instance inference

[Simon Peyton-Jones]

| > Yes, (1-GHC) certainly contradicts the FD paper that Martin and I
wrote.
| > I think (1) should be:
| >
| >    (1-fixed) Each assertion in the context must constrain type
| > variables, and
| >    those type variables must all be mentioned in the head.
| >
| > That is, there is no requirement that the type variables should be
| > distinct (they can be repeated).  But they must all appear in the
HEAD.
| 
| Ah, but you added the no-repetition requirement a few weeks ago in
| response to the example
| 
| 	class C a b
| 	instance C b b => C (Maybe a) b

Oh yes.  And in fact I now remember that this is a bug in the paper: the
Basic Conditions should require *distinct* type variables.  So I take
back that bit.

Simon