This can be done with existensial types as implemented in hbc. See
http://www.cs.chalmers.se/~augustss/hbc/decls.html
for the example and
http://www.cs.chalmers.se/~augustss/hbc/hbc.html
for the compiler.
Below is a part of the example from the webpage above:
Patrik Jansson
--------------------------------------------------------------------
Existentially quantified variables can also be constrained by giving a
context before a constructor. The following defines a type where the
argument to the constructor has to have a type belonging to class Show.
data TShow = (Show ?a) => T ?a
This can be used as follows:
l :: [TShow]
l = [T True, T 1, T (atan 1), T [True]]
s = map show l
and the value of s will be ["True","1","7.8539816339744828e-1","[True]"].
