On Sun, Mar 08, 2015 at 09:01:59PM +0000, Tom Ellis wrote:
On Sun, Mar 08, 2015 at 09:52:50PM +0100, martin wrote:
Hello all,
I was trying to construct a data type "Profile" which allows some set-like behavior. I started with functions to count, add and filter profiles.
type Count = Int
data Prof a = P { pCount :: Count, pFilter :: (a -> Bool) -> Prof a, pAdd :: (Prof a) -> (Prof a) } | NoProfile [...] What would be a good way to work with set-like behavior without being tied to one particular implementation?
If you really want the type parameter to be exposed you can use
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 }
Actually the above is the only version which works because pFilter mentions a. In that case you need the following package listProf :: ProfOps [] listProf = P { pCount = length, pFilter = filter, pAdd = (++) } I also made a mistake with the type of pAdd. It should be pAdd :: forall a. prof a -> prof a -> prof a