August 2007 - Haskell-Cafe

Indentation woes
by anon 01 Aug '07

01 Aug '07

Greetings, I wish to be able to indent my code like so: > longFunctionName various and sundry arguments > | guard1 = body1 > | guard2 = body2 > | ... > where declarations That is, with guards and where clauses indented to the same level as the function name. This seems like a perfectly reasonable indentation style to me. It also happens to be the preferred style in Clean, another layout-sensitive functional language. I believe it is not uncommon in ML dialects as well. So why is it that I'm not allowed to use it in Haskell?

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Re: [Haskell] View patterns in GHC: Request for feedback
by Simon Marlow 01 Aug '07

01 Aug '07

Dan Licata wrote: > Simon PJ and I are implementing view patterns, a way of pattern matching > against abstract datatypes, in GHC. Our design is described here: > > http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns > > If you have any comments or suggestions about this design, we'd love to > hear them. You can respond to this list (and we can take it to > haskell-cafe if the thread gets long) or, if you prefer, directly to me. At the risk of being a spoil-sport, I have a somewhat negative take on view patterns. Not because I think they're particularly bad, but because I don't think they're significantly useful enough to warrant adding to the language, at least if we also have pattern guards. All of the examples on the wiki page can be written in the same number of lines, sometimes with fewer characters, using pattern guards or some other existing Haskell idiom (e.g. the bit parsing example is much more nicely expressed using a bit-parsing monad). I believe adding yet another idiom will be detrimental: too much choice is bad. To my eyes, mixing bound and binding occurrences of variables in patterns in an arbitrarily nested way is sure to lead to confusion. The existing idioms all have a one-level deep notion of bound/binding scope, and in order to get more nesting you have to start naming things: this is good, IMO. Not that I think we should omit a language feature because it *could* be used to write obfuscated code; no, in this case I think nesting more than one level deep will *always* lead to obfuscated code. The use of the right-arrow is confusing, especially on the left of a case alternative or in a lambda expression. The main argument in favour of view patterns is: > it's possible that people would start routinely hiding the data > representation and exporting view functions instead, which would be an > excellent thing. My impression is that most of the time a data structure has a clever internal representation that you don't want to expose anyway. This is supported by the data structures we have in the base package, all these are abstract: Data.Set, Data.IntSet, Data.Map, Data.IntMap Data.Graph, Data.Sequence, Data.Bytestring, Data.Array, Data.HashTable, Data.Ratio(Rational) and most export view-like things (e.g. Data.Set.toList). The modules I found that export non-abstract types that should really be abstract: Data.Complex, Data.Tree So I don't think there's an overwhelming amount of stuff that would change if we had view patterns. In GHC itself most of our data structures are already abstract too. The View class is nice, even with just pattern guards. I'd be in favour of making it standard and actively encouraging its use. Cheers, Simon

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Re[6]: [Haskell-cafe] i need wxHaskell compiled for ghc 6.6.1 on Windows
by Bulat Ziganshin 01 Aug '07

01 Aug '07

Hello shelarcy, Wednesday, August 1, 2007, 4:03:52 AM, you wrote: >> problems. the only question that remains - does this version supports >> unicode? > Yes. Current darcs repository version support only unicode > enabled version. great, it's all what i need. but i'm still curious about other features enabled when building this package. can i see config.gcc or build.cfg or setup.h or any other file that shows feature list? also it will be useful to include such file with the next builds of wxHaskell -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Newbie question about Haskell skills progress
by peterv 31 Jul '07

31 Jul '07

Having only a couple of days of practice programming Haskell (but having read lots of books and docs), I find myself writing very explicit low level code using inner "aux" functions (accumulators and loops). Then I force myself to revise the code, replacing these aux functions with suitable higher-order functions from the library. However, I would like to use these higher order functions right away, without using low-level aux constructs, which is most likely caused by my very long history of imperative programming. Is this the "normal" way of progressing in Haskell, or should I consider a different approach? Thanks, Peter

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infinite list of random elements
by Chad Scherrer 31 Jul '07

31 Jul '07

I'm trying to do something I thought would be pretty simple, but it's giving me trouble. Given a list, say [1,2,3], I'd like to be able to generate an infinite list of random elements from that list, in this case maybe [1,2,1,3,2,1,3,2,3,1,2,...]. I'm using IO for random purely due to laziness (my own, not Haskell's). I was thinking the best way to do this might be to first write this function: randomElts :: [a] -> [IO a] randomElts [] = [] randomElts [x] = repeat (return x) randomElts xs = repeat r where bds = (1, length xs) xArr = listArray bds xs r = do i <- randomRIO bds return (xArr ! i) Then I should be able to do this in ghci: > sequence . take 5 $ randomElts [1,2,3] [*** Exception: stack overflow Any idea what's going on? I thought laziness (Haskell's, not my own) would save me on this one. Thanks! Chad

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Operational Semantics of Haskell
by Lewis-Sandy, Darrell 31 Jul '07

31 Jul '07

Is there a good source for the operational semantics of Haskell? I am trying to squeeze the most efficiency out of a bit of code and am looking to remove unnecessary reductions.

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Fwd: Re: Re[4]: [Haskell-cafe] i need wxHaskell compiled for ghc 6.6.1 on Windows
by shelarcy 31 Jul '07

31 Jul '07

Oops, I made mistake to send this mail only for Bulat. ------- Forwarded message ------- From: shelarcy <shelarcy(a)gmail.com> To: "Bulat Ziganshin" <Bulat.Ziganshin(a)gmail.com> Subject: Re: Re[4]: [Haskell-cafe] i need wxHaskell compiled for ghc 6.6.1 on Windows Date: Wed, 01 Aug 2007 09:03:52 +0900 Hello Bulat, On Wed, 25 Jul 2007 22:18:58 +0900, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > Tuesday, July 24, 2007, 2:32:01 AM, you wrote: > >> So I put newer Windows binary on my project's file space. > >> http://sourceforge.net/project/showfiles.php?group_id=168626 > > thank you very much!!! now i'm really happy - it works without any > problems. the only question that remains - does this version supports > unicode? Yes. Current darcs repository version support only unicode enabled version. http://article.gmane.org/gmane.comp.lang.haskell.wxhaskell.general/198 So you can use UTF-8 string in your program. Best Regards, -- shelarcy <shelarcy hotmail.co.jp> http://page.freett.com/shelarcy/

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