- Beginners - Haskell.org

Parsing an integer
by Tim Baumgartner 22 Nov '10

22 Nov '10

Hi folks, since I always get great answers here, I have another question. I'm currently parsing a (positive) integer using data Token = Number Int -- ... number :: Parser Token number = many1 digit ↠ (read >>> Number >>> return) -- = many1 digit ↠ return∘Number∘read But I wonder what an advanced haskeller would code instead. Particularly, I'd like to get rid of the 'return'. Thanks in advance Tim

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Ignoring the result of a monadic computation
by Tim Baumgartner 21 Nov '10

21 Nov '10

Hi, while learning about monads, I had something like do line <- getLine something putStrLn line and I wondered if I could write it in one line, without naming of parameters. I finally came up with getLine >>= ignore something >>= putStrLn using ignore :: Monad m => m a -> b -> m b ignore m a = m >> return a I'm satisfied with this solution but searching hoogle I didn't find a standard function for my ignore. Am I missing something? Cheers, Tim -- GRATIS! Movie-FLAT mit über 300 Videos. Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome

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Chain computations
by Tim Baumgartner 20 Nov '10

20 Nov '10

Hi, I really like fooling around with operators. In elementary school, we always had to make chain computations, e.g. teacher: 2 teacher: +1 teacher: *2 teacher: divided by 3 Again, I didn't find the following operator on hoogle (very similar to ($)): (->>) :: a -> (a -> b) -> b a ->> f = f a Using it, we can nicely write down the above computation: 2 ->> (+1) ->> (*2) ->> (`div` 3) Cheers Tim

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Rekommendation for select i GUI
by Anders Persson 19 Nov '10

19 Nov '10

Hi! i have installed Haskell Platform om a Windows Vista, I will also run Haskell on a OS/X machine. I have looked at a good GUI tool, tested wxHaskell, and hs2GTK but no one seems easy to be install, lot off errors, etc... Have i missed something and could i get some comments om a GUI toolkit to select. Best regards Anders -- med vänliga hälsningar Anders ______________________________________________ Anders Persson anders.u.persson(a)gmail.com

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Re: [Haskell-cafe] About "Fun with type functions" example
by Bastian Erdnüß 19 Nov '10

19 Nov '10

On Nov 19, 2010, at 1:07, Daniel Peebles wrote: > The best you can do with fromInt is something like Int -> (forall n. (Nat n) > => n -> r) -> r, since the type isn't known at compile time. Or you put it in the type class as fromInt :: Int -> n and do in Zero: fromInt _ = Zero and in the other: fromInt _ = Succ $ fromInt (undefined :: n). Shouldn't that work, too? > On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly <arnaud.oqube(a)gmail.com>wrote: > >> Thanks a lot, that works perfectly fine! >> Did not know this one... >> BTW, I would be interested in the fromInt too. >> >> Arnaud >> >> On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink <hesselink(a)gmail.com> >> wrote: >>> On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly <arnaud.oqube(a)gmail.com> >> wrote: >>>> Hello, >>>> I am trying to understand and use the Nat n type defined in the >>>> aforementioned article. Unfortunately, the given code does not compile >>>> properly: >>> >>> [snip] >>> >>>> instance (Nat n) => Nat (Succ n) where >>>> toInt _ = 1 + toInt (undefined :: n) >>> >>> [snip] >>> >>>> And here is the error: >>>> >>>> Naturals.hs:16:18: >>>> Ambiguous type variable `n' in the constraint: >>>> `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39 >>>> Probable fix: add a type signature that fixes these type variable(s) >>> >>> You need to turn on the ScopedTypeVariables extension (using {-# >>> LANGUAGE ScopedTypeVariables #-} at the top of your file, or >>> -XScopedTypeVariables at the command line). Otherwise, the 'n' in the >>> class declaration and in the function definition are different, and >>> you want them to be the same 'n'. >>> >>> Erik >>> >> _______________________________________________ >> 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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Package Documentation (Haddock)
by Bastian Erdnüß 18 Nov '10

18 Nov '10

Hi, I found the Haddock documentation to the packages that came with the Haskell Platform locally on my machine. 1) It's a little bit uncomfortable to always pick the right one by hand. Is there a more comfortable way like doing e.g. 'haddock Data.List' (that's not working) to direct to the documentation of a certain module? 2) I cannot find the documentation to the packages I post installed with cabal. I tried to build them using e.g. 'cabal haddock Yampa' but that's not working. How do I do it right? Thanks for your help in advance, Bastian

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arrow type signature question
by MH 18 Nov '10

18 Nov '10

I am looking at signatures for Arrow and Composable classes and I cannot understand some of them. Could you please explain me the following: Let's take for example the following: class FunAble h => FunDble h where resultFun :: (h b -> h b') -> (h (a->b) -> h (a->b')) class FunAble h where secondFun :: (h b -> h b') -> (h (a,b) -> h (a,b')) -- for 'second' in the signatures: resultFun :: (h b -> h b') -> (h (a->b) -> h (a->b')) secondFun :: (h b -> h b') -> (h (a,b) -> h (a,b')) if (h b -> h b') is the input of these functions where does 'a' comes from in the output? Thanks,

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Cost center annotations - where/let difference
by Daniel Seidel 18 Nov '10

18 Nov '10

Hi, I was doing some heap profiles and ran into the following issue. Consider the program fib.hs: module Main where fib i | i == 0 || i == 1 = 1 | i > 1 = fib (i-1) + fib (i-2) | otherwise = 0 testWhere = {-# SCC "Where" #-} res where res = fib 35 testLet = {-# SCC "Let" #-} let res = fib 35 in res If I choose main = print testLet compile with: ghc --make -prof -auto-all -caf-all -O2 fib.hs and run: ./fib +RTS -hc -hCLet -L60 the fib.hp file will contain entries as expected. If I choose main = print testWhere compile with: ghc --make -prof -auto-all -caf-all -O2 fib.hs and run: ./fib +RTS -hc -hCWhere -L60 the fib.hp file will contain the timestamps, but entries for measured heap consumption. Can anyone please tell me, if this is the expected behavior? I'm not very experienced in benchmarking and was a bit irritated by that difference. Cheers, Daniel.

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Random animals
by Amy de Buitléir 17 Nov '10

17 Nov '10

I have this function: -- | Choose an element at random from a list and return the element and its index -- Ex: runState (randomListSelection ['a', 'b', 'c', 'd', 'e']) (mkStdGen 1) randomListSelection :: (RandomGen s) => [a] -> State s (Int, a) randomListSelection xs = do i <- State $ randomR (0,length xs - 1) return (i, xs !! i) I want to repeatedly select random elements from a list. I know the code below is totally wrong, but it's the closest I've gotten so far. When I run it in ghci, I get the SAME random element each time, until I reload the module. I guess that's because g is always the same. I'm still struggling with monads, and would appreciate any advice on how to fix this. chooseCreatur = do g <- getStdGen return (evalState (randomListSelection ["cat", "dog", "lion", "mouse"]) g) FWIW, I'll be calling it from a loop within the IO monad. The real list will be growing and shrinking in size.

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Heterogeneous Lists
by John Smith 17 Nov '10

17 Nov '10

My program needs a list such as [IO 3, IO "f", IO 'z'] :: Show s => [IO s]. Is there any way to achieve this without wrapping all the values in existentially quantified types?

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