Explicit foralls in type family equations are pretty-printed inconsistently (and strangely, at times)

Load the following code into GHCi HEAD (8.7+):

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

import Data.Kind

type family F1 a
type instance forall a. F1 a = Maybe a

type family F2 a where
  forall a. F2 a = Maybe a

data family D a
data instance forall a. D a = MkD (Maybe a)

And make sure you have the -fprint-explicit-foralls flag enabled. Now let's see what happens when we look up the :info for each of these type families:

$ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive Bug.hs -fprint-explicit-foralls
GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Ok, one module loaded.
λ> :i F1
type family F1 a :: *   -- Defined at Bug.hs:7:1
type instance F1 a = Maybe a    -- Defined at Bug.hs:8:25
λ> :i F2
type family F2 a :: *
  where [a] F2 a = Maybe a
        -- Defined at Bug.hs:10:1
λ> :i D
data family D a         -- Defined at Bug.hs:13:1
data instance D a = MkD (Maybe a)       -- Defined at Bug.hs:14:25

There are two strange things of note here:

• The equations for F1 and D do not have any explicit foralls displayed at all, despite the fact that -fprint-explicit-foralls is enabled.
• The equation for F2 //does// have an explicit forall displayed, but in a rather bizarre fashion:

λ> :i F2
type family F2 a :: *
  where [a] F2 a = Maybe a
        -- Defined at Bug.hs:10:1

I certainly wasn't expecting to see the type variables in square brackets. I would have hoped to see something like this instead:

λ> :i F2
type family F2 a :: *
  where forall a. F2 a = Maybe a
        -- Defined at Bug.hs:10:1

Now that the "more explicit foralls" proposal is implemented, my hope is that it will be somewhat simple to change the way that this is pretty-printed (we already store the explicit forall information within the AST, after all).

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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