Re: [Haskell-cafe] Types
Hello, I´ve a small question about Haskell. It´s really vague explained in Haskell in 10 minutes and sometimes it's really detailed explained which is very hard for people who just got into Haskell to read. Can someone explain me something about the type Integral? What exactly is it? Integral isn't actually a type; it's a type class. Int is a fixed-size type; it's often 32 or 64 bits in practice, but you can't rely on that portably. Int operations are fast. Integer is an unbounded-size type. It's less efficient than Int, for obvious reasons, but it's often more correct because it won't over or underflow. Integral is a class of types that support integer-like operations. If you ask ghci: ePrelude :info Integral class (Real a, Enum a) = Integral a where quot :: a - a - a rem :: a - a - a div :: a - a - a mod :: a - a - a quotRem :: a - a - (a, a) divMod :: a - a - (a, a) toInteger :: a - Integer -- Defined in GHC.Real instance Integral Integer -- Defined in GHC.Real instance Integral Int -- Defined in GHC.Real Any type for which those operations (quot, rem, div, and so forth) make sense, can be made an instance of class Integral. Int and Integer are two types that are such instances. I know that 2 is something of the type Integral, however what does this differ from Int? The same question goes for Fractional and Floating, what exactly is their commonality and what exactly is their difference? Numeric literals are a special case. To make it possible to write 2 and mean it as an Int or an Integer (or any other numeric type), 2 is defined to *really* mean (fromIntegral 2). fromIntegral is a function that takes an Integral value (such as 2), and converts it to a type of your choice, so long as it's a member of the Num class. Thank you for explaining these basic things to me. This is one of those areas where Haskell beginners get quickly thrown a handful of things that are unfamiliar (type classes, defaulting, tricky numeric literals), while doing the most basic things (arithmetic). Don't despair; it'll make sense soon if it doesn't yet. Regards, John Dorsey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pattern Matching
Casey, Why in a pattern match like score (1 3) = 7 You probably mean score 1 3 = 7 which applies the function 'score' to two arguments. With the parentheses, it looks like an application of '1' to the argument '3'. But to answer your actual question... can I not have sizeMax = 3 score (1 sizeMax) = 7 When a variable name (such as 'sizeMax') appears in a pattern, it gets bound there. This is useful so you can refer to the variable on the right hand side, as in: successor x = x + 1 How would the compiler know whether to bind the variable (what actually happens), or match against the value represented by some earlier binding (what you're asking for)? What if the type of that second argument doesn't have a defined equality operation? There might be some reasonable way to do it, but I suspect it would be fragile and error-prone, at best. So it's always a new binding. It's easy enough to work around: score 1 x | x == sizeMax = 7 Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] NetSnmp problem.
I am trying to use NetSnmp to get some information from my switch. And I met this problem. After initialize, I used snmpWalk to get some information, and dealed with it. Everything is fine. Then I sleep for 5 mins. SnmpWalk again. But this time, all asnValues were unsupported format, which broke my dealing process. Are you on a 64-bit machine? I've made an update or two, but haven't pushed them to Hackage yet... I'll do that today. Please let me know if this fixes your problem. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Need help with ghc static compile for cgi using mysql
lambdabot is currently hosted on the lowest end linode. The biggest hurdle I [...] +1. I use linode for (most of) my Haskell work. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
Andrew Coppin said: Sure. But what is a computer program? then Richard O'Keefe said: A computer program, in short, is *whatever we want it to be*. (Within reasonable limits.) I agree with Richard's conclusion. From where I sit, the critical point is that, unless you're breadboarding, programming is working in the abstract, and we choose our abstractions. There's a strong tradition of sequential imperative programming, but that's as far as it goes. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] I read somewhere that for 90% of a wide class of computing problems, you only need 10% of the source code in Haskell, that you would in an imperative language.
Well, try this: Go ask a random person how you add up a list of numbers. Most of them will say something about adding the first two together, adding the third to that total, and so forth. In other words, the step by step instructions. You word the (hypothetical) question with a bias toward imperative thinking. You're asking How do you do this action? Why isn't the question What is the sum of a list of numbers?, which is biased toward the declarative? John (who's glad he's not a social scientist) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Typeclasses vs simple functions?
perimeter :: Geometry - Double perimeter (Sphere _ r) = 0.0 perimeter (Circle _ r) = 2.0 * pi * r The latter is even simpler because there is no need in extraction of Double value from Maybe. I'd strongly advise against this last one on style grounds. (0 :: Double) isn't nearly as suitable as a distinguished value indicating an invalid result as Nothing. It can be made to work, in the same way that you can write complex code in asm; instead, use a solution that gives you type-level help in getting it right. I'd use Maybe. Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Where does documentation get installed with cabal?
I'm trying to find the API documentation for happstack 0.3 (online is for 0.2). [...] Max In most cases it is installed in Max ~/.cabal/share/doc/happstack*/html. Is there any files at Max that directory? There are now, but only a very few. And very little contents in them. It's as if the --reinstall were partial. Perhaps it's in /usr/local/share/doc. Mine seem to end up there, and I don't recall whether I specified that at any point. It's not in my ~/.cabal/config, but I am using the very helpful user-install: False documentation: True I suspect /usr/local/share/doc is the default for documentation in global installs, but I can't go verify that at the moment. Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: TypeDirectedNameResolution
Cale et al, I have a concern about the implementation of the proposed TypeDirectedNameResolution. (I'm not familiar with the internals of any of the compilers, so it could be that my concern isn't well founded.) I'm assuming that name resolution is currently independent of type inference, and will happen before type inference. With the proposal this is no longer true, and in general some partial type inference will have to happen before conflicting unqualified names are resolved. My worry is that the proposal will require a compliant compiler to interweave name resolution and type inference iteratively. Give the Haskell source code: import X import Y import Z a = 1 + (1 :: Integer) b = x a c = y b d = z c . . . Assume X, Y, and Z all export x, y, and z. The compiler might do something like this: (do stuff) Infer type of 'a' Resolve unqualified use of 'x' in x a Infer type of 'b' from b = x a Resolve unqualified use of 'y' in y b Infer type of 'c' from c = y b Resolve unqualified use of 'z' in z c etc. If ambiguous unqualified names are used mutually recursively it may be that there's only one reasonable combination of name resolutions, but is this decidable? To my untrained eye it looks complicated and invasive, even without the mutually recursive case. Can anyone shed light on whether this would be a problem for, say, GHC? Thanks, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Circular pure data structures?
John, Is it possible to create a circular pure data structure in Haskell? For example: Creating the data structure is easy; as other respondents have pointed out. A simple example is this... ones = 1 : ones ones' = 1 : ones' Comparing these values is harder. All of (ones), (ones'), (tail ones), and so forth are equal values. But I don't know how to compare them. My Spidey-sense tells me there's probably a simple proof that you can't, if you care about the comparison terminating. Pointer equality (ie. testing if the values are represented by the same bits in memory) is no good, since it's entirely up to the compiler whether ones and ones' use the same bits. (They won't, but that's not important.) In general pointer equality of Haskell values, such as ones and (tail ones) interferes with referential transparency, which is held in high regard around here. Although, for the record, when pointer equality is really what you want, I'm sure there's ways to Force It. For your purposes, does it matter if you can actually do the comparison? Is it enough to know that ones and (tail ones) are equal? Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Re: Error message reform (was: Strange type error with associated type synonyms)
I like the expected/inferred vocabulary. Maybe it comes from being a native English speaker, but to me, it says this is what we expected to get, but instead (through type inference), we got this type for this term. As another native English speaker, I found expected/inferred very intuitive when I was new to GHC, and to Haskell. I even think that expected/inferred helped me form my intuition about Haskell's type inference. There was one hang-up; it wasn't at all clear which referred to the term, and which referred to the context. (Really both types are inferred.) This stopped bothering me when I decided it didn't matter which was which, and I could generally find the problem pretty quickly just knowing the location and the types involved. Of course, I can see how the messages are probably much less useful to non-native speakers, and that's quite important. Something along the lines of inferred type droozle for term, but expected type snidgit in context. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANNOUNCE] Bindings for libguestfs
Richard, I added some partial bindings for libguestfs[1] here: http://git.et.redhat.com/?p=libguestfs.git;a=blob;f=haskell/Guestfs.hs;hb=HEAD Terrific! Partial bindings are great. Thanks for releasing it. I haven't taken the time to look at your code, but... BTW, I found the documentation on writing FFIs very contradictory and incomplete. For example, I was completely defeated trying to find ways to do simple stuff like passing in integers or returning booleans. *Potentially* Haskell's FFI seems like it might be one of the best out of the languages I've used so far, but it needs way more documentation and examples. Can you be more specific about what needs improvement? I wrote a partial Haskell binding for Net-SNMP recently, and I got along pretty well using the API docs at http://www.haskell.org/ghc/docs/latest/html/libraries/index.html, and an example in RWH using hsc2hs. What did you see that was contradictory? Regards, John ___ 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?
