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return (user defined type) confusion?
by umptious 12 Apr '12

12 Apr '12

I'm bashing together a simple command line geometry thing that let's you create shapes, move them around them, delete and edit them, test for intersections, etc. The motivation is to get a grip on Haskell IO and how it interacts with user defined datatypes. I'm using GHC. The problem, which I've tried to show as simply as possible in the abstracted code below, is that something seems to go horribly wrong with un-boxing (is this the correct terminology?) user defined type X's from IO(X)'s. In the example below: - enterCircle makes a Shape using the Circle cstr and boxes it in a return. And the type checker is happy with the function. - But when the Shape is un-boxed in mainLoop then Haskell's type system (or GHC's?) has lost track of its Shapeness. Instead it sees it as a Pt2->Float. Which makes some sort of sense, but is useless (or at least needs ugly hacks to overcome) if you want to put your Circle in a List of Shapes. - Even worse then the loss of Shapeness is that Haskell is letting me type enterCircle as returning an IO(Shape) - indeed that's what :t gives me in GCHI - but then disagrees with this type in mainLoop! This isn't what I was expecting from the vaunted Haskell type system; can someone explain this contradiction? So. Does return fail to wrap up type info about user defined types in general? Is this correct Haskell behaviour? Or have I done something wrong? If I haven't done something wrong, is there a standard idiom for overcoming this? I suppose I could use a show to convert the Circle to a string and then a read to convert it back in mainLoop's where, but this will mean uglier branching than I planned (the idea was to have a Map of fns that return shapes and just one branch in mainLoop to handle every kind of shape.) And it seems to make something of a nonsense of Haskell's type system as the entire Shape hierarchy will be reduced to IO(String) for these operations. import Data.Map as Map data Pt2 = Pt2 {x::Float, y::Float} deriving (Show, Read) data Shape = Circle {origin::Pt2, radius::Float} | Square {origin::Pt2, side ::Float} | Rect {origin::Pt2, other ::Pt2} deriving (Show, Read) enterCircle :: IO(Shape) enterCircle = return (r) -- needs to rtn io (shape) because will take input here where r = Circle{origin=Pt2{x=1,y=2}, radius=4} mainLoop :: [Shape] -> IO (String) mainLoop shapes = do putStrLn "?" cmd <- getLine if cmd=="q" then return ("done") else do y <- enterCircle mainLoop shapes' where shapes' = y : shapes main = mainLoop []

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A "show" error
by bahadýr altan 12 Apr '12

12 Apr '12

Hi, I'm trying to process on a tree with this function : f (Branch x (Branch y y1 y2) (Branch z z1 z2)) = (x,y,y1,y2,z,z1,z2) and my tree declaration is this : data Tree = Empty | Branch Integer Tree Tree deriving (Show) And I'm getting this error : Main> f (Branch 12 (Branch 15 Empty Empty) (Branch 28 Empty Empty)) ERROR - Cannot find "show" function for: *** Expression : f (Branch 12 (Branch 15 Empty Empty) (Branch 28 Empty Empty)) *** Of type : (Integer,Integer,Tree,Tree,Integer,Tree,Tree) I'll be happy if you help me to get rid of this error..

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netwire and "dynamic" list of objects
by Nathan Hüsken 11 Apr '12

11 Apr '12

Hey, I have a simple SDL based graphic demo, which I am trying (for educational purposes) to rewrite utilizing netwire. The demo basically consists of a State which contains a list of object states. data ObjectState = ... data State = State [ObjectState] Then there is a function updateState which updates all objects, removing some and adding some new ones. updateState :: State -> State updateState (State oss) = State (catMaybes $ map updateObject $ newObjects ++ oss) where updateObject :: ObjectState -> Maybe ObjectState updateObject = ... newObjects :: [ObjectState] newObjects = ... OK, now I wrote a wire: type MyWire = Wire.Wire () (Kleisli IO) wire :: GameState -> MyWire () GameState wire initGS = proc _ -> do rec gs' <- Wire.delay initGS -< gs gs <- arr updateState -< gs' returnA -< gs Works wonderfully :). But I avoided rewriting "updateState" in wire format. But I have no Idea how to do that. How do I handle a dynamic list of objects in a wire? Thanks! Nathan

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Converting a particular warning into a fatal error.
by Michael Litchard 11 Apr '12

11 Apr '12

Is there a way to tell ghc to turn only `incomplete-pattern` warnings into fatal errors, as opposed to `-Werror`?

