#12468: GADTs don't refine hole types -------------------------------------+------------------------------------- Reporter: benjamin.hodgson | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: #11325 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * os: MacOS X => Unknown/Multiple * related: => #11325 Comment: As noted in #11325, this is actually a regression from GHC 7.10 to 8.0. In GHC 7.10.3, you get the behavior that you seek: {{{ $ /opt/ghc/7.10.3/bin/ghci Bug.hs GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:7:7: Found hole ‘_’ with type: Int Relevant bindings include f :: T a -> a (bound at Bug.hs:7:1) In the expression: _ In an equation for ‘f’: f I = _ Failed, modules loaded: none. }}} As opposed to the current behavior: {{{ $ /opt/ghc/8.0.1/bin/ghci Bug.hs GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:7:7: error: • Found hole: _ :: a Where: ‘a’ is a rigid type variable bound by the type signature for: f :: forall a. T a -> a at Bug.hs:6:6 • In the expression: _ In an equation for ‘f’: f I = _ • Relevant bindings include f :: T a -> a (bound at Bug.hs:7:1) Failed, modules loaded: none. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12468#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler