#10614: Show constraints in ``Found hole...'' -------------------------------------+------------------------------------- Reporter: bjmprice | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: 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 zyla): +1 for class constraints. This comes up a lot when working with existentials. Consider this (contrived) example: {{{#!hs {-# LANGUAGE GADTs #-} data AnyShow where AnyShow :: Show a => a -> AnyShow foo :: AnyShow -> String foo (AnyShow x) = _ }}} GHC currently (8.1.20161115) gives the following message: {{{ foo.hs:6:19: error: • Found hole: _ :: p Where: ‘p’ is a rigid type variable bound by the inferred type of foo :: AnyShow -> p at foo.hs:6:1-19 • In the expression: _ In an equation for ‘foo’: foo (AnyShow x) = _ • Relevant bindings include x :: a (bound at foo.hs:6:14) foo :: AnyShow -> p (bound at foo.hs:6:1) }}} The situation is even worse here than in OP's example, since `a` appears out of nowhere, and `Show a` isn't mentioned. But I think the decision whether to include them isn't that hard - we could have both `-fprint-equality-constraints` and `-fprint-class- constraints`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10614#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler