#16114: strange "instance .. => .. => .. where ..." -------------------------------------+------------------------------------- Reporter: j.waldmann | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 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):
Another option would be to re-investigate checkValidInstance
I don't think so. What I missed before is that ''the renamer needs to know'' which class is involved so it has the right "parent" for lookups. You can see this in the call to `getLHsInstDeclClass_maybe` in `RnNames.getLocalNonValBinders`. And it is also the cause of this ticket, because the instance got past the renamer only by remaming `(==)` not to the `(==)` from the `Eq` class, but to a new unrelated top-level binder. Yikes. I think putting up with marginally less-cool errors may be a price we have to pay. Rather than "mal-formed" we could say {{{ Class instances must be of the form context => C ty1 .. tyn where C is a class }}} or something more positive than "mal-formed". -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16114#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler