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, 4 insertions, 4 deletions

diff --git a/testsuite/tests/ghci/scripts/T15341.stdout b/testsuite/tests/ghci/scripts/T15341.stdout
index 0633ae5b39..e2555f9ac9 100644
--- a/testsuite/tests/ghci/scripts/T15341.stdout
+++ b/testsuite/tests/ghci/scripts/T15341.stdout
@@ -1,6 +1,6 @@
-type family Foo (a :: k) :: k
-  where Foo a = a
+type family Foo (a :: k) :: k where
+  forall k (a :: k). Foo a = a
   	-- Defined at T15341.hs:5:1
-type family Foo @k (a :: k) :: k
-  where Foo @k a = a
+type family Foo @k (a :: k) :: k where
+  forall k (a :: k). Foo @k a = a
   	-- Defined at T15341.hs:5:1