- Beginners - Haskell.org

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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Why does sequence (map print [1, 2, 3, 4, 5]) produce [(), (), (), (), ()] at the end?
by Olumide 03 Feb '16

03 Feb '16

Hello list, The question says it all. BTW, I'm studying LYH and I'm on the chapter on IO. The book offers and explanation but its not very clear -- to me at least. Thanks, - Olumide

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foldr on infinite list to decide prime number
by Chul-Woong Yang 03 Feb '16

03 Feb '16

Hi, all. While I know that foldr can be used on infinite list to generate infinite list, I'm having difficulty in understaind following code: isPrime n = n > 1 && -- from haskell wiki foldr (\p r -> p*p > n || ((n `rem` p) /= 0 && r)) True primes primes = 2 : filter isPrime [3,5..] primes is a infinite list of prime numbers, and isPrime does foldr to get a boolean value. What causes foldr to terminate folding? Any helps will be deeply appreciated. Thank you. Chul-Woong

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foldr with short circuit and accumulator
by Chul-Woong Yang 03 Feb '16

03 Feb '16

Hi, all. Can it be possible to do fold with short circuit and accumulator both? For example, can we find an element which is same value to adjacent one? findAdjacent [1,2..n, n, n+1, n+2.......] => n \__very long__/ Though there can be many ways to do it, Can we do it with fold[r|l]? I'll be happy to receive any comments. Chul-Woong

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Strange type error with hsc2hs and cabal
by PICCA Frederic-Emmanuel 30 Jan '16

30 Jan '16

Hello, I am still playing with FFI. Now I create a real project with cabal end build a library and an executable for my project. ---hkl.cabal--- name: hkl version: 0.1.0.0 executable ghkl main-is: ghkl.hs build-depends: base >=4.8 && <4.9 , containers >=0.5 && <0.6 , filepath , gtk >=0.13 && <0.14 , hkl hs-source-dirs: src default-language: Haskell2010 pkgconfig-depends: hkl build-tools: hsc2hs library exposed-modules: Hkl, Hkl.Types, Hkl.C, Hkl.DArray other-extensions: CPP, ForeignFunctionInterface, EmptyDataDecls, TypeFamilies, FlexibleInstances, FlexibleContexts, RecordWildCards build-depends: base >=4.8 && <4.9, containers >=0.5 && <0.6 hs-source-dirs: src build-tools: hsc2hs default-language: Haskell2010 pkgconfig-depends: hkl --- --- but when I compile my project I get this errormessage which is really strange. $ cabal build Package has never been configured. Configuring with default flags. If this fails, please run configure manually. Warning: The package list for 'hackage.haskell.org' is 17.1 days old. Run 'cabal update' to get the latest list of available packages. Resolving dependencies... Configuring hkl-0.1.0.0... Warning: 'data-files: ../../gui/ghkl.ui' is a relative path outside of the source tree. This will not work when generating a tarball with 'sdist'. Building hkl-0.1.0.0... Preprocessing library hkl-0.1.0.0... [1 of 4] Compiling Hkl.Types ( src/Hkl/Types.hs, dist/build/Hkl/Types.o ) [2 of 4] Compiling Hkl.C ( src/Hkl/C.hs, dist/build/Hkl/C.o ) [3 of 4] Compiling Hkl.DArray ( dist/build/Hkl/DArray.hs, dist/build/Hkl/DArray.o ) [4 of 4] Compiling Hkl ( src/Hkl.hs, dist/build/Hkl.o ) In-place registering hkl-0.1.0.0... Preprocessing executable 'ghkl' for hkl-0.1.0.0... [1 of 4] Compiling Hkl.Types ( src/Hkl/Types.hs, dist/build/ghkl/ghkl-tmp/Hkl/Types.o ) [2 of 4] Compiling Hkl.C ( src/Hkl/C.hs, dist/build/ghkl/ghkl-tmp/Hkl/C.o ) [3 of 4] Compiling Hkl ( src/Hkl.hs, dist/build/ghkl/ghkl-tmp/Hkl.o ) [4 of 4] Compiling Main ( src/ghkl.hs, dist/build/ghkl/ghkl-tmp/Main.o ) src/ghkl.hs:44:39: Couldn't match expected type ‘Hkl.Types.EngineList’ with actual type ‘Hkl.EngineList’ NB: ‘Hkl.Types.EngineList’ is defined in ‘Hkl.Types’ in package ‘hkl-0.1.0.0’ ‘Hkl.EngineList’ is defined at src/Hkl/Types.hs:22:1-74 In the first argument of ‘Hkl.engineListEnginesGet’, namely ‘engines’ In a stmt of a 'do' block: tmp <- Hkl.engineListEnginesGet engines if I add a other-modules to ghkl an remove the hkl dependecy it work great. so what is wrong with this project. PS: the DArray module is indeed the one using hsc2hs :) Thansk for your help Frederic

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Is it possible to use a <> constructor in analogy to [] ?
by martin 29 Jan '16

29 Jan '16

Hello all, I've written my own Read and Show instances for a type, which is essentially data Crust a = Open [a] | Closed [a] "Show" does something like *Main> Closed [1,2,3] <1,2,3> *Main> Open [1,2,3] <1,2,3..> And Read faithully parses each stings and turns it into a Crust again. Problem is: when I just type <1,2,3..> in GHCi I get a "parse error on input ‘<’". However when I spell out the custructor as above it parses it okay. Is it possible at all to write my own <> constructor, or something like it=

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