#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

#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: | -------------------------------------+------------------------------------- Comment (by simonpj): Fixing this infelicity isn't fundamentally hard, but it's a bit fiddly. I can advise if anyone (Ryan?) wants to take it up. The challenge is this: * In an instance declaration, the default method {{{ reflextive = $dgmreflective @ty }}} is generated as `HsSyn Name`; and then fed to the type checker. There are excellent reasons for doing this, rather than (say) generating something in already-typechecked form. * (Something similar happens for ordinary default methods, but it'd be `$dmreflective`. * When typechecking this binding the typechecker doesn't a-prori know that it's special, with a funny name in it. * So the typechecker needs to "know" somehow that this `Id` is special. One way to do that would be to add a form of `IdDetails` to say "I'm a default method Id". Then the "arising from ..." stuff could print a better message. * That would not be hard; these Ids are built in one place, by `mkDefaultMethodIds`. I have not worked out all the details, but am happy to advise. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13755#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler