At 2001-10-10 03:59, Marcin 'Qrczak' Kowalczyk wrote: >We need some type T such that it's possible to define a family of >functions for arbitrary choices of A: > upA :: A -> T > downA :: T -> Maybe A >satisfying downA (upA a) = Just a. We want to choose the type for T >before deciding the exact set of types for A.
I'm convinced extensible datatypes are the cleanest and most in-spirit extenstion to Haskell to solve this. data T = _; ... data T |= MkAT A; upA = MkAT; downA (MkAT a) = Just a; downA _ = Nothing; -- Ashley Yakeley, Seattle WA _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell