#9109: Improve error messages around "untouchable" type variables ------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- When I say {{{ {-# LANGUAGE GADTs #-} data G a where GBool :: G Bool foo GBool = True }}} I get {{{ Couldn't match expected type ‘t’ with actual type ‘Bool’ ‘t’ is untouchable inside the constraints (t1 ~ Bool) bound by a pattern with constructor GBool :: G Bool, in an equation for ‘foo’ at /Users/rae/temp/Bug.hs:6:5-9 ‘t’ is a rigid type variable bound by the inferred type of foo :: G t1 -> t at /Users/rae/temp/Bug.hs:6:1 Relevant bindings include foo :: G t1 -> t (bound at /Users/rae/temp/Bug.hs:6:1) In the expression: True In an equation for ‘foo’: foo GBool = True }}} My code is indeed bogus and the error message is accurate. But, it's really unhelpful in that the solution to my problem is "add a type signature". In general, I occasionally see folks complain about "untouchable" error messages. Most recently, see [http://www.haskell.org/pipermail/glasgow-haskell- users/2014-May/024993.html this thread], which inspired this report. Is the solution ''always'' to just add a type signature/annotation? Would it make sense to include a link to a stable wiki page about just what "untouchable" variables are? I don't necessarily have a solution to this problem, but I think it's worth thinking about. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9109 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler