slemi 0slemi0 at gmail.com
Tue Dec 22 09:13:27 EST 2009

```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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