#13754: ghc-8.0.2+ rejects more instance declarations -------------------------------------+------------------------------------- Reporter: zilinc | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 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): The error message is not great, but here is what is happening. The default method in the class decl generates {{{ $dmpretttyList :: forall a. Pretty a => [a] -> SDoc $dmprettyList = concatMap pretty }}} That seems reasonable. Now in the instance decl it's as if you had written {{{ instance (Pretty (e t)) => Pretty (Def e) where pretty = ... prettyList = $dmprettyList @(Def e) }}} From the call to `$dmprettyList` we get a "wanted" constraint `Pretty (Def e)`. That simplifies, via the instance decl itself, to `Pretty (e t0)`, where `t0` is a fresh unification varaible. Hence the message you see. But your instance declaration is deeply strange anyway, becuase the `t` is completely unconstrained. Having a "given" `(Pretty (e t)` isn't much use if we know nothing about `t`. (There is nothing wrong with the class, just the instance.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13754#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler