[Haskell-cafe] pointfree-trouble

[Kim-Ee Yeoh]

Here's another way of writing it:

data Matrix a = Matr {unMatr :: [[a]]} | Scalar a  deriving (Show, Eq)
-- RealFrac constraint removed

reMatr :: RealFrac a => ([[a]] -> [[a]]) -> (Matrix a -> Matrix a)
reMatr f = Matr . f . unMatr     -- this idiom occurs a lot, esp. with
newtypes

Affixing constraints to type constructors is typically deprecated. 



slemi wrote:
> 
> i have trouble making a function pointfree:
> 
> data RealFrac a => Matrix a = Matr [[a]] | Scalar a
>   deriving (Show, Eq)
> 
> unMatr :: RealFrac a => Matrix a -> [[a]]
> unMatr = (\(Matr a) -> a)
> 
> reMatr :: RealFrac a => ([[a]] -> [[a]]) -> (Matrix a -> Matrix a)
> reMatr a = Matr . (flip (.) unMatr) a
> 
> this works fine, but if i leave the 'a' in the last function's definition
> like this:
> reMatr = Matr . (flip (.) unMatr)
> it gives an error. can anybody tell me why? (i'm using ghci)
> 

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