#14938: Pattern matching on GADT does not refine type family parameters -------------------------------------+------------------------------------- Reporter: kcsongor | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Keywords: GADTs, Resolution: | TypeFamilies, TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Hm. Maybe it's only Agda that requires you to write a non-linear pattern here. Except that the pattern isn't ''really'' non-linear, because the non-linearity (at least in Agda) isn't checked. (Agda has a notation for an argument that has to be a certain way but that isn't checked for when matching.) In any case, you're right, though. `Refl` doesn't bring into scope any coercion. Instead, the appearance of `Refl` ''requires'' a coercion in order for the equation to match. But because type family equations are at the top level and that there is no phase separation between kind-checking and type-checking, this is all OK. After all, the same type checker that allowed `Refl` to be type checked at the type family application site is checking whether the equation matches; if one can prove the types equal, the other can, too. This design decision does have a few consequences, though. For example, you can't quantify a type family equation over an equality between type family applications: `type family F a (pf :: G a :~: H a)`. That just won't work (for several reasons). Of course, it's my hope that we won't have type families in a few years and can use normal GADT pattern matching instead! :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14938#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler