#10960: Closed type families don't reduce on data family instances -------------------------------------+------------------------------------- Reporter: exFalso | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: closed type | Operating System: Linux family data | Architecture: x86_64 | Type of failure: GHC rejects (amd64) | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code doesn't compile: {{{#!hs {-# LANGUAGE TypeFamilies #-} module Tmp where data family D a data instance D () = D type family T a where T () = () T a = Char try :: T (D ()) ~ Char => () try = () main :: IO () main = return try }}} giving {{{ Tmp.hs:15:15: Couldn't match expected type ‘Char’ with actual type ‘T (D ())’ In the first argument of ‘return’, namely ‘try’ In the expression: return try In an equation for ‘main’: main = return try }}} Is this a known limitation of closed type families? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10960 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler