#11511: Type family producing infinite type accepted as injective -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Keywords: TypeFamilies, Resolution: | Injective 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 rwbarton): I imagine you have some examples where you need the recursive assumption. For a boring example, imagine converting one (promoted) unary natural number type `data N1 = Z1 | S1 N1` to another `data N2 = Z2 | S2 N2`: {{{ type family F (a :: N1) :: N2 where F Z1 = Z2 F (S1 n) = S2 (F n) }}} That is injective but you need to use the injectivity inductively to prove it. The original example seems fine to me, since you can't prove `F Bool ~ F Char` (you would need both the "infinite type" `[[[...]]]` and an infinitely long proof in order to do so). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11511#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler