help wanted with type error message
Wed, 04 Apr 2001 18:17:15 +0200
Norman Ramsey wrote:
> > data Formula f => Row a = Row (f a, Weight)
> > type Weight = Float
> Hugs rejects this program:
> ERROR "hard.lhs" (line 14): Undefined type variable "f"
Right, f is not bound in the declaration of Row. Only the occurances of
type variables after the type constructor are binding occurances.
The fix is to turn Row into a binary constructor:
> data Formula f => Row f a = Row (f a, Weight)
BTW, contexts have no real meaning in data declarations - the
declaration above does not prevent you from writing
> type T = Row  Int
Andreas Rossberg, firstname.lastname@example.org
"Computer games don't affect kids.
If Pac Man affected us as kids, we would all be running around in
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