Does that also mean that you could write: if 3 - 4 then ... else ... (= if (fromInteger 3 :: Bool) - (fromInteger 4 :: Bool) then ... else ...) No. 3 - 4 is an Integer, the proposal is to convert Bools to Ints, not Ints to Bools. Rather, (3 - 4) is a (Num t) = t, so yes, this would work with instance Num Bool. *Main if 3 - 4 then yessirree else yep yep Yeah, the more people give examples of the power of Num Bool, the more it seems like a very bad idea! +1 John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] fromInteger for Lists
Paul, There's nothing better than making a data type an instance of Num. In particular, fromInteger is a joy. But how about lists? Do you mean something like Blargh below, only useful? John dor...@elwood:~/src/scratch$ cat list.hs class Blargh f where fromList :: [a] - f a data Foo a = Foo [a] deriving (Show) data Bar a = Bar [a] deriving (Show) instance Blargh Foo where fromList = Foo instance Blargh Bar where fromList l = Bar (reverse l) dor...@elwood:~/src/scratch$ ghci list.hs GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Main ( list.hs, interpreted ) Ok, modules loaded: Main. *Main fromList [1,2,3] :: Foo Int Foo [1,2,3] *Main fromList [1,2,3] :: Bar Int Bar [3,2,1] *Main ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] General function to count list elements?
Michael, I had a count function that worked fine for an enum type, but thought why not go polymorphic with it. So I changed it and got the error I originally inquired about. For variety, I'll go a slightly different direction. If you generalize count to use any predicate, instead of always equality... gcount :: (a - a - Bool) - a - [a] - Int gcount pred x0 xs = length (filter (pred x0) xs) count = gcount (==) This will work with any type that you can write a predicate for with the type (a - a - Bool). I can even use this with functions, if I'm careful. ghci gcount (\f g - True) (*2) [id,(const 1),(*3)] 3 ghci gcount (\f g - f 1 == g 1) (^2) [id,(const 1),(*3)] 2 By the way, do you see why everyone's bothing you about comparing functions? The type you gave count, which didn't have an Eq constraint, was an assertion that you could compare two values of *any* type. If there's a type that's not comparable, then count's type was wrong. Functions are the canonical example of an incomparable type. When you're bored some time, read a bit about the Curry-Howard correspondence. It's interesting, even if (like me) you don't grok all of its implications. Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Printing list of enum type
Michael, What do I need to add to this Color enum type to print a list of them? You can also easily print a list of /all/ of them. Regards, John scratch$ cat color.hs data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black deriving (Show,Enum,Bounded) scratch$ ghci color.hs *Main [minBound..maxBound] :: [Color] [Red,Blue,Green,Yellow,Orange,Brown,White,Black] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Configuring cabal dependencies at install-time
John Lato wrote: I think that the proper solution is to break up libraries into separate packages as Jeff suggests (buster, buster-ui, etc.), but then the total packages on hackage would explode. I don't feel great about I thought about this a while back and came to the conclusion that the package count should only grow by a small contant factor due to this, and that's a lot better than dealing with hairy and problematic dependencies. It should usually be: libfoo libfoo-blarg libfoo-xyzzy etc. and more rarely: libbar-with-xyzzy libbar-no-xyzzy etc. each providing libbar. Although I don't remember whether Cabal has 'provides'. The latter case could explode exponentially for weird packages that have several soft dependencies that can't be managed in the plugin manner, but I can't see that being a real issue. This looks manageable to me, but I'm no packaging guru. I guess it's a little harder for authors/maintainers of packages that look like leaves in the dependency tree, which could be bad. Am I missing something bad? Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Configuring cabal dependencies at install-time
Edward, Thanks for straightening me out; I see the problems better now. In particular I was missing: 1) Orphaned types (and related design issues) get in the way of splitting the package. 2) Viral dependencies work in two directions, since upstream packages must pick up your deps to include instances of your classes. I'm thinking out loud, so bear with me. The problem is there is no path to get there from here. Getting another library to depend on mine, they have to pick up the brittle dependency set I have now. Splitting my package into smaller packages fails because I need to keep the instances for 3rd party data types packed with the class definitions to avoid orphan instances and poor API design. So the option to Some class instances can go in three places: a) The source package for the type, which then picks up your deps. Bad. b) Your package, which then has a gazillion deps. Bad. c) Your sub-packages, in which case they're orphaned. Bad. I have to wonder whether (c) isn't the least of evils. Playing the advocate: - Orphaned instances are bad because of the risk of multiple instances. That risk should be low in this case; if anyone else wanted an instance of, say, a Prelude ADT for your library's class, their obvious option is to use your sub-package. - If you accept the above, then orphaning the intance in a sub-package that's associated with either the type's or the class's home is morally better than providing an instance in an unaffiliated third package. - Orphaning in sub-packages as a stopgap could make it much easier to get your class (and the instance) added to those upstream packages where it makes sense to do so. This clearly doesn't solve all parts of the problem. You may have other design concerns that make sub-packages undesirable. Even with instance definitions removed you may still have enough dependencies to deter integration. The problem probably extends beyond just class instances. The only other alternative that I seem to have at this point in the cabal packaging system is to create a series of flags for optional functionality. This sounds like rat hole of a different nature. You lose the ability to tell if an API is supported based on whether the package that implements it is installed. An installed and working package can cease to function after (possibly automatic) reinstallation when other packages become available. Complicated new functionality is required in Cabal. Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] high probability of installation problems and quality of the glorious implementation
Quoth FFT: My general null hypothesis is, as Alec Baldwin put it, that a loser is a loser, or a buggy project is buggy. I can't see the world in such black and white terms. GHC has strengths and weaknesses, as do other projects. GHC is changing over time, as are other projects. Formally verified software is still rare. Most of the useful stuff lies somewhere between buggy and bug-free. If GHC is robust overall (which I'm yet to find out), why is the installation so broken? History. Limited resources. Complexity and diversity of target environments. Moving targets. Day jobs. Of course, you have to determine what your needs and standards are for any product you use. Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Function to cast types
Anonymous One, I'm new to haskell, I'm wondering how can you write a function that will do the following: fromIntToString :: Int - String Is this a homework assignment, perchance? Please take a look at http://www.haskell.org/haskellwiki/Homework_help Everyone here is glad to help, but we're also sensitive on academic honesty. With that said, you've done the first step, which is writing down the function's type. You don't want to iterate over all possible Int values, so maybe a recursive definition is in order. What are the cases to consider? Like, what happens if you have a negative number... this is a cast function to cast an Int to a String. I know such function By the way, cast is probably the wrong word for what you want. You're building a string representation of an Integer; in my view it's not just a type conversion. Good luck, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Function to cast types
Rafael Cunha de Almeida wrote: This e-mail is offtopic, but I suppose being a little offtopic every now and then isn't that bad, specially if the subject is interesting. So, here I go. It's the cafe'. Very little is strictly off-topic here, and certainly not this. I'm happy to help anyone who's trying to learn, but I will not willingly subvert anyone's classroom rules, as a matter of academic honesty. I think that matters at any level of school. I accept that it can't be enforced perfectly, or anywhere near so. I didn't mean to accuse the original poster, and I trust I didn't come accross that way. I don't think it's a bad idea to point out that an anonymous question like this one may look suspicious, or that many instructors follow this forum. None of that should give offense. But I also provided some hints. I think a good and fun way to help with problems like that is to iteratively guide someone to their answer, instead of giving it to them straight-up. Just to be clear, I also have no criticism of the other replies. Anyone giving voluntary help should and will apply their own rules. The only reason people would want the answer of an assignment instead of actually doing it themselves is if they don't care about actually doing it. That is, they don't want to learn haskell, they just want to pass a class. In that case, do we really want to _make_ them learn? In that case I have no interest in forcing them to learn or in helping them get a grade. But I'm always optimistic that someone's willing to learn. I've been proven wrong, but not often. Cheers, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell-beginners] folds -- help!
Adrian, That's why I said for appropriate g and f. But I see that my wording was misleading. Thanks for following up! I had thought you were arguing that foldl and foldr were easily and intuitively interchangeable; they're surely not so for beginners. Now I think you're arguing that given strict oplus, each can be generally expressed in terms of the other, allowing for different space/time complexity. Thanks for clarifying. I should have paid more attention to your appropriate g and f qualifier. Cheers, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell users in the Houston area??
Vasili, Are there Haskell users in the Houston area? At least one, but I'm behind on my cafe' reading. smiley Cheers, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Time for a new logo?
Quoth Miguel Mitrofanov: You have to be unnecessarily strict to read the whole word Haskell and not just first one or two letters. Not true, if you're looking forward to what may follow Haskell. Cheers, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Linking and unsafePerformIO
Jules Bean wrote: I'm saying that we can change programs, and that changes their denotation, and that's fine, and anyone can do that. But the denotation of a program is supposed to be something independent of a particular compiler or OS or MAC address or RAM size or any of the millions of other things which probably don't change during the single run of a program. And native Int size. Should all Int's be in the IO Monad? Should all floating point numerals be in the IO Monad? Does your proposal push all non-portability into IO, making the non-IO subset of Haskell a portable virtual machine specification? Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [] vs [()]
dmehrtash: What is the difference between empty list [] and list with one unit element [()]? Prelude length [] 0 Prelude length [()] 1 Also, they differ in type. [()] is a list of unit elements, and happens to contain exactly one of them. [] is a (polymorphic) list of any kind of element, and happens not to contain any of them. Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Announcing OneTuple-0.1.0
Fellow Haskellers, Much attention has been paid over the years to the regrettable omission of singleton tuples from Haskell. I am pleased to announce OneTuple, a humble implementation of the singleton tuple for Haskell. Now you can: * Wrap a single value of any type in a OneTuple ! * Pattern match to retrieve your value ! * Solve any of the software problems that cannot be solved without the singleton tuple ! * Enjoy instances for all the classes normal tuples have, plus more ! * Proclaim feature parity with Python ! Note: the singleton tuple does not support tuple syntax. Contributions are welcome. The project could use a tutorial, and a decent test suite. Strict singleton tuples are planned for the next version. Enjoy! Regards, John Dorsey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Announcing OneTuple-0.1.0
All, I'm bundling responses to save paper. [EMAIL PROTECTED] wrote: I hope it has a Monad instance. Naturally! But more to the point: Can it send email? Can you give an example of a use case? Do the Haskell-98 standard tuples have a correspondence feature? I wasn't able to find one with Hoogle. Simon Brenner wrote: You could always use this one-tuple instead and get Functor, Monad and MonadFix for free: As Luke pointed out, that one seems to be too strict. It may simplify the strict implementation, though. The initial release did have Monad and Functor instances... I'll look into MonadFix (thanks!). Luke Palmer wrote: Hmm, it looks like you forgot to write a Traversable instance. Oops... I included the instance statement but retained the default, mutually recursive methods. Too bad GHC didn't warn me. (Pesky halting problem.) Your change is in 0.1.1 -- thanks! Benjamin L.Russell wrote: Wonderful! I'm intrigued Thank you. What is the syntax for the singleton tuple? [...] What is your solution? Haskell has no such syntax, of course. '(x)' is no good due to ambiguity with parens' usual associative use. '(x,)' has been discussed, I think. It's ugly; it's inconsistent with other tuples, which don't share its final comma; it looks a bit like a tuple section, which could cause confusion. My solution was to use a normal Algebraic Data Type: data OneTuple a = OneTuple a I think the need for singleton tuples is rare enough that the syntactic inconsistency is tolerable. Jon Fairbairn suggests using unicode 0x27e8 and 0x27e0 in place of parentheses for tuples. I like the idea, especially as an alternate syntax for the same tuple types, permitting the singleton. minh thu writes: I thought to this idea in another way : parenthesis could be used for s-expressions and [unicode 0x27e8 and 0x27e0] could be used for regular grouping. This would allow to switch in the same code between infix and s-expr (e.g. enabling SXML)... I don't think I fully understand your proposal, although it sounds interesting. Regards, John Dorsey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [haskell-cafe] Monad and kinds
Tim, A year or two ago, ISTR that *most* of the newbie-generated traffic in the cafe was about atrocious performance of naive programs due to strict/lazy concerns. I think it was scaring people away. I think it's debatable what the various causality relationships might be here. Certainly... Adding strictness can improve asymptotic space performance, as an example. Is there a reason to think this won't always be true? Honest question, since I don't know nearly enough about strictness analysis to guess how good it'll be some day. Adding strictness can also worsen asymptotic space (and time) performance. That's one reason why we use a lazy language at all. Strictness analysis is an approximation to the problem of determining what parts of a program can be evaluated strictly without changing their meaning, because if we had a perfect solution to that problem, we could solve the halting problem. No argument. I was responding to your comment that ... IMO, arguing that programmers should care at all amounts to conceding that default laziness is treacherous. ... which sounds like you're arguing against programmers giving due attention to lazy/strict choices. I was suggesting that there is good reason to think that we should pay attention to it; that it's a necessary part of learning Haskell; and that it will likely remain so. Newbies should be encouraged to think and experiment more with laziness and strictness. Regards, John (pondering an IDE mode for visualizing strictness properties) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [haskell-cafe] Monad and kinds
Tim Chevalier wrote: I'm no master, but I've never encountered a situation where strictness annotations would be useful as documentation, nor can I imagine one. That's because optimization *is* the only reason why programmers should care about strictness information. IMO, arguing that programmers should care at all amounts to conceding that default laziness is treacherous. I'm no master either, but I'd argue that if we promise new programmers that they don't need to care about strictness, we thereby ensure that default laziness is treacherous. A year or two ago, ISTR that *most* of the newbie-generated traffic in the cafe was about atrocious performance of naive programs due to strict/lazy concerns. I think it was scaring people away. Adding strictness can improve asymptotic space performance, as an example. Is there a reason to think this won't always be true? Honest question, since I don't know nearly enough about strictness analysis to guess how good it'll be some day. Regards, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeInterleaveIO, lazyness and sharing
Wolfgang, Haskell is non-strict but not necessarily lazy. So it's possible that an expression is reduced to WHNF although it is not used yet. Could this early reduction also happen to outputs of unsafeInterleaveIO actions (which might trigger the action too early)? While I'd expect those outputs to be evaluated lazily (reduced as late as possible), I cannot find anything in the docs that guarantees this. unsafeInterleaveIO allows IO computation to be deferred lazily. When passed a value of type IO a, the IO will only be performed when the value of the a is demanded. This is used to implement lazy file reading, see hGetContents. http://haskell.org/ghc/docs/latest/html/libraries/base/System-IO-Unsafe.html#v:unsafeInterleaveIO Is this the kind of guarantee you're looking for? I'd bet against getting any decent durable, portable guarantees for any unsafe* functions in Haskell; but the above behavioral description may be strong enough to suit you. In addition, I'd like to know whether unsafeInterleaveIO outputs are guaranteed to be evaluated at most once so that the interleaved action is executed at most once. Again, I suppose that this is the case while I cannot find a guarantee for it. I'd be surprised if an implementation didn't have that behavior. I'd also be wary of anyone claiming to guarantee it, beyond compiler X version Y. John Dorsey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal + yi + alex problem.
[problems with cabal install yi] Curious, I tried this out, on a Debian etch box with GHC 6.8.3. cabal update ; cabal install yi --global complained about needing alex =2.0.something and 3. Then cabal install alex happily installed alex version 2.2. Finally, cabal install yi --global worked fine. Is something amiss with cabal-install? Shouldn't it have automatically installed alex? Or does it only do that with libraries, by design? For that matter, ghc-pkg list | grep -i alex doesn't list anything, after I cabal-installed it. How does cabal verify the prerequisite alex version? (Or does it?) I'm enjoying cabal since I've begun using it, but it's still somewhat opaque to me. Regards, John Dorsey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Another optimization question
Jeroen, isPrime :: Int - Bool isPrime x = isPrime' x primes where isPrime' x (p:ps) | x == p = True | x p = isPrime' x ps | otherwise = False main = print $ length (filter (== True) (map isPrime [1..5000])) [...] isPrime x = elem x (takeWhile (= x) primes) Here's a couple of things, although I don't know if they account for what you're seeing. All code is untested. 1) A simpler main would be: main = print $ length $ filter isPrime [1..5000] This version manually fuses your map and filter. Of course it's not the same if you're doing anything else besides 'length'. 2) The takeWhile in your second isPrime creates a throwaway list, which doesn't exist in the explicit-recursion isPrime. Unless this gets optimized out, this could be the droid you're looking for. I'd compile with profiling (ghc -O2 --make -prof -auto-all experiment2), and run ./experiment2 +RTS -p -s Find profiling stats in experiment2.prof, and check whether the latter version isn't allocating a lot more. When you feel like Core-diving, it's something specific to look for. 3) Maybe you can get the best of your two versions -- meaning the relative speed of the first and functional style of the second -- by writing your own 'elem' replacement that works on a sorted list. Something like this, with suitably defined elemAsc: -- elemAsc: tests presence of element in an ascending list elemAsc :: (Ord a) = a - [a] - Bool elemAsc ... isPrime x = elemAsc x primes Here's a good habit: abstract things like this out. Read the libraries, and look for better and more abstract patterns. Rinse, repeat. 4) This doesn't explain why the version with real primes was 10x slower. Are you comparing apples to apples? Specifically, comparing both versions of isPrime above using real primes, so both of them have to create the primes list? Does your code for real primes still use [Int] and not [Integer] or (Num t) = [t] ? I haven't invested the time yet to stare at GHC Core until it clicks, excepting a few snippets that have been discussed here. I'm not sure how early in the learning curve it's advisable. Probably depends on your background. Good luck Eulering, John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] deriving
Paul, Hi data Bool = False | True deriving (Eq, Or, Show, Read) Bool is an instance of Eq, Ord, Show and Read. It is derived form these classes. No. deriving ... here does not mean that Bool is derived from those classes; it's not a statement about inheritance or anything similar. deriving ... means that the class instances for Bool, for those four type classes, are automatically derived from the definition of Bool. Does this make more sense? John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
(forw) Re: [Haskell-cafe] deriving
I meant to send this reply to the cafe. - Forwarded message - Date: Mon, 7 Apr 2008 15:05:00 -0400 To: PR Stanley [EMAIL PROTECTED] Subject: Re: [Haskell-cafe] deriving Paul, No, sorry. I'm not sure how this differs from my definition. Could you elaborate please? Gladly. I haven't tested any of the code below, so pardon any typos and thinkos. It sounded like you were trying to say that Bool derives from Eq, Ord, Show, and Read, and thus the keyword from would apply. Of course, I may have misunderstood your intent. I was countering by saying that the deriving keyword indicates something different. I'll try to describe what I meant in a little more detail. A type class definition