Functional dependency coverage check does not use type equalities
When checking the coverage condition to validate an instance, we should also consider equality constraints because they have a dependency in both directions.
class C a b | a -> b
instance (a ~ b) => C a b
-- Illegal instance declaration for ‛C a b’
-- The coverage condition fails in class ‛C’
-- for functional dependency: ‛a -> b’
-- Reason: lhs type ‛a’ does not determine rhs type ‛b’
-- Using UndecidableInstances might help
-- In the instance declaration for ‛C a b’
Trac metadata
Trac field | Value |
---|---|
Version | 7.6.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | iavor.diatchki@gmail.com |
Operating system | |
Architecture |