#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:2 diatchki]:
One (fairly simple?) way for GHC to check the validity of such constraints would be to simply evaluate them until it does find the ground type in question. In your example, this would result in GHC non- terminating during type checking, which is perfectly reasonable, especially since you need `UndecidableInstaces` to write such recursive types.
Except that means that type families can't work over abstract types that we don't know much about yet. For example: {{{ type family Equals a b where Equals a a = True Equals a b = False }}} I `Equals x x` to reduce to `True` even if I don't know what `x` is. Under your scheme, I would need to know what `x` is to be able to make progress, if only to ensure that it terminates. This whole thing is a sad story. But I don't know how to make it better. See also #10327 and [https://typesandkinds.wordpress.com/2015/09/09/what- are-type-families/ my blog post] on the subject ([https://www.reddit.com/r/haskell/comments/3kyfrl/richard_eisenberg_what_are_... reddit comments on blog post]). Bottom line: I think the Right Solution is to require all non-associated type families to be total (implying terminating) and force all non-total type families to be associated with a class constraint. (This actually makes them functional dependencies in disguise!) Then these problems go away. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11424#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler