#12564: Type family in type pattern kind -------------------------------------+------------------------------------- Reporter: int-index | Owner: goldfire Type: bug | Status: new Priority: high | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: TypeInType, Resolution: | TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: 14119 | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Generally I like this. But: * We'd need to update the dynamic semantics and subject-reduction proof for FC to show that the resulting system is sound. * I'm very troubled by the fact that the evidence is not used. It's all very similar to class instances, except that in class instances we use the evidence. * You way that the "evidence may potentially be mentioned" but how might that happen? * In the example here we know for certain that the evidence is redundant. The `(Len @a _1)` was not written by the user; it was inferred. GHC never guesses, so it must be uniquely determined. So it's really a waste of time to construct and pass that evidence at every call site. Surely there must be a solid criterion for what components of the type are fully determined, and hence redundant and don't need to be matched? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12564#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler