#12854: ghc-8 prints mangled names in error message: ‘GHC.Base.$dm<$’ -------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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 simonpj): I can explain what is going on. I'm not so sure how to fix it. You don't get an overlapping error for your `C` declarations because you aren't ''using'' them. Class overlap is reported lazily; see the user manual. For the `Functor` declarations, since you omit the default method, we fill it in fo you {{{ instance Functor (F String) where fmap = undefined ($<) = $dm< @(F String) }}} The default method comes from the original class decl, which looked like this {{{ class Functor f where fmap :: (a -> b) -> f a -> f b (<$) :: a -> f b -> f a (<$) = fmap . const }}} From this we get {{{ $dm< :: Functor f => a -> f b -> f a }}} So, in the instance decl we get the constraint `Functor (F String)`. But we don't know which of the two instance decls to use to solve it, alas, hence the error. It's not very clear what a good error message would be; and even less clear how to achieve it! Perhaps some more explanation in the user manual, along the lines above, would be a stop-gap. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12854#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler