts with the semigroup/group/ring setup, and good arbitrary-precision as well as approximate linear algebra support. I've been watching this thread for quite a while now, and it seems to me that there is quite a bit of interest in at least working on a new Prelude. I've also noticed a 'The Other Prelude' page on the wiki [http://haskell.org/haskellwiki/The_Other_Prelude] and they seem to have a start on this. So it seems that we should actually start this, because people will contribute. Can somebody with good Cabal skills and maybe access to darcs.haskell.org start a new library for people to start patching? Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Unresolved overloading error
On 3/31/07, Scott Brown [EMAIL PROTECTED] wrote: It's working now, thank you. I changed the definition to binom n j = div (fac n) ((fac j)*(fac (n - j))) bernoulli n p j = fromIntegral(binom n j)*(p ^ j) * ((1 - p)^(n - j)) As a matter of style suggestion, it might make 'binom' more clear if you use 'div' as an infix operator: binom n j = (fac n) `div` ( fac j * fac (n - j) ) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie concatenating monad question
My idea is to have a monad with a concatenating , so that I can: bulidStuff = do func1 func2 func3 func4 Leandro Penz I actually did this recently for a project I have been working on. First, an example: output label a@(I.Add a1 a2 a3) = do comment (show a) mov' label eax a1 add eax a2 mov a3 eax My monad in this example has a Writer or a WriterT in it somewhere. I of course had to include the 'tell' that you mention somewhere, but I hid it in the 'comment', 'mov', and 'add' functions in an attempt to make my own personal assembly DSL. Obviously, you would have to either include 'tell' in your func1, func2, etc., or create wrappers for them that include a 'tell'. Just my two cents. Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] flip fix and iterate (was: Lazy IO and closing of file handles)
On the topic of 'fix', is there any good tutorial for fix? I searched google, but mostly came up with pages including things like 'bug fix'. It's hard for me to get an intuition about it when 'fix' always stack overflows on me because I don't really know how to use it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Recursion
On 3/6/07, [EMAIL PROTECTED] [EMAIL PROTECTED] wrote: Does the following code increase the level of recursion once for each input line or is the recursive construct converted to an iteration? Thanks, Dave Feustel main :: IO () main = do line - getLine processIt line main processIt :: String - IO () processIt s = do print (length s) Dave, I would imagine it does not, but I will let somebody more knowledgeable tell you for sure. My thought, though, is that if this is your whole code and not a simplified version (eg, one that terminates on certain input), then you could consider using the Prelude's 'interact'[1] function, which performs a transformation on standard input. In your case, the code would simplify to: main = interact (unlines . map (show . length) . lines) The unlines/lines combo breaks up the whole input into a list of lines of input, and the (show . length) is the heart of your 'processIt' function. Alternately, you could also use something like: main = getContents = mapM_ processIt . lines Which takes everything from standard in, splits it up into lines, and then performs processIt for each line. [1] http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Ainteract Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Pulling a monad out of a functor
Haskellers, Recently, I have come across a couple of situations in which I need to pull a monad out of a functor so to speak. The first time it was needed, I didn't think much of it, but the second time started me thinking whether there is already an existing construct for this. The type signatures I have in mind are: pull :: (Monad m, Functor f) = f (m a) - m (f a) fmapM :: (Monad m, Functor f) = (a - m b) - f a - m (f b) fmapM f = pull . fmap f You may notice that these are already defined for the List functor as 'sequence' and 'mapM'. It seems to me that if this were a class, the implementor would be defining a traversing order by defining pull; in the list case this traversing sequence is clearly defined as head to tail. I am wondering if there is already a construct for this that I am not aware of (a class, a way to defined it using monads and functors that I am not aware of, an idiom)? Is this something that frequently happens for others, or is it a sign a few of my designs have been flawed recently? Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pulling a monad out of a functor
