Print explicit foralls in type family eqns when appropriate

Summary:
When `-fprint-explicit-foralls` is enabled, type family
equations are either printed without an explict `forall` entirely,
or with a bizarre square bracket syntax (in the case of closed type
families). I find neither satisfying, so in this patch, I introduce
support for printing explicit `forall`s in open type-family, closed
type-family, and data-family equations when appropriate. (By "when
appropriate", I refer to the conditions laid out in
`Note [When to print foralls]` in `IfaceType`.)

One tricky point in the implementation is that I had to pick a
visibility for each type variable in a `CoAxiom`/`FamInst` in order
to be able to pass it to `pprUserIfaceForAll` //et al.// Because
the type variables in a type family instance equation can't be
instantiated by the programmer anyway, the choice only really matters
for pretty-printing purposes, so I simply went with good ol'
trustworthy `Specified`. (This design choice is documented in
`Note [Printing foralls in type family instances]` in `IfaceType`.)

Test Plan: make test TEST=T15827

Reviewers: goldfire, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, carter

GHC Trac Issues: #15827

Differential Revision: https://phabricator.haskell.org/D5282

1 files changed, 10 insertions, 12 deletions

diff --git a/testsuite/tests/ghci/scripts/T7939.stdout b/testsuite/tests/ghci/scripts/T7939.stdout
index 82a8658906..4c2a602f4f 100644
--- a/testsuite/tests/ghci/scripts/T7939.stdout
+++ b/testsuite/tests/ghci/scripts/T7939.stdout
@@ -5,23 +5,21 @@ Bar :: k -> * -> *
 type family F a :: * 	-- Defined at T7939.hs:8:1
 type instance F Int = Bool 	-- Defined at T7939.hs:9:15
 F :: * -> *
-type family G a :: *
-  where G Int = Bool
+type family G a :: * where
+    G Int = Bool
   	-- Defined at T7939.hs:11:1
 G :: * -> *
-type family H (a :: Bool) :: Bool
-  where H 'False = 'True
+type family H (a :: Bool) :: Bool where
+    H 'False = 'True
   	-- Defined at T7939.hs:14:1
 H :: Bool -> Bool
-type family J (a :: [k]) :: Bool
-  where
-      J '[] = 'False
-      J (h : t) = 'True
+type family J (a :: [k]) :: Bool where
+    J '[] = 'False
+  forall k (h :: k) (t :: [k]). J (h : t) = 'True
   	-- Defined at T7939.hs:17:1
 J :: [k] -> Bool
-type family K (a1 :: [a]) :: Maybe a
-  where
-      K '[] = 'Nothing
-      K (h : t) = 'Just h
+type family K (a1 :: [a]) :: Maybe a where
+    K '[] = 'Nothing
+  forall a (h :: a) (t :: [a]). K (h : t) = 'Just h
   	-- Defined at T7939.hs:21:1
 K :: [a] -> Maybe a