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Wrong type inferred for polymorphic function in a case alternative
by Ramin Honary 11 Apr '12

11 Apr '12

Hi everyone, I am using GHC 7.0.4. I boiled-down my problem into a simple program which I attached to this message. I don't know how to get the program to compile. I need to pass arbitrary class member functions as a parameter to a very large, complex, polymorphic function called "apply", and have this class member function applied to abitrary fields of data constructors of a very large, complex data type. What I expect to happen is this: For each case alternative, GHC will infer the type of the polymorphic function from the matched pattern in the case alternative, and use that inference to select the correct instance function for that type. So for case alternative (TA x, TA y) -> TA (fn x y), the correct instance for "fn" is selected based on the type of "x" and "y" which are determined by the pattern match, and by the type of "TA". If that case alternative does not match, then it should try the next alternative (TB x, TB y) -> TB (fn x y), and since "fn" is polymorphic, it will still be able to select the correct instance for the type given by the pattern match. So I can pass "f1" or "f2" or "f3" to "apply" as the "fn" parameter, and have the correct instance of "f1" or "f2" or "f3" be selected depending on where in the "case" expression "fn" is used. What is actually happening is: GHC arbitrarily selects one of the case alternatives (usually the last case alternative, sometimes the first) and decides that from now on this one type is the type for "fn" for every case alternative. So for every other case alternative, it decides that "fn" is of the wrong type and rejects the program, rather than trying to infer a different type for "fn" specific to this case alternative. Said another way, the type inference algorithm is being too strict, and I would like it to be more lazy, computing the type once per each case alternative, rather than computing the type only once for the entire case statement. Is there a language option for that? I have tried using language options, like -XNoMonoPatBinds -XNoMonomorphismRestriction -XMonoLocalBinds -XPolymorphicComponents -XImpredicativeTypes, but none of those help. My current solution is to use Template Haskell and make the "apply" function a splice, so the whole case statement is copied to every place it is used (which takes a very, very long for it time to comple). Is there a way to do this without Template Haskell? Can I get GHC to behave as I expected it to? Thanks, Ramin

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Type of function with constant pattern
by j.romildo＠gmail.com 11 Apr '12

11 Apr '12

Hello. Given the following function definitions f 0 = True g False = True ghc infers the following types for the functions: f :: (Eq a, Num a) => a -> Bool g :: Bool -> Bool Why f has "Eq a" in the context in ts type, and g does not? As both are defined using a constant pattern, I expected none of them should require the type of the argument to be instance of Eq. Romildo

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PutStrLn
by Pierre Michallet 11 Apr '12

11 Apr '12

I have the impression that putStrLn adjusts its intercharacter spacing according to the length of the string to print. Can someone tell me : 1 Is it right 2 if yes , is there a function which does not apply that kind of proportionality Thank you for your help Pierre Michallet

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self recursive lambda
by sarfraz 09 Apr '12

09 Apr '12

Hi, Can't seem to be able to debug the following: import System.IO nHead :: Int -> String -> String nHead n str = if (length str) < n then str else if n ==0 then str else (\x n str -> if n == 0 -- this lambda does not work then str else x (n - 1) (tail str)) main = interact (nHead 5) I see where the problem is. But surely it is possible to make a recursive lambda. (I want to use a lambda, i know how to do otherwise) -- Sarfraz K.

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install wxHaskell on osx 10.7 64bit
by Miguel Negrao 09 Apr '12

09 Apr '12

Hi list, I’m very Haskell newbie, and I’m trying to install wxHaskell in order to try out reactive-banana-wx. What I did so far: 1 ) Installed lastest Haskell platform 2) Installed wxWidgets from source, latest dev version from http://sourceforge.net/projects/wxwindows/files/2.9.3/wxWidgets-2.9.3.tar.b… compiled with ../configure --with-osx_cocoa 3 ) darcs get http://darcsden.com/kowey/wxhaskell-osx64-dave built this one But then doing 'cabal install reactive-banana-wx’ will start installing wxcore-0.12.1.7 when the step 3) already installed wxcore-0.13.1 and that is the repository that should have the wxHaskell which works in osx lion. So is it possible to tell reactive-banana-wx to use wxcore-0.13.1 instead of wxcore-0.12.1.7 ? best, Miguel Negrão

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Knights exercies in <Learn your good haskell>
by Haisheng Wu 06 Apr '12

06 Apr '12

Hi there, There is one trivial exercise in chapter 12 <List Monad> saying 1. As an exercise, you can change this function so that when you can reach one position from the other, it tells you which moves to take. 2. how to modify this function so that we also pass it the number of moves to take instead of that number being hard-coded. I have one answer (find the URL below) but I'm not happy with it. https://github.com/freizl/dive-into-haskell/blob/master/monad/knight-queue.… Do you have better ideas? thanks. -Haisheng

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