help wanted with type error message

[Andreas Rossberg]

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

Cheers,

	- Andreas

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

"Computer games don't affect kids.
 If Pac Man affected us as kids, we would all be running around in
 darkened rooms, munching pills, and listening to repetitive music."