- Beginners - Haskell.org

A simple function
by Ezequiel Hernan Di Giorgi 30 Aug '12

30 Aug '12

Hallo liebe Leute. Hola gente. I'm starting with Haskell and i have some problems: *intercalate :: [t] -> [t] -> [t]* *intercalate [a,b] [c,d] = [a,c,b,d]* *intercalate (_) (_) = error "JOJO u cant do this"* Are there any form to restrict the parmaters that only allow to call the function *intercalate* with two arrays of two elements, in compilation time? Cause i cant write* intercalete :: [t,t] -> [t,t]->[t,t,t,t]*. Sorry for my bad english. I'm Argentinean. Thanks!

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associative arrays
by Christopher Howard 30 Aug '12

30 Aug '12

What is typically used in Haskell circles to provide associative array functionality? (I.e., key-value type arrays.) I'm not really looking for efficiency so much as interface convenience. Obviously I could code it myself based on lists or something, but I don't want to reinvent the wheel. -- frigidcode.com indicium.us

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complex typeclass/constraint question
by Dennis Raddle 30 Aug '12

30 Aug '12

{- I'm writing code to do some combinatorial construction via test-and-evaluate algorithms. And I want to write a very generic algorithm. The idea is that the item being constructed has a current state, which is represented by a data type of typeclass EvalState. And the algorithm will consider a set of new elements to add to the object being constructed, represented by a data type of typeclass ElemSet. Here are the class definitions: -} class ElemSet a where elemSetNumElements :: a -> Int class EvalState s where isCompleteState :: s -> Bool newElemSet :: ElemSet a => s -> a integrateElem :: ElemSet a => a -> Int -> s -> s -- given an elem set, and an index of the elem to choose , compute a score scoreElem :: ElemSet a => a -> Int -> s -> EvalScore type EvalScore = Float type RandomnessChooser = [(EvalScore,Int)] -> ErrorRand Int {- here's my generic search algorithm written using the typeclasses the problem I'm having relates to the fact that I need to call 'integrateElem' which is a function in typeclass EvalState, and one of the arguments has the constraint of being of typeclass ElemSet, but I don't know how to put that constraint in. The specific error is "Ambigous type variable 'a' in the constraint: 'ElemSet a' arising from a use of 'integrateElem'" -} search :: (EvalState s) => RandomnessChooser -> s -> ErrorRand ssearch chooser state = do if isCompleteState state then return state else do let elemSet = newElemSet state n = elemSetNumElements elemSet scores = map g [0..n-1] g i = (scoreElem elemSet i state,i) chosenElemIdx <- chooser scores let newState = integrateElem elemSet chosenElemIdx state search chooser newState

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How to load multiple source files in GHCi?
by Yang 29 Aug '12

29 Aug '12

Hi, I have a few source files in current directory to load as modules of my library in GHCi. How should I do with :load command? I tried ghci>:load a.hs where module a is defined and exported for testing purpose, but failed for a.hs:1:1: File name does not match module name: Saw: `Main' Expected: `a' Failed, modules loaded: none. However, I do not need Main to run for testing. I just want to test exported functions. -- Best Regards Yang Zhao

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IO vars
by Corentin Dupont 29 Aug '12

29 Aug '12

Hi all, there is something very basic that it seems escaped me. For example with the following program f and g have type IO () and I can thread a value between the two using a file. Can I do the exact same (not changing the types of f and g) without a file?* f,g :: IO () **f = withFile "toto" WriteMode (flip hPutStr "toto") g = withFile "toto" ReadMode hGetLine >>= putStrLn main = f >> g* Thanks and cheers, Corentin

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Re: [Haskell-beginners] IO vars & ACID
by Corentin Dupont 29 Aug '12

29 Aug '12

To give you more context, I have a soft using ACID state to store and retrieve values. For example I have a function like that: newPlayer :: PlayerName -> IO () newPlayer name = update $ AddPlayer name It seems that ACID uses a global state or a file to store the events... I'd like to suppress the ACID state for the moment, how should I do? Change all the* IO () *types to *StateT Game IO ()*? Indead the ACID state is started with: c <- localStartSystemState (Proxy :: Proxy Game) Best, C On Wed, Aug 29, 2012 at 12:00 PM, Corentin Dupont <corentin.dupont(a)gmail.com > wrote: > Thanks Eugene and Ozgur. > I also looked on the side of IORef, but it doesn't looked to be a good > solution since we have to use unsafePerformIO. > > I have a big program to modify, and I want to pass some new data to > existing functions of type IO(). I'd like to avoid changing all the > function's types down the chain... What is the best way to do that? > > > > > On Wed, Aug 29, 2012 at 11:43 AM, Ozgur Akgun <ozgurakgun(a)gmail.com>wrote: > >> On 29 August 2012 10:21, Corentin Dupont <corentin.dupont(a)gmail.com>wrote: >> >>> *f,g :: IO () >>> f = withFile "toto" WriteMode (flip hPutStr "42") >>> g = withFile "toto" ReadMode hGetLine >>= (\s -> putStrLn $ "Answer:" ++ >>> s) >>> main = f >> g* >>> >>> Is it possible to do the same without files (the types must remain IO())? >>> >> >> One can use an IORef to get a similar effect. >> >> import Data.IORef >> import System.IO.Unsafe >> >> {-# NOINLINE toto #-} >> toto :: IORef String >> toto = unsafePerformIO (newIORef "") >> >> f,g :: IO () >> f = writeIORef toto "42" >> g = readIORef toto >>= (\s -> putStrLn $ "Answer:" ++ s) >> >> main = f >> g >> >> HTH, >> Ozgur >> > >

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Are tuples really needed?
by Carlos J. G. Duarte 29 Aug '12

29 Aug '12

Sorry if this question is too insane, but I was wondering if tuples are really needed in Haskell. I mean, could tuples be generally replaced by variables unroll (f x y z) and explicit data types, or are there some things only possible to do via tuples? Thx in advance (and sorry if this looks silly).

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"system" call uses a different shell, or does not pick up the whole environment
by Hong Yang 28 Aug '12

28 Aug '12

Hi, I am trying to mimic mapM() at shell command line. I define the interface as "mapm cmd2 cmd1," so cmd2 will be run for each of the cmd1 results. "$_" can be used inside cmd2 to represent the current cmd1 result. For example, the command mapm 'cp -pr $_ destination_dir/$_' ls copies everything under the current directory to the destination directory. The code is as follows: -- module Main where import System.Environment ( getArgs ) import System.Exit import System.IO import System.Process import Text.Regex import Text.Regex.Posix main = do hs_argv <- getArgs if length hs_argv /= 2 then putStrLn "wrong arguments!" >> exitFailure else do let [cmd2, cmd1] = hs_argv (_, hOut, hErr, _) <- runInteractiveCommand cmd1 err <- hGetContents hErr hClose hErr if null err then do out <- hGetContents hOut mapM (f cmd2) (lines out) else putStr err >> exitFailure f :: String -> String -> IO ExitCode f cmd2 item = system cmd2' where cmd2' = if cmd2 =~ "\\$\\_"::Bool then subRegex (mkRegex "\\$\\_") cmd2 item else cmd2 -- It works, except one issue that is bothering me. If I issue mapm 'lt $_' ls, I get a bunch of /bin/sh: lt: command not found, while I expect it act the same as mapm 'ls -Alrt --color=auto $_' ls, because "lt" is aliased to "ls -Alrt --color=auto." Notice "/bin/sh" above. My shell is actually tcsh. All the aliases are in ~/.cshrc. I tried replacing "system cmd2'" with system ("source ~/.cshrc; " ++ cmd2') and system ("tcsh -c " ++ "'source ~/.cshrc; " ++ cmd2' ++ "'"), but they did not solve the problem. Can someone please help me? Thanks, Hong

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Errors involving rigid skolem types
by Matthew Moppett 27 Aug '12

27 Aug '12

The following code is intended as a first step towards creating a cyclical enumerable type, such that: (e.g.) [Cyc Friday .. Cyc Tuesday] would yield [Friday, Saturday, Sunday, Monday, Tuesday] module Cycle where newtype Cyc a = Cyc a deriving (Eq, Ord, Bounded, Show, Read) fromCyc :: Cyc a -> a fromCyc (Cyc a) = a instance (Enum a, Bounded a) => Enum (Cyc a) where fromEnum = fromEnum . fromCyc toEnum n = Cyc x where (x, max) = (x', maxBound) :: (a, a) x' = toEnum $ n `mod` ((fromEnum max) - 1) This yields a kind of error message that I've often bashed my head against in other code I've written, without ever really understanding what the problem is exactly: Couldn't match type `a0' with `a1' because type variable `a1' would escape its scope This (rigid, skolem) type variable is bound by an expression type signature: (a1, a1) The following variables have types that mention a0 x' :: a0 (bound at Cycle.hs:12:15) In the expression: (x', maxBound) :: (a, a) In a pattern binding: (x, max) = (x', maxBound) :: (a, a) In an equation for `toEnum': toEnum n = Cyc x where (x, max) = (x', maxBound) :: (a, a) x' = toEnum $ n `mod` ((fromEnum max) - 1) The problem comes up when I'm trying to give hints to the compiler about the type that a particular expression should have. My questions are: (1) what exactly is going on here, and (2) is there any general technique for specifying types in situations like this that gets around this problem?

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IO / State / Do Syntax
by Christopher Howard 27 Aug '12

27 Aug '12

So, I'm still working with my resource tracker idea -- keeping images stored and organized inside a resource tracker structure that I can pass around to functions that need them. Let's say I want a function like so: code: -------- initResources :: IO ResourceTracker -------- The idea being that initResources loads the image files, stores them in the resource tracker (RT) structure, and returns the RT. I know I can do something like so: code: -------- initResources = do pic1 <- loadImage "someimage.png" -- IO function pic2 <- loadImage "someimage2.png" -- ... and so on ... let rt = emptyResourceTracker in let rt' = storeImage rt "pic1keyword" pic1 in let rt'' = storeImage rt' "pic2keyword" pic2 in -- ... and so on, until finally: ... rt'''''''''''''''''''''''''' -------- Obviously, all the let statements and apostrophes are undesirable. So, presumably what I need is to being using the State monad, yes? (I must confess I have only a vague understanding of the State monad, even after reading several tutorials.) But in my initResources function, how do I mix use of the IO and State do syntax, and still get what I want? I think this has something to do with Monad transformers, but I'm even less clear on how those work. -- frigidcode.com indicium.us

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