“Illegal type synonym family application in instance” is too strict in the presence of functional dependencies
GHC rejects the following instance, complaining about the use of a type synonym family in the instance head:
type family Fam a
class Multi a b | a -> b
instance Multi (Maybe a) (Fam a)However, this seems too strict to me, since the second parameter of Multi is determined by a functional dependency. If I tweak the instance slightly to lift the second parameter out of the instance head into an equality constraint, GHC accepts the instance:
instance (b ~ Fam a) => Multi (Maybe a) bIt seems to me that, due to the functional dependency, these two instances are completely equivalent. Therefore, GHC should accept the first instance.
This is similar to an old ticket, #3485 (closed), but that did not involve fundeps, so GHC’s error was correct. I’m pretty sure the addition of functional dependencies changes things, making the two actually equivalent.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
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| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |