There you have it: fully- and semi-pointfree versions of reMatr. A heads up: aggressively pursuing pointfreeness without type signatures guarantees a courtesy call from the monomorphism restriction, pace ()-garlic aficionados. As for your question on why the original code doesn't typecheck: if you explain how you arrived at it, perhaps we can figure out where you tripped up. Daniel Fischer for instance, *calculated* for you the right answer. Habeas calculus and all that. slemi 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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