On Tue, 22 Apr 2014 06:44:26 +0000, Simon Peyton Jones wrote:

| Independent of language extensions, what about types and classes whose | names are not in scope. Is there an implicit "... if you import all | the relevant symbols" and the end of the rule?

Good point. I'm honestly unsure how far to push this one! (It'd be relatively easy to check whether they were in scope and complain if not, but ...)

Simon

My stance on this is that, without the appropriate names in scope, you are actually specifying a different type signature to the one that GHC inferred (or none at all, if the names are unbound) - so this rule doesn't cover. The ruling that it would be valid, had you specified the precise signature GHC inferred is not broken, because you're actually specifying a different signature. Meanwhile, in the case of FlexibleContexts etc. you are actually specifying precisely the signature that GHC infers, but it's getting rejected due to other rulings.