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
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