http://haskell.org/ghc/dist/current/docs/libraries/base/Data-Traversable.html pull is sequence, fmapM is mapM (also see the sequenceA, traverse functions for the more general case of applicative functors vs. monads) Stefan Ah yes, thank you! Saying It seems to me that if this were a class, the implementor would be defining a traversing order by defining pull should have tipped me off to check out the 'Traversable' class before asking (but, having probably only seen that class once or twice before in passing, it was probably too deep in my head for quick retrieval). Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Memoisation
On 2/25/07, Tony Morris [EMAIL PROTECTED] wrote: I have a backtracking algorithm that I need to memoise with. Rather than go into the intricacies of the algorithm, I figure (and hope) the factorial function is trivial enough to point out my problem. Simply, suppose I wish to calculate the factorial of 10, then later the factorial of 5. I have already calculated the factorial of 5, but now I must do it again. I have thought of various ways of preventing this; perhaps passing an Array in a state monad. I'm wondering if there is a general solution for this kind of problem. Thanks for any tips. -- Tony Morris You may be able to glean some ideas from a previous discussion at: http://comments.gmane.org/gmane.comp.lang.haskell.cafe/19623 Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Summer of Code
Hello, Yes, I realize it's mid-February right now and the summer is still months away, but it's probably not too early to think about the future. I am wondering if there are any Summer of Code projects that I would be able to do for the Haskell community. I will be graduating from my undergrad program this semester and am hoping to go on to graduate school in the fall, and I think a good Haskell project would be the perfect way to spend my transitional summer. I looked at http://hackage.haskell.org/trac/summer-of-code/query?status=newstatus=assignedstatus=reopenedgroup=topictype=proposed-projectorder=priority to see if there was anything that people already had in mind, but that page looks old and unkempt (judging by the abundance of advertisements) so I am not sure which of those are still available or needed. So, are there any projects that will need a student for the summer? If this is not the place to ask, where should I be asking? Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] It matters how Type Synonyms are defined?
Today, I was plugging away on a program and I ran into a problem. It
seems that ErrorT can or can not take a type synonym as its monad,
depending on how the type synonym was defined. For example, consider
this GHCi interactive run:
:k Maybe
Maybe :: * - *
let { a :: ErrorT String Maybe Bool; a = undefined }
:t a
a :: ErrorT String Maybe Bool
:k State (Scope VVar)
State (Scope VVar) :: * - *
let { a :: ErrorT String (State (Scope VVar)) Bool; a = undefined }
:t a
a :: ErrorT String (State (Scope VVar)) Bool
ScopeState is defined in a file as:
type ScopeState a = State (Scope VVar) a
:k ScopeState
ScopeState :: * - *
let { a :: ErrorT String ScopeState Bool; a = undefined }
interactive:1:6:
Type synonym `ScopeState' should have 1 argument, but has been given 0
In the type signature: a :: ErrorT String ScopeState Bool
Now, I was going to ask something like, How can I define my type
synonym so I can do this, but I figured out while writing this email
that if I define ScopeState a different way:
type ScopeState = State (Scope VVar)
:k ScopeState
ScopeState :: * - *
let { a :: ErrorT String ScopeState Bool; a = undefined }
:t a
a :: ErrorT String ScopeState Bool
So, my new question is: Why does it matter how ScopeState is defined?
Bryan Burgers
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combine list of sorted lists
I am not sure how to express f1 with map? how do I say (lambda (ls) (map (lambda (x) (list x)) ls)) in Haskell? map ([]) ? map (:[]), :[] takes a single element and puts it into a list. Some people refer to this as box Another way to express f1 with map is: f1 xs = map (\x - [x]) xs The (\x - [x]) is a lambda that takes an x and puts it in a list. This is semantically the same as (\x - x:[]), where (:) puts x at the front of the empty list ([]). So, this is where Niel gets his method (:[]) -- ie, just like (\x - x+1) is semantically the same as (+1), so (\x - x:[]) is semantically the same as (:[]). Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] constant functions
I'm trying to learn Haskell and translating some Lisp functions as exercises. How would I write a Haskell function named ALWAYS that behaves like this: one = always 1 bozo = always clown map one [2,3,4,5,6] [1,1,1,1,1] one 62 1 map bozo [2,3,4,5,6] [clown,clown ,clown, clown, clown] bozo 62 clown i.e. ALWAYS returns a function with a single parameter that is ignored, returning instead the value given to ALWAYS when the function was created. This is what I've been trying: always :: (a - a) - a - a always x = (\y - x) one = always 1 Michael First, you want 'one' to take an integer and return 1. So, one :: Integer - Integer since one = always 1, then always 1 :: Integer - Integer So, 'always' takes an Integer and returns an Integer - Integer always :: Integer - (Integer - Integer) But that's the same as always :: Integer - Integer - Integer You actually have the implementation correct, you just didn't have the right type signature. always first = (\second - first) Of course, neither of these implementations need to be tied to Integers; they can be polymorphic. So, we end up with: always :: a - b - a -- no reason the second parameter has to be the same type as the first, so use 'b' instead of 'a'. always first = (\_ - first) -- replace 'second' with '_', because we don't need to bind anything to the second parameter. Does that makes sense? Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell Side Effect
On 12/20/06, Jason Dagit [EMAIL PROTECTED] wrote: Am I the only one that doesn't get it? Jason No, you are not the only one that doesn't get it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] I'd like start with Haskell, but...
That said, if I was writing a GUI+database thing, which doesn't do a lot of substantial processing (more just Add/Edit/Delete buttons), I'd definately use C# over Haskell. Haskell can do this, but you are walking a relatively new path. On the other hand C# developers do this day in, day out, and the language is optimised towards it. I agree with Neil here. I've been working with Haskell for a while and I wouldn't have the foggiest idea of how to start a GUI / Database project. After working with C# for about a week, I had a substantial GUI / Database program going for a company I interned with last summer. Haskell is good for a lot of things, and I'm sure somebody someday will show me that Haskell is even great for GUI/Database, but you should always use the right tool for the job, and for your job, I'd say C# is the right tool. Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Name that function =)
On 12/12/06, Louis J Scoras [EMAIL PROTECTED] wrote: I have some IO actions that I want to map over a list of pairs -- these are just directory names and their down-cased versions. It wasn't difficult to actually get the right behavior by just doing mapM twice. -- putDirs just outputs something like mv fst snd mapM_ putDirs pairs mapM_ (uncurry renameFile) pairs This bothered me though, because I suspected that this could be done in one pass. Naively I proceeded to this. mapM_ (putDirs (uncurry renameFile)) pairs Which didn't work. I thought about it a little more before realizing that putDirs wouldn't get any parameters this way: I needed some way to distribute the pair over both operations. Here's the higher-order function I needed: foo h f g i = h (f i) (g i) which could then be curried and we get: mapM_ (foo () putDirs $ uncurry renameFile) pairs Works great. So my question: is there a established name for foo? What about foo partially applied to ()? This was a fun exercise, but I'd like to use the standard implementations if they exist. Before we get too far down the obfuscation road, I'd like to offer what I think is more readable than a liftM2 solution: mapM_ (\p - putDirs p uncurry renameFile p) pairs I haven't tested it, but I hope that does the same thing. To me, this explicitely shows what each is doing, moreso than with a point-free 'foo' combinator. The way my mind worked to get to this solution: mapM_ putDirs pairs mapM_ (uncurry renameFile) pairs == mapM_ (\p - putDirs p) pairs mapM_ (\p - uncurry renameFile p) pairs == mapM_ (\p - putDirs p uncurry renameFile p) pairs Is that a reasonable solution? Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] smallest double eps
Hang on, hang on, now I'm getting confused. First you asked for the smallest (positive) x such that 1+x /= x which is around x=4.5e15. 1 + 0 /= 0 0 is smaller than 4.5e15 So I don't understand this at all... But then 0 isn't positive. Why not? In any case every positive number nust satisfy the above inequation so what about 0.1, which is certainly smaller than 4500? In math, every positive number must satisfy the above inequation, that is true. But as Chad said, the smallest number in Haskell (at least according to my GHC, it could be different with different processors, right?) that satisfies the equation is 2.2e-16. 1 + 2.2e-16 /= 1 True 1 + 2.2e-17 /= 1 False This is because the Double type only holds so much precision. After getting small enough, the type just can't hold any more precision, and the value is essentially 0. last $ takeWhile (\x - 1 + x /= 1) (iterate (/2) 1) 2.220446049250313e-16 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Numeric type classes
It seems we are at a point, where we have to define what is a 'number'. More precisely: Can you tell me the difference between numbers and more complex mathematical objects? Is a complex number a number? Is a quaternion a number? Is a residue class a number? We can calculate with integers modulo some other integer like with integers - is that considered computation with numbers? Shall we distinguish between matrices of numbers and matrices of more complex mathematical objects? In signal theory matrices of polynomials are very common. My question would be why is it so important to determine what is or isn't a number? Whether something is a number or not does not determine what operations and properties it has. Rather, we should try to determine what is a field, a ring, a group, etc. If we know that matrices of polynomials form a group, then we can perform the operations of the group on those objects. That being said, I'll have to play the other side of the coin: it would probably be a little bit of a pain to have to define instances of each data declaration (Integer, Int, Float, Matrix, Complex, etc.) on each of these seperate classes--especially when being in a certain class usually implies being in another (ie, the definition of a set being a field requires that that set is a group, right?) And another problem I can see is that, for example, the Integers are a group over addition, and also a group over multiplication; and in my small bit of thinking about this, it seems that having to keep track of all of this might get a bit unruly. Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: class [] proposal Re: [Haskell-cafe] One thought: Num to 0 as ? to list?
On 8/22/06, Bulat Ziganshin [EMAIL PROTECTED] wrote: what i propose is not full replacement of existing syntax - quite the contrary it is just a syntax sugar for most frequent cases of using classes in function signatures. the key idea is that in most cases we use only one type class for each type variable, and the same type for each occurrence of type class in the type: (+) :: Num - Num - Num [...] so, while this proposal is rather minor, i think that it is Good thing I disagree. As a new learner to Haskell, I already have a hard time keeping Constructors, Types, and Classes straight. I know what they all are and what they all do, but sometimes I really have to think hard to remember which is which in a piece of code. What helps my understanding is that each has a specific place in the type signature (which I guess includes 'nowhere' regarding constructors). Being able to put Classes where Types go would just serve to muddle that understanding. Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] REALLY simple STRef examples
On 7/21/06, S C Kuo [EMAIL PROTECTED] wrote: Not totally relevant to what the discussion has evolved to, but I wrote a factorial function using STRefs (in the spirit of the Evolution of a Haskell programmer) and I think it qualifies as a really simple example. Code follows: import Data.STRef import Control.Monad.ST foreach :: (Monad m) = [a] - (a - m b) - m () foreach = flip mapM_ -- Bryn Keller's foreach, but with type restrictions fac :: (Num a, Enum a) = a - a fac n = runST (fac' n) fac':: (Num a, Enum a) = a - ST s a fac' n = dor - newSTRef 1 foreach [1..n] (\x - modifySTRef r (*x)) x - readSTRef r return x Forgive me for not understanding, but I was hoping you would explain a choice you made in your code. Why did you define foreach and then use foreach [1..n] (\x - modifySTRef r (*x)) Instead of simply using mapM_ (\x - modifySTRef r (*x)) [1..n] ? I tried it out in GHCi, and it worked fine, and I have seen code that has been defined as a flip to take advantage of partial application. But your code doesn't seem to take advantage of partial application, so why did you define 'foreach' and then use it instead of using 'mapM_'? I am just curious, and have always been interested in reasons behind coding style. Bryan Burgers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?
On 6/15/06, Vladimir Portnykh [EMAIL PROTECTED] wrote:
Suppose there is a data definition in Haskell:
data MyType = MyType { date :: Double,
weight :: Double,
height:: Double
} deriving (Eq, Ord, Show)
Is it possible to check if the field height, for example, is filled
in(defined)? Can we give default values in Haskell?
Many thanks and sorry fro so sily questions. Vladimir
You could make date, weight, and height Maybe Doubles, then
isDefined = isJust and isNull = isNothing.
Bryan
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?
Vladimir,
I think you forgot to put Haskell-cafe as a recipient of this email,
so first I'll repost what you wrote.
On 6/15/06, Vladimir Portnykh [EMAIL PROTECTED] wrote:
many thanks. i have the follwoing code:
module MyType (DataContainer(..)) where
import Maybe
data DataContainer =
MyType {
date :: [Maybe Double],
weight ::[Maybe Double],
height ::[Maybe Double]
}
deriving (Eq, Ord, Show)
myType = MyType {
date =
[38548,38562,38576,38590,38604,38618,38632,38646,38660,38674],
weight = [],
height = []
}
reportProblem = if isJust (height myType !! (length (date myType) - 1))
then 0 else 1
it failed to compiled saying No instance for (Num (Maybe Double)) arising
from the literal 38674 .
After reading your code for a while, I still don't know if I quite
understand. Can you explain exactly what you are trying to do?
My thought is that maybe you just want a list of objects, since you
are indexing height at the same index as date. Then you just want a
list of the structures that Duncan described.
myObjects = [default {weight = 3.2}, default {weight = 7.9, date=38458},...]
If not, hopefully somebody a little more knowledgable than I can help you.
Bryan
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] elementsinlist
On 6/13/06, Jenny678 [EMAIL PROTECTED] wrote: Hallo I search a code for elements_in_List([1,2],[1,2]). True elements_in_List([1,8],[1,2,3,4,8]). True elements_in_List([2,1],[1,2]). True elements_in_List([1,1],[1]). False I have a code elements_in_List :: Eq a = [a] - [a] - Bool elements_in_List [] _ = True elements_in_List _ [] = False elements_in_List (x:xs) (y:ys) | x == y = elements_in_List xs ys | True = elements_in_List (x:xs) ys but it failed at elements_in_List([2,1],[1,2]). True I hope somebody can help me Please don't use built-in-Functions. Thanks for Help I haven't looked too far into this, but how about something like this: elements_in_list a b = and $ map (\x - element_in_list x b) a where element_in_list = ... Maybe if you don't want to use built-in functions, define your own 'and' as well. Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Library Repository?
Recently, I have seen a few messages to Haskell Cafe (Like Marc Weber's Is there any url lib? and Ferenc Wagner's LDIF output library, as well as many replies like oh, you can find that library at this link...) that have prodded me to ask what I've been wondering for a long time. What are the pros and cons of having a library repository for Haskell libraries. At current, I'm thinking of Perl's CPAN, which many Perl experts claim is the most important aspect of Perl (Don't reinvent the wheel is a common reply at Perlmonks.org), and I believe Ruby has something similar (RubyGems?). It seems to me that with the emergence of Cabal as a standard way to package Haskell libraries, this could be possible. Here's a short, non-exhaustive list of pros and cons I came up with. Pros: * Localized place to find libraries. If I want to know if a urllib exists, I can go to the server and search for 'url' and come up with relevant libraries. * Encourages programmers to share their libraries. If there's a place to put libraries, I would imagine that some people who have good libraries lying around might put them there. Or if they have something close, they might generalize it into a library and put it there. Cons: * It obviously needs a server, which costs money, and other things like that cost money. * Personel. It would take a bit of initiative to start a project like this, as well as continued upkeep. That would either require some knowledgable volunteers, or, once again, money. I'd love to hear the thoughts of the community on this. Bryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
