- Beginners - Haskell.org

latest Haskell Platform build fails
by Fábio Roberto Teodoro 05 Aug '15

05 Aug '15

---------- Mensagem encaminhada ---------- De: "Fábio Roberto Teodoro" <fr.teodoro(a)gmail.com> Data: 04/08/2015 11:04 Assunto: latest Haskell Platform build fails Para: <beginner(a)haskell.org> One way I found to get rid of the "package(s) with this id already exist" problem was by doing a "ghc-pkg --force unregister <package>"

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An unsigned int class.
by derek riemer 01 Aug '15

01 Aug '15

Is there anything in haskell that represents an unsigned Int class? namely, something bounded below at 0? If not, is it possible to define an Int in this way to add the bounded constraint (I can't immagine trying to define Int like this, there has got to be a better way that I haven't quite understood in my study of haskell). data UnsignedInt = 0 | 1 | 2 | (...) for a long time doing that ... deriving (Show, Eq, Ord, Bounded, Read) Thanks, Derek

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How to define ord manually.
by derek riemer 31 Jul '15

31 Jul '15

Hi, I was playing around with haskell, and decided to implement a RockPaperScissors type. data RPS= Rock | Paper | Scissors Then I manually derived show for fun. instance Show RPS where show Rock = "rock" show Paper = "paper" show Scissors = "scissors" But, I have a problem when defining ord. How do I look in the sourcecode for ord? In ghci I did prelude> :i Ord <snip> Ord (a, b, c, d, e, f, g, h, i, j) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i) => Ord (a, b, c, d, e, f, g, h, i) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h) => Ord (a, b, c, d, e, f, g, h) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g) => Ord (a, b, c, d, e, f, g) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e, Ord f) => Ord (a, b, c, d, e, f) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d, Ord e) => Ord (a, b, c, d, e) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c, Ord d) => Ord (a, b, c, d) -- Defined in `GHC.Classes' instance (Ord a, Ord b, Ord c) => Ord (a, b, c) -- Defined in `GHC.Classes' instance (Ord a, Ord b) => Ord (a, b) -- Defined in `GHC.Classes' instance Ord () -- Defined in `GHC.Classes' Prelude> Where is ghc.classes? I looked under the ghc directory and didn't find a classes.hs file. I tried defining it as such. instance Ord RPS where ord Scissors `compare` Rock = LT ord Paper `compare` Scissors = LT ord Rock `compare` Paper = LT But this fails with verry little errors, and I really don't know how I define an instance of Ord for this. I want rock to be less than paper, but greater than scissors, scissors to be less than rock but greater than paper, and paper to be greater than rock but less than scissors. Kind of geeky, but it's one way of learning. Also, What is the best way to write useful programs, One example I tried is to make a simple guess what I am guessing game, but I couldn't make heads or tails of the IO typeclasses, or the System.Random module. How can I possibly get random numbers and do things on them, and does this mean main must be imperative with a do statement? Thanks, Derek

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Traversing generic trees
by Ali Baharev 24 Jul '15

24 Jul '15

Dear All, I have started learning Haskell recently, and I try to reimplement interesting algorithms that are challenging to do in other languages. I would like to ask you for your kind help with a problem that I am stuck with. A generic tree is given in the form of nested sequences (nested lists or nested tuples) and I would like to traverse it in (1) depth-first order, (2) lazily and (3) without copying the data into some other datastructure first. I test the algorithm by pretty printing the tree. My goal is learning, so Data.Tree is not an option (although I did look into its implementation). Here is my first attempt: data Tree a = Leaf a | Node [Tree a] tree = Node [Leaf 'a', Node [Leaf 'b', Leaf 'c', Node []], Leaf 'd'] toString :: (Show a) => Tree a -> String toString (Leaf leaf) = " " ++ show leaf ++ " " toString (Node nodes) = " [ " ++ concatMap toString nodes ++ " ] " main = print $ toString tree It (more or less) satisfies the above requirements. The pretty printing is not very pretty but the part that I am really not happy with is the lot of code duplication in giving the tree. I would like to enter it as: tree = ['a', ['b', 'c', []], 'd'] which of course won't work. Is there a (not too obscure) way to help the compiler so that it understands that 'a' means Leaf 'a', etc? I have also tried tree = ('a', ('b', 'c', ()), 'd') but then I do not know how to destructure the nested tuples. Any help is greatly appreciated. Ali

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(->) reader/environment
by Martin Vlk 23 Jul '15

