[Haskell-cafe] Computing sums
andrewcoppin at btinternet.com
Sat Feb 20 06:11:46 EST 2010
>> Anyway, for yours: try to implement (.) :: Property b c -> Property a
>> b -> Property a c, and first :: Property a b -> Property (a,c) (b,c).
>> Then you will have an arrow.
> (.) = flip (>==>)
> first p = p >==< pure id
No, not quite.
\p -> p >==< pure id :: Property a b -> Property a (b, a)
What you want is
first p = (pure fst >==> p) >==< (pure snd)
which has the required type.
More information about the Haskell-Cafe