#9479: Report required constraints when reporting the type of a hole -------------------------------------+------------------------------------- Reporter: | Owner: dominiquedevriese | Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.8.3 checker) | Resolution: | Keywords: holes 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 thomie): This is fixed in HEAD. {{{ $ ghc-7.11.20151213 Test.hs [1 of 1] Compiling Test ( Test.hs, Test.o ) Test.hs:4:8: error: • Ambiguous type variable ‘a0’ arising from a use of ‘show’ prevents the constraint ‘(Show a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Show Module -- Defined in ‘GHC.Show’ instance Show Ordering -- Defined in ‘GHC.Show’ instance Show TrName -- Defined in ‘GHC.Show’ ...plus 25 others (use -fprint-potential-instances to see them all) • In the expression: show _h In an equation for ‘test’: test = show _h Test.hs:4:13: error: • Found hole: _h :: a0 Where: ‘a0’ is an ambiguous type variable Or perhaps ‘_h’ is mis-spelled, or not in scope • In the first argument of ‘show’, namely ‘_h’ In the expression: show _h In an equation for ‘test’: test = show _h • Relevant bindings include test :: String (bound at Test.hs:4:1) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9479#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler