January 2010 - Beginners

Could not deduce (Matrix m (Maybe a)) from the context (Matrix m a)
by Lyndon Maydwell 29 Jan '10

29 Jan '10

Hi Beginners. I'm writing a matrix class for a game of life implementation. When I try to compile it I get the error "Could not deduce (Matrix m (Maybe a)) from the context (Matrix m a)" for the method vicinityMatrix. However, when I query the type of an identical implementation to vicinityMatrix in ghci it is successful: :t \m x y -> fromRows $ vicinityRows m x y \m x y -> fromRows $ vicinityRows m x y :: forall (m :: * -> *) (m1 :: * -> *) a. (Matrix m (Maybe a), Matrix m1 a) => m1 a -> Integer -> Integer -> m (Maybe a) What might be preventing the class from compiling? Thanks guys. --- My Matrix class definition follows below: module Matrix (Matrix) where import Data.Array import Data.Maybe (catMaybes) import Control.Monad (guard) class Matrix m a where fromRows :: [[a]] -> m a toList :: m a -> [a] rows :: m a -> Integer columns :: m a -> Integer row :: m a -> Integer -> [a] column :: m a -> Integer -> [a] at :: m a -> Integer -> Integer -> a (!!!) :: m a -> Integer -> Integer -> a vicinityRows :: m a -> Integer -> Integer -> [[Maybe a]] vicinityMatrix :: m a -> Integer -> Integer -> m (Maybe a) neighbours :: m a -> Integer -> Integer -> [a] toList m = do x <- [0 .. columns m - 1] y <- [0 .. rows m - 1] return $ at m x y row m n = [at m x n | x <- [0 .. columns m - 1]] column m n = [at m n y | y <- [0 .. rows m - 1]] at = (!!!) (!!!) = at vicinityRows m x y = do x' <- [x - 1 .. x + 1] return $ do y' <- [y - 1 .. y + 1] return cell where cell | x < 0 = Nothing | y < 0 = Nothing | x >= columns m = Nothing | y >= rows m = Nothing | otherwise = Just $ at m x y vicinityMatrix m x y = fromRows $ vicinityRows m x y -- neighbours = catMaybes . toListN . vicinityMatrix toListN :: Matrix m a => m a -> [a] toListN m = do x <- [0 .. columns m - 1] y <- [0 .. rows m - 1] guard $ x /= 1 && y /= 1 return $ at m x y

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subset - a little add
by Luca Ciciriello 29 Jan '10

29 Jan '10

Just a little add to may previous mail. The solution I've found from myself is: subset :: [String] -> [String] -> Bool subset xs ys = and [elem x ys | x <- xs] My question is if exists a more elegant way to do that. Luca. _________________________________________________________________ Do you have a story that started on Hotmail? Tell us now http://clk.atdmt.com/UKM/go/195013117/direct/01/

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Could not deduce (Matrix m (Maybe a)) from the context (Matrix m a)
by Lyndon Maydwell 29 Jan '10

29 Jan '10

Hi Beginners. I'm trying to write a matrix class, for a game of life implementation, and am having trouble with the vicinityMatrix function. When I define it inside the class I get the error "Could not deduce (Matrix m (Maybe a)) from the context (Matrix m a)". However when I define it inside ghci, there is no problem: :t \m x y -> fromRows $ vicinityRows m x y \m x y -> fromRows $ vicinityRows m x y :: forall (m :: * -> *) (m1 :: * -> *) a. (Matrix m (Maybe a), Matrix m1 a) => m1 a -> Integer -> Integer -> m (Maybe a) How would I modify my class to allow the definition? The class is defined below. module Matrix (Matrix) where import Data.Array import Data.Maybe (catMaybes) import Control.Monad (guard) class Matrix m a where fromRows :: [[a]] -> m a toList :: m a -> [a] rows :: m a -> Integer columns :: m a -> Integer row :: m a -> Integer -> [a] column :: m a -> Integer -> [a] at :: m a -> Integer -> Integer -> a (!!!) :: m a -> Integer -> Integer -> a vicinityRows :: m a -> Integer -> Integer -> [[Maybe a]] vicinityMatrix :: m a -> Integer -> Integer -> m (Maybe a) neighbours :: m a -> Integer -> Integer -> [a] toList m = do x <- [0 .. columns m - 1] y <- [0 .. rows m - 1] return $ at m x y row m n = [at m x n | x <- [0 .. columns m - 1]] column m n = [at m n y | y <- [0 .. rows m - 1]] at = (!!!) (!!!) = at vicinityRows m x y = do x' <- [x - 1 .. x + 1] return $ do y' <- [y - 1 .. y + 1] return cell where cell | x < 0 = Nothing | y < 0 = Nothing | x >= columns m = Nothing | y >= rows m = Nothing | otherwise = Just $ at m x y vicinityMatrix m x y = fromRows $ vicinityRows m x y

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how to specify type class in type of function?
by george young 28 Jan '10

28 Jan '10

[complete newbie using ghci 6.10.4 I understand that I should indicate that 'a' is in class Ord, since I use lessthan. But what is the syntax? Other style suggestions are welcome. mysort :: [a] -> [a] mysort [] = [] mysort [x] = [x] mysort [x, y] | (x <= y) = [x, y] | otherwise = [y, x] mysort l = (mysort s1) ++ (mysort s2) where (s1, s2) = splitAt (div (length l) 2) l Could not deduce (Ord a) from the context () arising from a use of `<=' at /home/gry/foo.hs:53:17-22 Possible fix: add (Ord a) to the context of the type signature for `mysort' In the expression: (x <= y) In a stmt of a pattern guard for the definition of `mysort': (x <= y) In the definition of `mysort': mysort [x, y] | (x <= y) = [x, y] | otherwise = [y, x]

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Re: [Haskell-beginners] Brainfuck interpreter stack overflow
by Edgar Klerks 28 Jan '10

28 Jan '10

He Felipe, I think you are right. Now I essentially have 2 data tapes, while I wanted a grid. The results of up and down are independent of left and right. I have to redesign it again. With kind regards, Edgar

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function nesting again
by Zbigniew Stanasiuk 28 Jan '10

28 Jan '10

Hallo all, I have two functions f and g of one arguments: f :: (Num a) => [a] -> [a] g :: (Num a) => [a] -> [a] I can do composition (f .f .f.) [a] or (g .g .g.) [a] for a few repeated functions. And what about if I want to nest f, say, many many times? The matter is more complicated (from my prespective :-) ) if I would like to get e.g.: the composition (g .f .g. g.f .f) [a] if t == 0 then f else g and having the list [1,0,1,1,0,0] as a pattern to construct above composition. How to make above, generally for arbitrary length ???

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single responsibility principle vs functional programming
by Joe Van Dyk 28 Jan '10

28 Jan '10

So I'm still pretty new to functional programming. In OOP, you have the concept of each class having a single-responsibility (or reason for changing). See http://en.wikipedia.org/wiki/Single_responsibility_principle I'm having problems seeing how that concept maps to functional programming. Perhaps it doesn't? -- Joe Van Dyk http://fixieconsulting.com

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Getting random values
by Tyler Hayes 28 Jan '10

28 Jan '10

First of all, how do you get a simple random integer in Haskell? Second, How does one get a random value from a list of non-integer values? For example, I would like to pull a random musical note from a list of notes using the Haskore library. Thanks, Tyler

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Programmers File Editor
by willingly950＠aol.com 27 Jan '10

27 Jan '10

Hi! Apologies for asking what is probably a very simple question... ...how do I map the drives/create a BAT file/etc so I can write and save programmes in the Programmers File Editor to use them in HUGS? Is there an idiots guide? Thanks. W.

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Brainfuck interpreter stack overflow
by Edgar Klerks 26 Jan '10

26 Jan '10

He Stephen Thanks for the tip. I will test the performance of Data.Sequence against Immutable arrays. I also have another question about deriving monads. I have modified Data.List.Zipper, so it supports 2 extra directions (up and down): data Zipper a = Zip [a] [a] [a] [a] Now I made a functor out of it: ** instance Functor Zipper where -- fmap :: (a -> b) -> f a -> f b f `fmap` (Zip ls rs ds us) = Zip (f `fmap` ls) (f `fmap` rs) (f `fmap` ds) (f `fmap` us) This works. Now I try to make an monad out of it: instance Monad Zipper where return a = Zip [a] [] [] [] (Zip ls rs ds us) >>= f = Zip (ls >>= f) (rs >>= f) (ds >>= f) (us >>= f) This doesn't work like expected, but when using the newtype derive mechanism it works. Thanks for all your help so far. Best Wishes, Edgar

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