23 Jul '15

Hi, this one is a bit of a mystery to me. I am working on solutions to cis194/NICTA Haskell exercises and among others I am asked to work with (->). I understand it refers to the reader/environment design patterns, but I am stumped as for what exactly it is. It doesn't seem to be an ordinary type, I know it is used as type constructor in type declarations, so that suggests it's a function, but somehow special. Looking in Hoogle it seems to be Haskell keyword - e.g. not an ordinary function. So what is it - type constructor, a keyword, how is it related to the reader/environment pattern? >From GHCI's :info (->) I get: data (->) a b -- Defined in ‘ghc-prim-0.4.0.0:GHC.Prim’ instance P.Monad ((->) r) -- Defined in ‘GHC.Base’ instance P.Functor ((->) r) -- Defined in ‘GHC.Base’ instance P.Applicative ((->) a) -- Defined in ‘GHC.Base’ instance P.Monoid b => P.Monoid (a -> b) -- Defined in ‘GHC.Base’ Cheers Martin

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Re: [Haskell-beginners] Traversing generic trees
by Ali Baharev 23 Jul '15

23 Jul '15

Dear Elliot, The question is whether there is a (not too obscure) way to eliminate the code duplication, or how the code duplication can be avoided using nested tuples instead. Please try to answer the question. Ali On Wed, Jul 22, 2015 at 11:51 PM, elliot <elliot(a)citrusbytes.net> wrote: > In general you don't produce large trees (or any data structure) by hand. It's slightly frustrating to test of course (as you've discovered) but for the most part you'll only need to produce small structures. > > -- > e: elliot(a)citrusbytes.net > p: +44 751 070 5158 > ---- Original Message ---- > From: "Ali Baharev" > To: beginners(a)haskell.org > Sent: Wed, Jul 22, 2015, 10:24 PM > Subject: [Haskell-beginners] Traversing generic trees > Dear All, > > I have started learning Haskell recently, and I try to reimplement > interesting algorithms that are challenging to do in other languages. > I would like to ask you for your kind help with a problem that I am > stuck with. > > A generic tree is given in the form of nested sequences (nested lists > or nested tuples) and I would like to traverse it in (1) depth-first > order, (2) lazily and (3) without copying the data into some other > datastructure first. I test the algorithm by pretty printing the tree. > My goal is learning, so Data.Tree is not an option (although I did > look into its implementation). > > Here is my first attempt: > > data Tree a = Leaf a | Node [Tree a] > > tree = Node [Leaf 'a', Node [Leaf 'b', Leaf 'c', Node []], Leaf 'd'] > > toString :: (Show a) => Tree a -> String > toString (Leaf leaf) = " " ++ show leaf ++ " " > toString (Node nodes) = " [ " ++ concatMap toString nodes ++ " ] " > > main = print $ toString tree > > It (more or less) satisfies the above requirements. The pretty > printing is not very pretty but the part that I am really not happy > with is the lot of code duplication in giving the tree. I would like > to enter it as: > > tree = ['a', ['b', 'c', []], 'd'] > > which of course won't work. > > Is there a (not too obscure) way to help the compiler so that it > understands that 'a' means Leaf 'a', etc? > > I have also tried > > tree = ('a', ('b', 'c', ()), 'd') > > but then I do not know how to destructure the nested tuples. > > Any help is greatly appreciated. > > Ali > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org (mailto:Beginners@haskell.org) > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners (http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners)

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seq vs deepseq
by Ryan Warner 22 Jul '15

22 Jul '15

I'm trying to grok Haskell's laziness. In this example, why does the seq not force youTyped to wait for s to be fully evaluated, before returning a result to putStrLn? -------------------- import System.IO (hSetBuffering,stdin,BufferMode(..)) youTyped :: String -> String youTyped s = s `seq` "you typed: " ++ s main = do hSetBuffering stdin NoBuffering s <- getContents let l = map youTyped $ lines $ takeWhile ( /= '\04' ) s mapM_ putStrLn ("Start Typing (Ctrl-D to exit):":l) -------------------- The output looks like: -------------------- $ runhaskell seqLazy.hs Start Typing (Ctrl-D to exit): Hyou typed: Heelllloo wwoorrlldd fyou typed: faaiill ^D -------------------- Changing seq, to deepseq does the trick though. -------------------- import System.IO (hSetBuffering,stdin,BufferMode(..)) import Control.DeepSeq youTyped :: String -> String youTyped s = s `deepseq` "you typed: " ++ s main = do hSetBuffering stdin NoBuffering s <- getContents let l = map youTyped $ lines $ takeWhile ( /= '\04' ) s mapM_ putStrLn ("Start Typing (Ctrl-D to exit):":l) -------------------- Output: $ runhaskell deepSeqLazy.hs Start Typing (Ctrl-D to exit): Hello world you typed: Hello world success you typed: success ^D When does it make sense to use seq then?

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Re: [Haskell-beginners] Cabal dependencies are per target or global?
by Dimitri DeFigueiredo 19 Jul '15

19 Jul '15

I guess my example was incomplete. I can make GHC run. Here's a modified version where I do have the ghc-options line. I can build an executable for the second target (myPersonalMain) by commenting the "build-depends: section" for the first target (agent). My .cabal file with (commented lines) is: ------------------------------------------------ name: agent version: 0.1.0.0 build-type: Simple cabal-version: >=1.20 ------------------------------------------------ executable agent main-is: Main.hs hs-source-dirs: ./src -- build-depends: base >=4.6 && <4.7 -- , unordered-containers >= 0.2.3.0 -- , unix >= 2.6.0.1 -- , process >= 1.1.0.2 -- , stm >= 2.4.2 -- Base language which the package is written in. default-language: Haskell2010 ---------------------------------------------- executable myPersonalMain main-is: Mpm.hs hs-source-dirs: ./src ghc-options: -main-is Mpm build-depends: base >=4.4 default-language: Haskell2010 ---------------------------------------------- The contents of Mpm.hs are: module Mpm where main = putStrLn "Hi!" But if I remove the comments in the .cabal file above, I get this: Dis-machine:cabal-tests dimitri$ cabal build myPersonalMain ./agent.cabal has been changed. Re-configuring with most recently used options. If this fails, please run configure manually. Resolving dependencies... Configuring agent-0.1.0.0... cabal: At least the following dependencies are missing: process >=1.1.0.2, stm >=2.4.2, unix >=2.6.0.1, unordered-containers >=0.2.3.0 Is this a bug? Or am I missing something? Cheers, Dimitri On 17/07/15 15:10, Imants Cekusins wrote: > this could shed some light: > > http://stackoverflow.com/questions/14238729/producing-multiple-executables-… > > answer #2 was marked as answered > > ghc-options: -O2 -threaded -with-rtsopts=-N -main-is FirstExecutable

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Re: [Haskell-beginners] basic Functor, Applicative and Monad instances
by amindfv＠gmail.com 18 Jul '15

18 Jul '15

Right, you'll want to say eg fmap :: (a -> b) -> Val a -> Val b Tom El Jul 17, 2015, a las 12:23, Imants Cekusins <imantc(a)gmail.com> escribió: >> InstanceSigs > > Thank you Tom. > > this sig does not work though: > fmap ::(a -> b) -> f a -> f b

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basic Functor, Applicative and Monad instances
by Imants Cekusins 18 Jul '15

18 Jul '15

Here are complete working snippets with Functor, Applicative and Monad instances. This is my first attempt to write own instances rather than use ready ones. Here we go: module Part.Monad where import Control.Applicative {-# ANN module ("HLint: ignore Use let"::String) #-} {- Val' { val = b1 } can also be written as Val' b1 -} data Val a = Val' { val::a } deriving Show data Weather = Cold | Warm deriving Show instance Functor Val where -- (a -> b) -> f a -> f b fmap ab fa = let a1 = val fa b1 = ab a1 in Val' { val = b1 } instance Applicative Val where -- a -> f a pure a = Val' { val = a } -- f (a -> b) -> f a -> f b (<*>) fab fa = let ab1 = val fab a1 = val fa b1 = ab1 a1 in Val' { val = b1 } instance Monad Val where -- a -> m a return a = Val' { val = a } -- m a -> (a -> m b) -> m b (>>=) ma amb = let a1 = val ma in amb a1 -- pure and return in this example are interchangeable main::Int -> IO() main i = do -- do: Val as monad v1 <- pure Val' { val = i } -- pure: applicative v2 <- return $ over20 <$> v1 -- <$> : functor print v2 v3 <- return $ Val' weather <*> v2 -- <*> : applicative print v3 over20::Int-> Bool over20 i | i > 20 = True | otherwise = False weather::Bool-> Weather weather False = Cold weather True = Warm

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