#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