defines properties of the type class. Eg., class Example a where foo :: a bar :: a - a The class is then related to specific types by giving them instance declarations. The instance declarations can be explicit, as in data Xyzzy = Xy | Zzy instance Example Xyzzy where foo = Xy bar = const Zzy Sometimes the instance declaration can be automatically derived instead, which looks like: -- non-working code, see below data Xyzzy = Xy | Zzy deriving (Example) This lets the compiler derive the obvious class instance declaration. But it won't work for just any class, because the compiler needs to know how to derive obvious instances. A few classes have deriving machinery built-in: Eq, Ord, Enum, Bounded, Show, Read. deriving (Example) won't work, since that machinery doesn't exist. With effort, you could implement this automatic derivation machinery for your own types, at least in GHC. I've never needed it, so I've never learned exactly how to do it. In short, so data Foo [...] deriving (Eq) means: define a new type Foo, and automatically derive its class instance for Eq. Does this help? Regards, John - End forwarded message - ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: GHC version 6.8.2
Judah, Thanks for your reply, and your help. (Although it looks like the libraries/readline/configure script might recognize these, I can't get an option to pass through.) Actually, this is supposed to work. When running the top-level ghc configure, you should be able to just say ./configure --with-readline-libraries=/opt/local/lib --with-readline-includes=/opt/local/include [...] I just checked that this works on my machine (Tiger x86) when building 6.8.2 against MacPorts' readline. If it doesn't work for you, what errors are you getting? I was slow to reply, in part so I could try this again with a fresh start from the tarballs. Sure enough, it now works for me. What I /was/ seeing was a successful compile and install of ghc, but no readline functionality in ghci (i.e. backspaces gave me junk). Also, a couple of other generic questions: - Are you on x86 or PPC? (The latter is not working right yet with Leopard.) I have both, but I've been using the x86. - How are you bootstrapping the build of 6.8.2: are you using a pre-built binary of ghc-6.8.1, or some other way? Yep, a binary distro of 6.8.1 was right. I must have failed to dot an 'i' somewhere before. Thanks for pointing me in the right direction. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: GHC version 6.8.2
(Moving to the cafe) On a related topic, I've been trying to build 6.8.2 on Leopard lately. I've been running up against the infamous OS X readline issues. I know some builders here have hacked past it, but I'm looking for a good workaround... ideally one that works without changes outside the GHC build area (besides installing a real readline). Here's what I noticed before I started drowning in the build platform. (I'm no gnu-configure expert nor GHC insider.) I can get gnu-readline installed from Macports, no problem. The top-level configure in GHC doesn't respond to my various attempts: o using --with-readline-libraries and --with-readline-includes (Although it looks like the libraries/readline/configure script might recognize these, I can't get an option to pass through.) o setting LDFLAGS and CPPFLAGS environment variables (with -L/opt/local/lib and -I/opt/local/include resp.) in my shell before running configure o playing with the above settings and others in a mk/build.mk Until Apple fixes their broken-readline issue (maybe when the readline compatibility of libedit improves)... maybe the top-level configure can pass through flags or settings somehow? For those who've built with readline on OS X: have you had to resort to blasting the existing readline library link, or is there a configuration option within the GHC tree that you've gotten to work? Should I be filing a trac bug instead of asking here? Thanks for any help. There's no urgency for me; I'm just trying to get a working environment at home; I'd prefer to be able to bootstrap from the ground up; and I'd like to be able to contribute to testing/debugging on OSX. John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
