Hello, I didn't try to understand what the function is doing, but just quickly noticed that
reMatr a = Matr . (flip (.) unMatr) a
can be written as
reMatr a = Matr . ((flip (.) unMatr) a)
but that
reMatr = Matr . (flip (.) unMatr)
can be written as
reMatr a = (Matr . (flip (.) unMatr)) a
so because of precedence rules a different function is being applied to 'a' in the 2 versions Best Keith On Tue, Dec 22, 2009 at 9:13 AM, slemi <0slemi0@gmail.com> wrote:
thanks, that's a really neat syntactic sugar :)
however, my original question was how to make the reMatr function pointfree, as reMatr = Matr . (flip (.) unMatr) is not working. any ideas/explanation why it doesnt work?
Kim-Ee Yeoh wrote:
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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