#11207: GHC cannot infer injectivity on type family operating on GHC.TypeLits' Nat, but can for equivalent type family operating on user-defined Nat kind -------------------------------------+------------------------------------- Reporter: duairc | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 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 duairc): Could it not be possible to introduce some way of pattern-matching on {{{GHC.TypeLits}}}'s {{{Nat}}}s independently of generalised injectivity? Maybe it's a question that would involve a lot of bikeshedding... but surely the compiler ultimately has enough information to prove that {{{Replicate}}} is injective, I just can't destructure type-level {{{Nat}}}s. Could there be a magic built-in pattern synonym {{{Succ}}} for {{{Nat}}}s? Can pattern synonyms work on the type level? I don't really know much about the internals to know how to implement that, but there must be a way. Like if full generalised injectivity is just around the corner, then maybe we should just wait for that, but from what I've read it sounds like there are still a lot of kinks that need to be worked out first and it might turn out to be too difficult altogether? Right now pretty much every package that does type-level stuff defines its own {{{Nat}}} type, and I'd imagine the lack of the ability to pattern match on {{{GHC.TypeLits}}}'s {{{Nat}}} is a big part of the reason for that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11207#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler