Simon Peyton-Jones:
| > * GHC says that these constraints must be obeyed only | > *after* the programmer-written type has been normalised | > by expanding saturated type synonyms | > ... | > I regard this as a kind of pre-pass, before serious type checking | > takes place, so I don't think it should interact with type checking | > at all. | > | > I don't think this normalisation should include type families, | > although H98 type synonyms are a kind of degenerate case of type | > families. | > | > Would that design resolve this particular issue? | | Not quite, but it refines my proposal of requiring that type synonyms | in the rhs of type instances need to be saturated. Let me elaborate.
Why not quite?
Maybe I was thinking too much in terms of GHC's implementation, but due to the lazy expansion type synonyms, the expansion is interleaved with all the rest of type checking. But I think I now know what you meant: the outcome should be *as if* type synonym expansion was done as a pre-pass.
| So, the crucial point is that, as you wrote, | | > I don't think this normalisation should include type families, | > although H98 type synonyms are a kind of degenerate case of type | > families.
Exactly! Just to state it more clearly again:
Any programmer-written type (i.e one forming part of the source text of the program) must obey the following rules: - well-kinded - type synonyms saturated - arguments of type applications are monotypes (but -> is special)
However these rules are checked ONLY AFTER EXPANDING SATURATE TYPE SYNONYMS (but doing no reduction on type families)
I agree.
The above checks are performed by checkValidType in TcMType. In particular, the check for saturated synonyms is in check_type (line 1134 or thereabouts). I'm not sure why these checks are not firing for the RHS of a type family declaration. Maybe we aren't calling checkValidType on it.
I'll check that. Might be an oversight.
So I think we are agreed. I think the above statement of validity should probably appear in the user manual.
Yes, I'll take care of that. Manuel