[GHC] #15827: Explicit foralls in type family equations are pretty-printed inconsistently (and strangely, at times)

[GHC]

#15827: Explicit foralls in type family equations are pretty-printed inconsistently
(and strangely, at times)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.7
      Resolution:                    |             Keywords:  TypeFamilies
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 goldfire):

 The covars are always empty. That comment is right. Along the way to
 `-XTypeInType`, there were some scenarios where the covars weren't empty.
 Mutterings to self are memorialized
 [https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Internal#Matchingaxioms
 here].

 As for pretty-printing behavior, I have a few observations:
  - The AST indeed stores whether or not the user wrote a `forall`, because
 some of the warnings, etc., are different based on this choice. However,
 once a type family equation is accepted, the presence/absence of the
 `forall` is immaterial.
  - The order of variables in the `forall` is immaterial to users. Not sure
 if this is a problem at all, but there's never a way to manually
 instantiate the variables, so we don't have to care about order.
  - Visibility/specificity is simply not a thing for the variables bound in
 a type family equation, also because the user can't ever instantiate these
 manually.
  - The funny `[a]` syntax was invented because using `forall` here is a
 tiny bit of a lie: `forall` is a specific construct in Core that does not
 exist in type family equations (i.e. coercion axioms). I don't think this
 is useful or friendly to users, though.

 Do these design points help?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15827#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler