f1cd1611
by sheaf at 2026-01-28T06:56:38-05:00
Improve defaulting of representational equalities
This commit makes the defaulting of representational equalities, introduced
in 1e53277a, a little bit more robust. Now, instead of calling the eager
unifier, it calls the full-blown constraint solver, which means that it can
handle some subtle situations, e.g. involving functional dependencies and
type-family injectivity annotations, such as:
type family F a = r | r -> a
type instance F Int = Bool
[W] F beta ~R Bool
25edf516
by sheaf at 2026-01-28T06:56:38-05:00
Improve errors for unsolved representational equalities
This commit adds a new field of CtLoc, CtExplanations, which allows the
typechecker to leave some information about what it has done. For the moment,
it is only used to improve error messages for unsolved representational
equalities. The typechecker will now accumulate, when unifying at
representational role:
- out-of-scope newtype constructors,
- type constructors that have nominal role in a certain argument,
- over-saturated type constructors,
- AppTys, e.g. `c a ~R# c b`, to report that we must assume that 'c' has
nominal role in its parameters,
- data family applications that do not reduce, potentially preventing
newtype unwrapping.
Now, instead of having to re-construct the possible errors after the fact,
we simply consult the CtExplanations field.
Additionally, this commit modifies the typechecker error messages that
concern out-of-scope newtype constructors. The error message now depends
on whether we have an import suggestion to provide to the user:
- If we have an import suggestion for the newtype constructor,
the message will be of the form:
The data constructor MkN of the newtype N is out of scope
Suggested fix: add 'MkN' to the import list in the import of 'M'
- If we don't have any import suggestions, the message will be
of the form:
NB: The type 'N' is an opaque newtype, whose constructor is hidden
Fixes #15850, #20289, #20468, #23731, #25949, #26137