QuantifiedConstraints not fully described in manual

As pointed out in ticket:15351#comment:156470, the section in the manual on -XQuantifiedConstraints says that an => is essential in a quantified constraint. However, the following is accepted:

class C a

foo :: (forall x. C x) => ()
foo = ()

Note that there is no => in the quantified constraint. We need to update the manual accordingly.

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Documentation
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mentioned in issue #15351

changed milestone to %8.6.1

changed weight to 5

added Tbug Trac import documentation labels

mentioned in commit 8ec29460

Replying to [#15354 (closed) Simon Peyton Jones <simonpj@…>]:

This is a documentation-only fix, addressing Trac #15354.

Thanks Simon, is there a typo?

    class ::= ...
             | [forall tyavrs .] [context =>] qtycls inst1 ... instn
             | [forall tyavrs .] [context =>] tyvar inst1 ... instn

should be | [forall tyvars .] ...? (I think this has come through from the github proposal.)

So to know if some code is using QuantifiedConstraints: either there's a context => or a forall (or both). If neither appear (despite having {-# LANGUAGE QuantifiedConstraints #-}), then it's a regular class appearing in a context, whatever "regular" means ;-).

Yes there's some overlap (with the existing productions) in the grammar. I considered having rules for all combinations of with and without foralls and context (for each of the two forms of head) leading to six productions instead of two, but I decided against.

The grammar isn't trying to tell you which extension you are using, just what forms are legal.

I don't feel strongly; it does not seem v important.

changed milestone to %8.8.1

These won't be fixed for in GHC 8.6.

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

added Pnormal label

removed milestone

This was fixed in 8ec29460, the typo was fixed in 68530b1c.

closed