- Haskell-Cafe - Haskell.org

[ANN] Haskell Cheatsheet v1.0
by Justin Bailey 14 Oct '08

14 Oct '08

All, I've created a "cheat sheet" for Haskell. It's a PDF that tries to summarize Haskell 98's syntax, keywords and other language elements. It's currently available on hackage[1]. Once downloaded, unpack the archive and you'll see the PDF. A literate source file is also included. If you install with "cabal install cheatsheet", run "cheatsheet" afterwards and the program will tell you where the PDF is located. The audience for this document is beginning to intermediate Haskell programmers. I found it difficult to look up some of the less-used syntax and other language stumbling blocks as I learned Haskell over the last few years, so I hope this document can help others in the future. This is a beta release (which is why I've limited the audience by using hackage) to get feedback before distributing the PDF to a wider audience. With that in mind, I welcome your comments or patches[2]. Justin [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CheatSheet [2] git://github.com/m4dc4p/cheatsheet.git

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gtk2hs and GTK+OSX
by Jeroen Baekelandt 14 Oct '08

14 Oct '08

Hi, I was wondering if anybody managed to get gtk2hs working with GTK+OSX from http://www.gtk-osx.org/ I'm trying but there seems to be a quite long dependency list that keeps on building up. So I was wondering if it ends and if it works in the end. Thanks, Jeroen

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Re: What I wish someone had told me...
by John Lato 14 Oct '08

14 Oct '08

While this may be true, it's missing the point. The claim that type classes are like interfaces (the usual argument is that they define an interface, but not an implementation) misleads OO-grounded programmers into thinking of type classes in terms of an already-familiar concept. This is bad because said programmers then attempt to apply familiar OO-styles, when they would be better off abandoning most of what they already know about design. I don't mean in the sense of "abstraction good, separation of processing good, repetition bad", but rather how to develop a useful/stable API, how to divide up the different processing the module should do, etc. In short, how to approach API design, which is very different in Haskell than the practice that most OO disciples are familiar with. IMHO, stating that type classes act like interfaces in argument types, but not return types, is too complicated a relationship to be useful to somebody's first introduction to FP. Better to avoid the whole issue completely. John Lato Alexey Romanov wrote: > > Well, they act like interfaces in argument types, just not variable or > return types. > > Yours, Alexey Romanov >

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Congrats. to wxHaskell
by Don Stewart 14 Oct '08

14 Oct '08

I just want to congratulate the wxHaskell team on their new release. With GHC 6.10 RC, cabal 1.6, and cabal-install 0.6, I was able to simply, cabal install wxcore wx And it all worked. Well done! -- Don

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hackage warning: Exposed modules use unallocated top-level names: ... what am I doing wrong?
by Thomas Hartman 14 Oct '08

14 Oct '08

However the package generated the following warnings: * A 'license-file' is not specified. * Exposed modules use unallocated top-level names: Algorithms everything fetchable with darcs get http://code.haskell.org/external-sort external-sort is on hackage and seems to work (you can run demo.hs after cabal installing) but I'd still like to understand if this warning is something I should pay attention to anyway.

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Interesting new user perspective
by Don Stewart 14 Oct '08

14 Oct '08

http://sandersn.com/blog/index.php?title=two_weeks_of_haskell Has some good thoughts on what tripped him up in his first two weeks of Haskell programming. Anything people want to work on there? :-) Worth a read if you're thinking about how to make Haskell yet more accessible. -- Don

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Associated Types and several Classes
by Martin Hofmann 14 Oct '08

14 Oct '08

> {-# OPTIONS_GHC -fglasgow-exts #-} > module Test where >import qualified Data.Set as S Hi. I try to model the following: Hypotheses are build up from Rules, which itself are made of the type Rule. Because I may change the implementation later, I want to use type classes, which define the signature of my functions I will use in other modules. >class CRule r >class (CRule (CRulesRule r) ) => CRules r where > type CRulesRule r >class (CRule (CHypoRule h), CRules (CHypoRules h) ) => CHypo h where > type CHypoRules h > type CHypoRule h > hypo :: > CHypoRules h -> > CHypoRule h -> > h -- | Rule >data Rule = Rule Int deriving(Eq,Ord) >instance CRule Rule -- | Rules >type Rules = S.Set Rule >instance CRules (S.Set Rule) where > type CRulesRule (S.Set Rule) = Rule -- | Hypothese >data Hypo = Hypo { open :: Rules > , closed :: Rules > } >instance CHypo Hypo where > type CHypoRules Hypo = Rules > type CHypoRule Hypo = Rule > hypo ro rc = Hypo { open=ro, closed=(S.singleton rc)} So far so good. But why does now the last of the following lines not type check? It says: Couldn't match expected type `CHypoRules h' against inferred type `S.Set Rule' Couldn't match expected type `CHypoRule h' against inferred type `Rule' >rule1 = Rule 1 >rule2 = Rule 2 >rule3 = Rule 3 >rules = S.fromList [rule1,rule2,rule3] >ahypo = hypo rules rule1 Shouldn't be (CHypoRules Hypo) be associated with Rules and similar (CHypoRule) with Rule? Thanks a lot, Martin -- --------------------------------------------------------------- Dipl.-Wirtsch.Inf. (E.M.B.Sc.) Martin Hofmann Cognitive Systems Group Faculty Information Systems and Applied Computer Science University of Bamberg http://www.cogsys.wiai.uni-bamberg.de/members/hofmann http://www.inductive-programming.org

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couple of questions on monads
by Daryoush Mehrtash 13 Oct '08

13 Oct '08

Question 1: Why are there lazy and strict modules of some monads? (e.g. Control.monad.State) Question 2: If I define a new monad (say xyz), does it have to be as control.monad.xyz module? daryoush

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Query regarding Type classes
by Arun Suresh 13 Oct '08

13 Oct '08

Hello folks, Im kinda new to haskell. Ive only been fiddling around with it for bout 3 - 4 weeks now. And for the life of me... i cant seem to figure out why this doesnt work : class Foo a where fooFunc :: a -> Int data FooData = FData instance Foo FooData where fooFunc _ = 10 class Bar a where barFunc :: (Foo b) => a -> b -> Int data BarData = BData instance Bar BarData where barFunc _ FData = 20 When I compile I get this : Couldn't match expected type `b' against inferred type `FooData' `b' is a rigid type variable bound by the type signature for `barFunc' at Sample.hs:16:20 In the pattern: FData In the definition of `barFunc': barFunc _ FData = 20 In the definition for method `barFunc' Think Im missing something really big... Could somebody kindly help me out here... Regards Arun Could so

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Re: [Haskell-cafe] Still stacking monad transformers
by Tillmann Rendel 13 Oct '08

13 Oct '08

Dino Morelli wrote: > I was wishing I could do this: > > let foo = str `or-if-empty` default > > > If it was a Maybe, this works with mplus: > > (Just "foo") `mplus` (Just "bar") == Just "foo" > Nothing `mplus` (Just "bar") == Just "bar" > > > But not so much for list, mplus just ain't defined that way, instead > doing concatination: > > "foo" `mplus` "bar" == "foobar" > "" `mplus` "bar" == "bar" The difference between Maybe-style MonadPlus and List-style MonadPlus is discussed on http://www.haskell.org/haskellwiki/MonadPlus_reform_proposal. With that proposal, you could use morelse with both Maybe and lists. Personally, I would like to call these beasts <|> (for Maybe-style mplus) and <+> (for List-style mplus). One could even have a <| b = a <|> pure b a <+ b = a <+> pure b a +> b = pure a <+> b in the style of <* and *>. Note that I assume Applicative as a superclass of Monad, while I'm talking about a better world anyway. That said, I would consider using something like the <+> sketched here for Strings as a hack, because I do not see Strings as Chars under the List functor, even if they technically may be exactly that. Since "abc" does not represent multiple results, I don't think "" should represent failure. Instead, I would wrap optional strings in Maybe, and then use fromMaybe to handle the default values: fromMaybe "bar" (Just "foo") = "foo" fromMaybe "bar" Nothing = "bar" Now, with the operators sketched above, fromMaybe = flip (<|), so one could use Just "foo" <| "bar" = "foo" Nothing <| "bar" = "bar" which seems to closely correspond to what you want. Tillmann

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