Generalise macro type-checking #296
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This commit generalises macro type-checking, making class constraints
more general by allowing what GHC calls FlexibleContexts and
FlexibleInstances.
Key changes:
- A special monad for solving, TcSolveM.
This monad keeps track of a work list and a set of
inert (= fully processed) constraints.
- Solving a constraint using a top-level instance may now add
additional constraints to the work list.
- Instances are keyed using a TrieMap, similar to GHC's RoughMap,
which avoids traversing all instances when doing instance matching.
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The solver didn't handle equalities properly, as it didn't account for the result of unification. Now, we try to unify, and if the unifier makes no progress we make the constraint into an inert constraint.
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This commit generalises macro type-checking, making class constraints more general by allowing what GHC calls FlexibleContexts and FlexibleInstances.
Key changes:
A special monad for solving, TcSolveM. This monad keeps track of a work list and a set of inert (= fully processed) constraints.
Solving a constraint using a top-level instance may now add additional constraints to the work list.
Instances are keyed using a TrieMap, similar to GHC's RoughMap, which avoids traversing all instances when doing instance matching.