#12468: GADTs don't refine hole types -------------------------------------+------------------------------------- Reporter: benjamin.hodgson | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: MacOS X | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I don't think this is a bug. Yes we have an equality in scope; but a value of type `a` and one of type `Int` are both equally valid things to fill the hole with. E.g. {{{ f :: T a -> a -> a f I x = _ }}} I could fill the hole with `(3::Int)` or with `x`. Neitehr is better than the other. Another way to say this is that type equalities are not normalising; they are equalities, not left-to-right rewrite rules. Suppose we had `a ~ b` in scope. Should report the hole as having type `a` or `b`? What if it was `a ~ [b]`? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12468#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler