Hi Lenny,
i am not quite sure how to do this in the most elegant way:
I have some data structures:
data A = A Double data B = B Double data C = C Double ...
and i want to allow only a subset in another data structure, so i did something like this:
data SubSet = SubSetA A | SubSetC C
and use it in Foo:
data Foo = Foo [SubSet]
No i want to perform a polymorphic operation on the contents of A,B,C, e.g.
doSomething :: Foo -> [Double] doSomething (Foo s) = map doSomethingElse s
You can do things similar to this using one of the many generics libraries for Haskell [1,2]. I'm not sure if this is exactly what you're after, but here is a possibility using EMGM [3]. {-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE UndecidableInstances #-} {-# OPTIONS -fglasgow-exts #-}
import Generics.EMGM import Generics.EMGM.Derive
data A = A Double data B = B Double data C = C Double
data Subset = SubsetA A | SubsetC C
data Foo = Foo [Subset]
$(deriveMany [''A,''B,''C,''Subset,''Foo])
doSomething :: Foo -> [Double] doSomething = collect
In GHCi, I get the following: *Main> doSomething (Foo [SubsetA (A 5.0),SubsetC (C 9.9)]) [5.0,9.9] Other libraries to look at include SYB [4] and Uniplate [5]. Regards, Sean [1] http://hackage.haskell.org/packages/archive/pkg-list.html#cat:generics [2] http://www.cs.uu.nl/research/techreps/UU-CS-2008-010.html [3] http://www.cs.uu.nl/wiki/GenericProgramming/EMGM [4] http://www.cs.uu.nl/wiki/GenericProgramming/SYB [5] http://community.haskell.org/~ndm/uniplate/