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  • Warning: newtype `CInt' is used in an FFI declaration,

    - by vivian
    When building gtk2hs-buildtools with ghc 7.4.2, I get the following warning: c2hs/toplevel/C2HSConfig.hs:110:1: Warning: newtype `CInt' is used in an FFI declaration, but its constructor is not in scope. This will become an error in GHC 7.6.1. When checking declaration: foreign import ccall safe "static bitfield_direction" bitfield_direction :: CInt I get similar warnings with FFI calls, even though I have import Foreign.C.Types(CInt). What is the correct way of getting rid of this warning?

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  • Defining your own Ord for a data type

    - by mvid
    I am attempting to make some data structures to solve a graph puzzle. I am trying to define an edge's comparison criteria, but I am not sure how. So far: data Edge = Edge (Set String) Bool How do I tell let the compiler know that I want edges to be declared equal if they have identical sets of strings, and not have equality have anything to do with the boolean value?

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  • Modified map2 (without truncation of lists) in F# - how to do it idiomatically?

    - by Maciej Piechotka
    I'd like to rewrite such function into F#: zipWith' :: (a -> b -> c) -> (a -> c) -> (b -> c) -> [a] -> [b] -> [c] zipWith' _ _ h [] bs = h `map` bs zipWith' _ g _ as [] = g `map` as zipWith' f g h (a:as) (b:bs) = f a b:zipWith f g h as bs My first attempt was: let inline private map2' (xs : seq<'T>) (ys : seq<'U>) (f : 'T -> 'U -> 'S) (g : 'T -> 'S) (h : 'U -> 'S) = let xenum = xs.GetEnumerator() let yenum = ys.GetEnumerator() seq { let rec rest (zenum : IEnumerator<'A>) (i : 'A -> 'S) = seq { yield i(zenum.Current) if zenum.MoveNext() then yield! (rest zenum i) else zenum.Dispose() } let rec merge () = seq { if xenum.MoveNext() then if yenum.MoveNext() then yield (f xenum.Current yenum.Current); yield! (merge ()) else yenum.Dispose(); yield! (rest xenum g) else xenum.Dispose() if yenum.MoveNext() then yield! (rest yenum h) else yenum.Dispose() } yield! (merge ()) } However it can hardly be considered idiomatic. I heard about LazyList but I cannot find it anywhere.

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  • Use 'let' in 'if' expression

    - by demas
    I need a function that works like this: foo :: Integer -> Integer -> [Integer] foo a b = do let result = [] let Coord x y = boo a b if x > 0 let result = result ++ [3] if y > 0 let result = result ++ [5] if x < a let result = result ++ [7] if y < b let result = result ++ [9] result I can not use the guards because the result can have more then one element. But as I see I can not use 'let' in the 'if' expression: all_possible_combinations.hs:41:14: parse error on input `let' How can I check multiple expressions and add new elements in the list? I search not only imperative solution, but the functional one.

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  • Why would an image (the Mandelbrot) be skewed and wrap around?

    - by Sean D
    So I just wrote a little snippet to generate the Mandelbrot fractal and imagine my surprise when it came out all ugly and skewed (as you can see at the bottom). I'd appreciate a point in the direction of why this would even happen. It's a learning experience and I'm not looking for anyone to do it for me, but I'm kinda at a dead end debugging it. The offending generation code is: module Mandelbrot where import Complex import Image main = writeFile "mb.ppm" $ imageMB 1000 mandelbrotPixel x y = mb (x:+y) (0:+0) 0 mb c x iter | magnitude x > 2 = iter | iter >= 255 = 255 | otherwise = mb c (c+q^2) (iter+1) where q = x --Mandelbrot --q = (abs.realPart $ x) :+ (abs.imagPart $ x) --Burning Ship argandPlane x0 x1 y0 y1 width height = [(x,y)| y<-[y1,(y1-dy)..y0], --traverse from x<-[x0,(x0+dx)..x1]] --top-left to bottom-right where dx = (x1 - x0)/width dy = (y1 - y0)/height drawPicture :: (a->b->c)->(c->Colour)->[(a,b)]->Image drawPicture function colourFunction plane = map (colourFunction.uncurry function) plane imageMB s = createPPM s s $ drawPicture mandelbrotPixel (\x->[x,x,x]) $ argandPlane (-1.8) (-1.7) (0.02) 0.055 s' s' where s' = fromIntegral s And the image code (which I'm fairly confident in) is: module Image where type Colour = [Int] type Image = [Colour] createPPM :: Int -> Int -> Image -> String createPPM w h i = concat ["P3 ", show w, " ", show h, " 255\n", unlines.map (unwords.map show) $ i]

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  • Step by Step / Deep explain: The Power of (Co)Yoneda (preferably in scala) through Coroutines

    - by Mzk
    some background code /** FunctorStr: ? F[-]. (? A B. (A -> B) -> F[A] -> F[B]) */ trait FunctorStr[F[_]] { self => def map[A, B](f: A => B): F[A] => F[B] } trait Yoneda[F[_], A] { yo => def apply[B](f: A => B): F[B] def run: F[A] = yo(x => x) def map[B](f: A => B): Yoneda[F, B] = new Yoneda[F, B] { def apply[X](g: B => X) = yo(f andThen g) } } object Yoneda { implicit def yonedafunctor[F[_]]: FunctorStr[({ type l[x] = Yoneda[F, x] })#l] = new FunctorStr[({ type l[x] = Yoneda[F, x] })#l] { def map[A, B](f: A => B): Yoneda[F, A] => Yoneda[F, B] = _ map f } def apply[F[_]: FunctorStr, X](x: F[X]): Yoneda[F, X] = new Yoneda[F, X] { def apply[Y](f: X => Y) = Functor[F].map(f) apply x } } trait Coyoneda[F[_], A] { co => type I def fi: F[I] def k: I => A final def map[B](f: A => B): Coyoneda.Aux[F, B, I] = Coyoneda(fi)(f compose k) } object Coyoneda { type Aux[F[_], A, B] = Coyoneda[F, A] { type I = B } def apply[F[_], B, A](x: F[B])(f: B => A): Aux[F, A, B] = new Coyoneda[F, A] { type I = B val fi = x val k = f } implicit def coyonedaFunctor[F[_]]: FunctorStr[({ type l[x] = Coyoneda[F, x] })#l] = new CoyonedaFunctor[F] {} trait CoyonedaFunctor[F[_]] extends FunctorStr[({type l[x] = Coyoneda[F, x]})#l] { override def map[A, B](f: A => B): Coyoneda[F, A] => Coyoneda[F, B] = x => apply(x.fi)(f compose x.k) } def liftCoyoneda[T[_], A](x: T[A]): Coyoneda[T, A] = apply(x)(a => a) } Now I thought I understood yoneda and coyoneda a bit just from the types – i.e. that they quantify / abstract over map fixed in some type constructor F and some type a, to any type B returning F[B] or (Co)Yoneda[F, B]. Thus providing map fusion for free (? is this kind of like a cut rule for map ?). But I see that Coyoneda is a functor for any type constructor F regardless of F being a Functor, and that I don't fully grasp. Now I'm in a situation where I'm trying to define a Coroutine type, (I'm looking at https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/coroutines-for-streaming/part-2-coroutines for the types to get started with) case class Coroutine[S[_], M[_], R](resume: M[CoroutineState[S, M, R]]) sealed trait CoroutineState[S[_], M[_], R] object CoroutineState { case class Run[S[_], M[_], R](x: S[Coroutine[S, M, R]]) extends CoroutineState[S, M, R] case class Done[R](x: R) extends CoroutineState[Nothing, Nothing, R] class CoroutineStateFunctor[S[_], M[_]](F: FunctorStr[S]) extends FunctorStr[({ type l[x] = CoroutineState[S, M, x]})#l] { override def map[A, B](f : A => B) : CoroutineState[S, M, A] => CoroutineState[S, M, B] = { ??? } } } and I think that if I understood Coyoneda better I could leverage it to make S & M type constructors functors way easy, plus I see Coyoneda potentially playing a role in defining recursion schemes as the functor requirement is pervasive. So how could I use coyoneda to make type constructors functors like for example coroutine state? or something like a Pause functor ?

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  • mapping list of different types implementing same function?

    - by sisif
    I want to apply a function to every element in a list (map) but the elements may have different types but all implement the same function (here "putOut") like an interface. However I cannot create a list of this "interface" type (here "Outputable"). How do I map a list of different types implementing the same function? main :: IO () main = do map putOut lst putStrLn "end" where lst :: [Outputable] -- ERROR: Class "Outputable" used as a type lst = [(Out1 1),(Out2 1 2)] class Outputable a where putOut :: a -> IO () -- user defined: data Out1 = Out1 Int deriving (Show) data Out2 = Out2 Int deriving (Show) instance Outputable Out1 where putOut out1 = putStrLn $ show out1 instance Outputable Out2 where putOut out2 = putStrLn $ show out2 I cannot define it this way: data Out = Out1 Int | Out2 Int Int putOut Out1 = ... putOut Out2 = ... because this is a library and users should be able to extend Out with their own types

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  • What is wrong with this simple type definition? (Expecting one more argument to...)

    - by fluteflute
    basic.hs: areaCircle :: Floating -> Floating areaCircle r = pi * r * r Command: *Main> :l basic.hs [1 of 1] Compiling Main ( Sheet1.hs, interpreted ) Sheet1.hs:2:15: Expecting one more argument to `Floating' In the type signature for `areaCircle': areaCircle :: Floating -> Floating Failed, modules loaded: none. I see that areaCircle :: Floating a => a -> a loads as expected. Why is the above version not acceptable?

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  • Pattern matching for lambda expressions

    - by alphomega
    21 --Primitive recursion constructor 22 pr :: ([Int] -> Int) -> ([Int] -> Int) -> ([Int] -> Int) 23 pr f g = \xs 0 -> f xs 24 pr f g = \xs (y+1) -> g xs y ((pr f g) xs y) I want the function this function creates to act differently on different inputs, so that it can create a recursive function. As expected, the above code doesn't work. How do I do something like pattern matching, but for the function it creates? Thanks

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  • Why does s ++ t not lead to a stack overflow for large s?

    - by martingw
    I'm wondering why Prelude> head $ reverse $ [1..10000000] ++ [99] 99 does not lead to a stack overflow error. The ++ in the prelude seems straight forward and non-tail-recursive: (++) :: [a] -> [a] -> [a] (++) [] ys = ys (++) (x:xs) ys = x : xs ++ ys So just with this, it should run into a stack overflow, right? So I figure it probably has something to do with the ghc magic that follows the definition of ++: {-# RULES "++" [~1] forall xs ys. xs ++ ys = augment (\c n -> foldr c n xs) ys #-} Is that what helps avoiding the stack overflow? Could someone provide some hint for what's going on in this piece of code?

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  • GADTs and Scrap your Boilerplate

    - by finnsson
    I'm writing a XML (de)serializer using Text.XML.Light and Scrap your Boilerplate (at http://github.com/finnsson/Text.XML.Generic) and so far I got working code for "normal" ADTs but I'm stuck at deserializing GADTs. I got the GADT data DataBox where DataBox :: (Show d, Eq d, Data d) => d -> DataBox and I'm trying to get this to compile instance Data DataBox where gfoldl k z (DataBox d) = z DataBox `k` d gunfold k z c = k (z DataBox) -- not OK toConstr (DataBox d) = toConstr d dataTypeOf (DataBox d) = dataTypeOf d but I can't figure out how to implement gunfold for DataBox. The error message is Text/XML/Generic.hs:274:23: Ambiguous type variable `b' in the constraints: `Eq b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Show b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Data b' arising from a use of `k' at Text/XML/Generic.hs:274:18-30 Probable fix: add a type signature that fixes these type variable(s) It's complaining about not being able to figure out the data type of b. I'm also trying to implement dataCast1 and dataCast2 but I think I can live without them (i.e. an incorrect implementation). I guess my questions are: Is it possible to combine GADTs with Scrap your Boilerplate? If so: how do you implement gunfold for a GADT?

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  • Strange pattern matching with functions instancing Show

    - by Sean D
    So I'm writing a program which returns a procedure for some given arithmetic problem, so I wanted to instance a couple of functions to Show so that I can print the same expression I evaluate when I test. The trouble is that the given code matches (-) to the first line when it should fall to the second. {-# OPTIONS_GHC -XFlexibleInstances #-} instance Show (t -> t-> t) where show (+) = "plus" show (-) = "minus" main = print [(+),(-)] returns [plus,plus] Am I just committing a motal sin printing functions in the first place or is there some way I can get it to match properly? edit:I realise I am getting the following warning: Warning: Pattern match(es) are overlapped In the definition of `show': show - = ... I still don't know why it overlaps, or how to stop it.

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  • Adding class constraints to typeclass instance

    - by BleuM937
    I'm trying to implement the Cantor Pairing Function, as an instance of a generic Pair typeclass, as so: module Pair (Pair, CantorPair) where -- Pair interface class Pair p where pi :: a -> a -> p a k :: p a -> a l :: p a -> a -- Wrapper for typing newtype CantorPair a = P { unP :: a } -- Assume two functions with signatures: cantorPair :: Integral a => a -> a -> CantorPair a cantorUnpair :: Integral a => CantorPair a -> (a, a) -- I need to somehow add an Integral a constraint to this instance, -- but I can't work out how to do it. instance Pair CantorPair where pi = cantorPair k = fst . cantorUnpair l = snd . cantorUnpair How can I add the appropriate Integral constraint to the instance? I have a vague feeling I might need to modify the Pair interface itself, but not sure how to go about this.

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  • breadth-first traversal of directory tree is not lazy

    - by user855443
    I try to traverse the diretory tree. A naive depth-first traversal seems not to produce the data in a lazy fashion and runs out of memory. I next tried a breadth first approach, which shows the same problem - it uses all the memory available and then crashes. the code i have is: getFilePathBreadtFirst :: FilePath -> IO [FilePath] getFilePathBreadtFirst fp = do fileinfo <- getInfo fp res :: [FilePath] <- if isReadableDirectory fileinfo then do children <- getChildren fp lower <- mapM getFilePathBreadtFirst children return (children ++ concat lower) return (children ++ concat () else return [fp] -- should only return the files? return res getChildren :: FilePath -> IO [FilePath] getChildren path = do names <- getUsefulContents path let namesfull = map (path </>) names return namesfull testBF fn = do -- crashes for /home/frank, does not go to swap fps <- getFilePathBreadtFirst fn putStrLn $ unlines fps I think all the code is either linear or tail recursive, and I would expect that the listing of filenames starts immediately, but in fact it does not. Where is the error in my code and my thinking? where have I lost lazy evaluation?

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  • understanding syb boilerplate elimination

    - by Pradeep
    In the example given in http://web.archive.org/web/20080622204226/http://www.cs.vu.nl/boilerplate/ -- Increase salary by percentage increase :: Float -> Company -> Company increase k = everywhere (mkT (incS k)) -- "interesting" code for increase incS :: Float -> Salary -> Salary incS k (S s) = S (s * (1+k)) how come increase function compiles without binding anything for the first Company mentioned in its type signature. Is it something like assigning to a partial function? Why is it done like that?

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  • What is considered bleeding edge in programming these days?

    - by iestyn
    What is "bleeding edge" these days? has it all been done before us, and we are just discovering new ways of implementing mathematical constructs within programming? Functional Programming seems to be making inroads in all areas, but is this just marketing to create interest in a programming arena where it appears that the state of the art has climaxed too soon. have the sales men got hold of the script, and selling ideas that can be sold, dumbing down the future? I see very old ideas making their way into the market place....what are the truly new things that should be considered fresh and new in 2010 onwards, and not some 1960-1980 idea being refocused.

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