February 2016 - Beginners

GLUT bindings on Mac OS X 10.11
by C Gosch 15 Feb '16

15 Feb '16

Hello Haskellers, I have used Haskell for a bit on Linux and Windows, and now started using OS X 10.11. I wanted to run a program using the GLUT bindings, and it complains about "user error (unknown GLUT entry glutSetOption)” when I set certain options with ($=), for example actionOnWindowClose. A bit of googling did not gain me much other than that glutSetOption is available in freeGlut, which is not the standard GLUT on Mac OS X, it seems. Does anyone have an idea how to get around this? Can I replace OS X’s GLUT with FreeGLUT? Thanks a bundle, Christian

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Random Numbers with the State Monad
by Thomas Jakway 13 Feb '16

13 Feb '16

I'm having a bad time using the State monad to generate random numbers without carrying around a lot of StdGens manually. I have this snippet in the IO monad: ... IO stuff ... gen <- getStdGen let (numPlayers, numMatches) = (evalState genRandVariables gen) :: (Integer, Integer) ... More IO stuff ... where maxRandPlayers = 10 :: Integer minRandMatches = 10 :: Integer maxRandMatches = 100 :: Integer genRandVariables = (do np <- randomR (1, maxRandPlayers) --minimum 1 other player nm <- randomR (minRandMatches, maxRandMatches) return (np, nm)) :: State StdGen (Integer, Integer) I get this error message: test/Jakway/Blackjack/Tests/IntegrationTests/MatchTests.hs:53:23: Couldn't match expected type ‘StateT StdGen Data.Functor.Identity.Identity Integer’ with actual type ‘g0 -> (Integer, g0)’ Probable cause: ‘randomR’ is applied to too few arguments In a stmt of a 'do' block: np <- randomR (1, maxRandPlayers) In the expression: (do { np <- randomR (1, maxRandPlayers); nm <- randomR (minRandMatches, maxRandMatches); return (np, nm) }) :: State StdGen (Integer, Integer) test/Jakway/Blackjack/Tests/IntegrationTests/MatchTests.hs:54:23: Couldn't match expected type ‘StateT StdGen Data.Functor.Identity.Identity Integer’ with actual type ‘g1 -> (Integer, g1)’ Probable cause: ‘randomR’ is applied to too few arguments In a stmt of a 'do' block: nm <- randomR (minRandMatches, maxRandMatches) In the expression: (do { np <- randomR (1, maxRandPlayers); nm <- randomR (minRandMatches, maxRandMatches); return (np, nm) }) :: State StdGen (Integer, Integer) What's really baffling to me is I feel like this is how it *should* look--that the whole point of the state monad is to *not* have to explicitly pass the StdGen to randomR. What am I doing wrong?

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Unequal types constraint
by Imants Cekusins 12 Feb '16

12 Feb '16

class C a b instance C Int a -- when a /= Int instance C a a -- when a == a how would you do this without IncoherentInstances?

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while loop
by PICCA Frederic-Emmanuel 11 Feb '16

11 Feb '16

Hello, I am playing with FFI and I need to extract from a C API a list of pointers. I have two methods available c_get_first_item :: Ptr List -> IO (Ptr Item) c_get_next_item :: Ptr List -> Ptr Item -> IO (Ptr Item) I would like to obtain a [Ptr Item] I try to used whileM but I did not find how to inject the first item in the loop. whileM (return . ( /= nullPtr)) (c_get_next_item list item) the c_get_next_item return a nullPtr when there is no remaining item. what is the haskell way in order to extract a list of pointer using these C methods ? thanks for your help Frederic

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Understanding wrap function in Connor McBride's Faking It
by Ben Rogalski 08 Feb '16

08 Feb '16

Hello, I am working on a Vector class, heavily based on the paper "Faking It: Simulating Dependent Types in Haskell" by Connor McBride. Here is a stripped down version of my code: --- {-# LANGUAGE RankNTypes #-} import Control.Applicative import Data.Functor infixr 5 :. data Cons u t = (:.) t (u t) deriving (Show) data Nil t = Nil deriving (Show) class (Applicative v, Functor v) => Vector v where wrap :: (forall u. Vector u => s -> f (u t) -> f (Cons u t)) -> f (Nil t) -> v s -> f (v t) instance (Vector u) => Vector (Cons u) where wrap f n (x:.xs) = f x (wrap f n xs) instance Vector Nil where wrap _ n Nil = n instance (Applicative u) => Applicative (Cons u) where pure x = x:.(pure x) (f:.fs) <*> (x:.xs) = (f x):.(fs <*> xs) instance Applicative Nil where pure _ = Nil _ <*> _ = Nil instance (Functor u) => Functor (Cons u) where fmap f (x:.xs) = (f x):.(fmap f xs) instance Functor Nil where fmap _ Nil = Nil transpose :: (Vector w, Vector u) => w (u t) -> u (w t) transpose v = wrap (\x xs -> (:.) <$> x <*> xs) (pure Nil) v --- I am having trouble understanding the type signature of 'wrap'. This is how I interpret it: (function that takes an 's' and container of 'u t' and returns a container of 'Cons u t') -> (container of 'Nil') -> (a vector of 's') -> (a container with the same type as the container of Nil, but containing v t instead) What doesn't make sense to me is how the signature seems to imply that the structure of the container in the return type is the same as the structure of the container holding the Nils, but in 'transpose' the container holding Nil seems to be infinite. Can someone help me to understand what is going on here? Thanks, Ben

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Returning Maybe Float -> Float -> Float
by Olivier Duhart 08 Feb '16

08 Feb '16

hello, I am starting with Haskell and trying some little exercices on my own. I successfully implemented a reverse polish notation evaluator and I want to improve it a little using *Maybe* All i want is to implement a function that can returned available functions according to its string name i.e return (+) when gived "+" I also want to return Nothing if the operation is not available (not implemented yet). So my function should Maybe return a (float -> float -> float) My current implementation is operation :: String -> Maybe Float -> Float -> Float operation op | op == "+" = Just (+) | op == "-" = Just (-) | op == "*" = Just (*) | op == "/" = Just (/) | otherwise = Nothing but it failed to compile with the following error : rpn.hs:64:18: Couldn't match expected type `Maybe Float -> Float -> Float' with actual type `Maybe a0' In the expression: Nothing In an equation for `operation': operation op | op == "+" = Just (+) | op == "-" = Just (-) | op == "*" = Just (*) | op == "/" = Just (/) | otherwise = Nothing I don't understand the error :( Do I have to explicitly type the Nothing return ? Could you guide me to a solution or explain me what I am doing wrong, please ? Thanks in advance Olivier

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Got "Non type-variable argument in the constraint" error for a simple function
by wizard 08 Feb '16

08 Feb '16

Dear all, I just started to learn Haskell with learnyouahaskell.com and at the very beginning, I met a strange issue with following simple function: -- why does work with "toZero 10" but not for "toZero -10"?toZero :: (Integral t) => t -> [t]toZero 0 = [0]toZero x = if x > 0 then x : toZero (x - 1) else x : toZero (x + 1) This function works as expected for positive arguments, e.g., "toZero 10" gives me [10,9,8,7,6,5,4,3,2,1,0]. However, GHCI will raise following error if I give it a negative argument, e.g., "toZero -10": *Main> toZero -10 <interactive>:12:1: Non type-variable argument in the constraint: Num (t -> [t]) (Use FlexibleContexts to permit this) When checking that ‘it’ has the inferred type it :: forall t. (Integral t, Num (t -> [t])) => t -> [t] This seems strange to me as 10 and -10 has exactly the same type "Num a => a". I've done with chapter 1~10 of learnyouahaskell.com but still has no idea on why this error. Anybody can help to explain this? Thanks a lot. Regards Zhiyi Xie

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How to convert ST.Trans Array to ByteString efficiently?
by John Ky 06 Feb '16

06 Feb '16

Anyone know of an efficient way to convert a ST.Trans Array to a ByteString? Or is this it? https://github.com/haskell-works/conduit-succinct-json/commit/1427f10ce67de… Cheers, -John

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generating object file for ARM
by Fabien R 06 Feb '16

06 Feb '16

Hello, I would like to link my Haskell module with existing object files for armv7-m. I use ghc 7.4.1 on debian/amd64. This command seems incorrect: ghc -pgmcarm-none-eabi-gcc -pgmParm-none-eabi-cpp -pgmaarm-none-eabi-as -pgmlarm-none-eabi-ld -keep-tmp-files HaskellModule.hs A lot of errors appeared: [1 of 1] Compiling HaskellModule ( HaskellModule.hs, HaskellModule.o ) /tmp/ghc13654_0/ghc13654_0.s: Assembler messages: /tmp/ghc13654_0/ghc13654_0.s:5:0: Error: unrecognized symbol type "" /tmp/ghc13654_0/ghc13654_0.s:17:0: Error: unrecognized symbol type "" /tmp/ghc13654_0/ghc13654_0.s:29:0: Error: bad instruction `leaq -40(%rbp),%rax' /tmp/ghc13654_0/ghc13654_0.s:30:0: Error: bad instruction `cmpq %r15,%rax' Any hints ? -- Fabien

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tuple space
by Mike Houghton 06 Feb '16

06 Feb '16

Hi, I’m thinking about how to write a Tuple Space (TS) in Haskell. A tuple can have many fields of different types, in pseudo code something like T = (1, ”A string”, 3.4) i.e. an int, string and double. How can this (and the many variations) be done in Haskell? (In Java it would be a list of Object) Thanks Mike

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