#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 diatchki): I am not sure what you mean by "abstract types". If by `x` you mean a type variable, then I don't think there is any 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. I do agree that if we can't prove that a type family is total, it essentially should have a well-formedness check on every use---in this example this is `F a ~ b`, but one may have some other class/constraint encoding the same thing too. One may also think of this the other way: we always emit a well-formedness constraint on use, but if we've proved that a type function is total, then we can solve this constraint trivially. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11424#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler