If by `x` you mean a type variable, then I don't think there is any
#11424: "Occurs check" not considered when reducing closed type families -------------------------------------+------------------------------------- Reporter: diatchki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:4 diatchki]: problem in reducing `Equals x x` to `True`.
Otoh, if you encountered `Equals (F a) (F a)`, then you can still reduce
that to `True`, but you'd also have to emit the constraint `F a ~ b`, to make sure that `F a` is well-defined.
But this doesn't support the substitution lemma. You say that we can reduce `Equals x x`. But what if we later learn that `x` is really `F a`? So if we're going to do the well-definedness check in the `F a` case, we have to delay reducing the `x` case. I agree with the other points in comment:4. Instead of a general well- formedness check, I'm asking the users to provide the name of a class that represents well-formedness. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11424#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler