On Mon, Mar 09, 2015 at 09:30:27PM +0100, martin wrote:
Am 03/08/2015 um 10:05 PM schrieb Tom Ellis: How would I use this? If I have a listProf and I want to perform pCount, must I then choose the pCount from listProf? Something like
pCount listProf aProfile
But then my code knows that I am using a list representation, which is the thing I wanted to avoid.
I tried to pair the ops with the representation, but this also got me into trouble, because when I e.g. do a pAdd then it gets strange when the two operands do not use the same Ops.
You use it like this. You can write operations on your Prof without having to know its concrete type. If you don't like threading the dictionary `p` around then you could use a reader monad or a typeclass. {-# LANGUAGE Rank2Types #-} type Count = Int data ProfOps prof = P { pCount :: forall a. prof a -> Count, pFilter :: forall a. (a -> Bool) -> prof a -> prof a, pAdd :: forall a. prof a -> prof a -> prof a } listProf :: ProfOps [] listProf = P { pCount = length, pFilter = filter, pAdd = (++) } example :: ProfOps prof -> prof Integer -> prof Integer -> Count example p profs1 profs2 = pCount p (pAdd p (pFilter p even profs1) profs2) Tom