#13755: GHC-8.0.2+ spits out $dm names in error messages -------------------------------------+------------------------------------- Reporter: zilinc | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #10087 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * related: => #10087 Comment: #10087 is very closely related to this ticket. The context of #10087 is a bit different since it was talking about `DefaultSignatures`, e.g., {{{#!hs {-# LANGUAGE DefaultSignatures #-} class C a where reflexive :: a -> Bool default reflexive :: Eq a => a -> Bool reflexive x = x == x data D instance C D where }}} {{{ /home/abel/play/haskell/bugs/DefaultSig.hs:10:10: No instance for (Eq D) arising from a use of ‘Main.$gdmreflexive’ In the expression: Main.$gdmreflexive In an equation for ‘reflexive’: reflexive = Main.$gdmreflexive In the instance declaration for ‘C D’ }}} The consensus on that ticket is that we should instead print an error message to the effect of: {{{ No instance for (Eq D) arising from the generic default method for `reflexive` In the instance declaration for ‘C D’ }}} Following the same principle, I think an ideal error message for your particular example would be: {{{ • Overlapping instances for Arbitrary Int arising from a default implementation of `shrink` ... • In the instance declaration for ‘Arbitrary Int’ }}} Do you agree, zilinc? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13755#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler