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HaL4: Local Haskell meeting, Halle/Saale, Germany, June 12
by Henning Thielemann 29 Apr '09

29 Apr '09

Hi all, we like to remind the potential participants of our Local Haskell meeting HaL4 to submit proposals for talks. See the recent announcement in German: http://www.haskell.org/pipermail/haskell/2009-April/021220.html --------------------------------------------- HaL4 : Haskell - Tutorial + Workshop + Party am Freitag, dem 12. Juni 2009, in Halle/Saale --------------------------------------------- Das traditionsreiche HaL-Treffen bietet eine gute Mischung von Haskell-bezogenen Themen aus Forschung, Anwendung und Lehre mit vielen Möglichkeiten zu Diskussion und Unterhaltung bei der anschließenden Party. Der Workshop wird in diesem Jahr ergänzt durch Tutorien für Haskell-Ein- und Umsteiger. Diesmal findet das Treffen in Halle/Saale im Institut für Informatik der Martin-Luther-Universität Halle-Wittenberg statt. Wir planen: 10 - 13 Uhr: Tutorien 15 - 19 Uhr: Fachvorträge 19 - 22 Uhr: Grillparty Wir freuen uns auf rege Teilnahme sowie spannende Vorträge mit heißen Diskussionen und bitten um Vortragsanmeldungen bis zum 8. Mai. Gedacht sind 4 mal je 30 min Vortrag + 30 min Diskussion. Weitere Informationen auf http://www.iba-cg.de/hal4.html Beste Gruesse Henning Thielemann

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chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Hi, My Prelude docs must be out of date because chr and ord don't seem to be there. How do I access these functions? Michael =============== [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> chr 65 <interactive>:1:0: Not in scope: `chr' Prelude> ord 'A' <interactive>:1:0: Not in scope: `ord' Prelude>

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Re: [Haskell-cafe] chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Yep, that fixed it.. Thanks, Brandon. Michael --- On Tue, 4/28/09, Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> wrote: From: Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "Tim Wawrzynczak" <inforichland(a)gmail.com>, "Lennart Augustsson" <lennart.augustsson(a)gmail.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Tuesday, April 28, 2009, 11:44 PM On Apr 28, 2009, at 23:32 , michael rice wrote:Thank guys, Now what am I misunderstanding in the code below? lst = [('A',65),('B',66),('C',67),('D',68)] You didn't give a type for lst, so it defaulted to [(Char,Integer)]. This is a manifestation of the Monomorphism Restriction, invoked because lst doesn't take any arguments: it forces lst to have a definite type, so defaulting is used to fix the numeric part to Integer. If you add a type specification for lst, things will work as you expect. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.comsystem administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.eduelectrical and computer engineering, carnegie mellon university KF8NH

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Typing efficient folds
by Keith Battocchi 29 Apr '09

29 Apr '09

I'm trying to write some code to do folds on nested datatypes as in http://web.comlab.ox.ac.uk/people/Jeremy.Gibbons/publications/efolds.pdf but running into trouble getting things to typecheck. Given the types data Nest a = Nil | Cons(a, (Nest (Pair a))) type Pair a = (a,a) and the following function to map over pairs: pair f (a,b) = (f a, f b) the paper indicates that an efficient fold over a nest is defined as follows efold :: forall n m b. (forall a. n a) -> (forall a . (m a, n (Pair a)) -> n a) -> (forall a. Pair (m a) -> m (Pair a)) -> (forall l z. (l b -> m (z b)) -> Nest (l b) -> n (z b)) efold e f g h Nil = e efold e f g h (Cons(x, xs)) = f(h x, efold e f g (g . pair h) xs) However, when I try to compile this, I get the error "Couldn't match expected type `l' against inferred type `z'". I'm new to Haskell so I'm probably missing something obvious, but this code looks to me like it ought to work. Any thoughts on what I'm doing wrong? Thanks, Keith

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Compiling on windows - again
by Gü?nther Schmidt 29 Apr '09

29 Apr '09

Hi guys, I'm sorry, I asked this before ... What is the flag you have to pass during ghc --make .... in order to produce an exe on Windows that doesn't open a DOS window. Günther

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Question about implementing an off-side rule in Parsec 2
by Bas van Gijzel 29 Apr '09

29 Apr '09

Hello everyone, I'm doing a bachelor project focused on comparing parsers. One of the parser libraries I'm using is Parsec (2) and I'm going to implement a very small subset of haskell with it, with as most important feature the off-side rule (indentation based parsing) used in function definitions and possibly in where clauses. But I'm still a bit stuck on how to implement this cleanly. I tried to search for some examples on blogs but I haven't found anything yet. As far as I can see the way to go would be using getState and updateState methods defined in Parsec.Prim and to use the methods in Parsec.Pos to compare the difference in indendation for tokens. But I haven't completely wrapped my head around any state monad yet and I don't understand Parsec enough yet to see how to use the methods Parsec.Pos and state easily. Some examples or pointers to something to read would really be helpful. Thanks in advance, Bas van Gijzel

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Re: [Haskell-cafe] chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Thank guys, Now what am I misunderstanding in the code below? I would think that *Main> comb (Just 65) foo and *Main> comb (lookup 'A' lst) foo would return the same result Just 'A' Michael ===========Haskell code============= import Data.Char lst = [('A',65),('B',66),('C',67),('D',68)] -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x foo :: Int -> Maybe Char foo n = Just (chr n) ================Ghci=========== [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :load test1 [1 of 1] Compiling Main ( test1.hs, interpreted ) Ok, modules loaded: Main. *Main> comb (Just 65) foo Just 'A' *Main> comb Nothing foo Nothing *Main> lookup 'A' lst Just 65 *Main> (lookup 'A' lst) Just 65 *Main> comb (lookup 'A' lst) foo <interactive>:1:22: Couldn't match expected type `Integer' against inferred type `Int' In the second argument of `comb', namely `foo' In the expression: comb (lookup 'A' lst) foo In the definition of `it': it = comb (lookup 'A' lst) foo *Main> --- On Tue, 4/28/09, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: From: Lennart Augustsson <lennart.augustsson(a)gmail.com> Subject: Re: [Haskell-cafe] chr/ord? To: "Tim Wawrzynczak" <inforichland(a)gmail.com> Cc: "michael rice" <nowgate(a)yahoo.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Tuesday, April 28, 2009, 10:53 PM You can also use the more general toEnum and fromEnum. -- Lennart (iPhone) On Apr 29, 2009, at 2:13, Tim Wawrzynczak <inforichland(a)gmail.com> wrote: Michael, those functions are not in the Prelude, they're in Data.Char. On Tue, Apr 28, 2009 at 8:08 PM, michael rice <nowgate(a)yahoo.com> wrote: Hi, My Prelude docs must be out of date because chr and ord don't seem to be there. How do I access these functions? Michael =============== [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> chr 65 <interactive>:1:0: Not in scope: `chr' Prelude> ord 'A' <interactive>:1:0: Not in scope: `ord' Prelude> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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name for monad-like structure?
by Michael Vanier 29 Apr '09

29 Apr '09

I've stumbled upon a structure that is like a weaker version of a monad, one that supports return and >> but not >>=. Has anyone seen this before, and if so, does it have a standard name? Mike

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breaking too long lines
by Christian Maeder 28 Apr '09

28 Apr '09

Hi, according the several style guides, lines shouldn't be too long (longer than 78 characters). http://www.cs.caltech.edu/courses/cs11/material/haskell/misc/haskell_style_… http://www.haskell.org/haskellwiki/Programming_guidelines However, I miss hints how to break lines best. Therefore I make some suggestions here and ask for comments. If a "one-liner" does not fit on one line I break the line after "=" rather than breaking the following infix- or prefix-expression. (I break in the same way after "->" or "<-" in case- or do- expressions) (Some people move "=" to the next line, but I only suggest this for proper infix operators, below.) If a "do" or a short "case ... of" or "let ... in" fits, I leave it at the end of the previous line behind "=" ("->" or "<-"). A line should be broken after "do" or "of" (from "case") in order to allow insertions without breaking the layout, provided there is preceding text at all. The following is fine, because there's no text before "do") c x = do y <-... z ... But this is fine, too, if not better: c x = do y <- ... z ... It's not necessary to put "do" (or "let") on a separate line (and care about the indentation of that keyword): c x = do y <-... z ... Because layout may easily break (if b is renamed) and the layout block starts already too far to the right, this is really bad: c b x = unless b $ do y <- ... z ... In many cases a "do" can be moved to the end of the previous line: c b x = unless b $ do y <- ... z What applies to "do" could be applied to "let" as well (and vice versa), but: f x = let y = x + x z = y + y in z does not look as nice as: f x = let y = x + x z = y + y in z And also a "let" (without "in") within a "do" should not be broken after "let". A long infix-expression should be broken _before_ an infix symbol to better indicate it's continuation: f ++ g ++ h f . g $ h x (One should also put spaces around infix operators and should not put unnecessary brackets around prefix applications.) Surely a long prefix expression can be broken anywhere, but I try to break expressions on the top-level and not within too deeply nested sub-expressions: f arg1 arg2 arg3 (longArg4 sa1 sa2 sa3 sa4) arg5 arg6 arg7 After a line break (following "=", "do", "<-", or "->") I try to stay as far to the left as layout permits (using 2 spaces as minimal indentation). I don't care if "=", <-", or "->" of one block line up (which surely is difficult for the top-level module block) and I may put parts of the rhs below parts of the lhs: let longPattern = case bla of Nothing -> "don't know" Just b -> "a longer expression" vN = ... The standard breaking of if-then-else (within "do") is: if ... then ... else ... but I think the following variations are also fine: ... <- if ... then ... else ... if ... then ... else ... By chance I rarely use guards and "where", therefore I give no examples for those expressions. Somewhat tricky I've found are 2 do-expressions connected by an infix-operator within an outer do-expression: do c <- letter do d <- digit return [c, d] <|> do u <- char '_' return [c, u] The line containing "<|> do" is critical to indentation. If "<|> do" is moved (2 columns) to the left it'll be wrong (by chance c will not be in scope). If "<|> do" is moved (3 columns) to the right it'll mean something different, namely parsing and returning a letter and a digit _or_ parsing a letter, a digit, and an underscore, but only return the letter and the underscore. In order to more clearly put the infix operator into the middle I've tried to insert one more space: do c <- letter do d <- digit return [c, d] <|> do u <- char '_' return [c, u] Also the indentation of data types is an issue, because I think, code shouldn't be indented due to long (constructor) names. I've nothing against long names, but one shouldn't try to put blocks to the right of them: data TName = LongConstructorName { selector1 :: C1 , ... } | LongSecondConstructor .... deriving ... Cheers Christian

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Is curryfication practical with dynamic scope?
by Loup Vaillant 28 Apr '09

28 Apr '09

Hi, can this code (\a -> \b -> a) 1 2 eval to normal form in a strict language with dynamic scope? Thanks, Loup PS: some context: I am currently implementing a toy language. The AST is this one: data whnf = -- Weak Head Normal Form | Unit | Int of int | Bool of bool | Prim of (whnf -> whnf) -- curried | Fun of string * expr -- curried data expr = | Whnf of whnf | Var of string | App of expr * expr | If of expr * expr * expr Dynamic typing, dynamic scope, and, as you can see, functions have to be curried. I plan to implement lexical scope.

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