[core libraries] Re: Tightening up on inferred type signatures

[Niklas Haas]

On Tue, 22 Apr 2014 06:44:26 +0000, Simon Peyton Jones <simonpj at microsoft.com> 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.