Re: [Haskell-cafe] How to abstract away set representation

8 Mar 2015


      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?
How about

    data ProfOps prof = P {
        pCount  :: prof -> Count
        pFilter :: (a -> Bool) -> prof -> prof
        pAdd    :: prof -> prof
    }

Then you can instantiate this package of operations at different types, for
example

    listProf :: ProfOps [a]
    listProf = P {
      pCount  = length,
      pFilter = filter,
      pAdd    = (++)
    }

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
    }

Tom

Re: [Haskell-cafe] How to abstract away set representation

Tom Ellis