#9479: Report required constraints when reporting the type of a hole -------------------------------------+------------------------------------- Reporter: dominiquedevriese | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 7.8.3 checker) | Operating System: Keywords: holes | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- For the following code {{{ module Test where test :: String test = show _h }}} GHC currently reports: {{{ Found hole ‘_h’ with type: a0 Where: ‘a0’ is an ambiguous type variable Relevant bindings include test :: String (bound at /tmp/Test.hs:4:1) In the first argument of ‘show’, namely ‘_h’ In the expression: show _h In an equation for ‘test’: test = show _h }}} It correctly does not report the lack of a `Show _a` instance as a separate type error. However, it would be useful if the report containing the type of the hole would also contain the constraints that apply to its type. Something like: {{{ Found hole ‘_h’ with type: a0 Where: ‘a0’ is an ambiguous type variable Applicable constraints: Show a0 Relevant bindings include test :: String (bound at /tmp/Test.hs:4:1) In the first argument of ‘show’, namely ‘_h’ In the expression: show _h In an equation for ‘test’: test = show _h }}} I am explicitly *not* suggesting to report a type like `Show a0 => a0` for the hole, because that might mistakenly suggest that we are looking for a value of type `forall a0. Show a0 => a0`, which we are not. A possible alternative is to use an imaginary exists type like `exists a0. Show a0 => a0` but that's probably just even more confusing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9479 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler