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

10 files changed, 54 insertions, 27 deletions

diff --git a/testsuite/tests/ghci/scripts/T13420.stdout b/testsuite/tests/ghci/scripts/T13420.stdout
index e6b81adc41..c6dbaf2755 100644
--- a/testsuite/tests/ghci/scripts/T13420.stdout
+++ b/testsuite/tests/ghci/scripts/T13420.stdout
@@ -1,6 +1,5 @@
-type family F a :: *
-  where
-      F [Int] = Bool
-      F [a] = Double
-      F (a b) = Char
+type family F a :: * where
+    F [Int] = Bool
+    F [a] = Double
+    F (a b) = Char
   	-- Defined at T13420.hs:4:1
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
diff --git a/testsuite/tests/ghci/scripts/T15827.hs b/testsuite/tests/ghci/scripts/T15827.hs
new file mode 100644
index 0000000000..ba67534e62
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T15827.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+module T15827 where
+
+import Data.Kind
+import Data.Proxy
+
+type family F1 (a :: k)
+type instance forall k (a :: k). F1 a = Proxy a
+
+type family F2 (a :: k) :: Type where
+  forall k (a :: k). F2 a = Proxy a
+
+data family D (a :: k)
+data instance forall k (a :: k). D a = MkD (Proxy a)
diff --git a/testsuite/tests/ghci/scripts/T15827.script b/testsuite/tests/ghci/scripts/T15827.script
new file mode 100644
index 0000000000..a05fcd8a2d
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T15827.script
@@ -0,0 +1,4 @@
+:load T15827
+:info F1
+:info F2
+:info D
diff --git a/testsuite/tests/ghci/scripts/T15827.stdout b/testsuite/tests/ghci/scripts/T15827.stdout
new file mode 100644
index 0000000000..50df504e58
--- /dev/null
+++ b/testsuite/tests/ghci/scripts/T15827.stdout
@@ -0,0 +1,9 @@
+type family F1 (a :: k) :: * 	-- Defined at T15827.hs:9:1
+type instance forall k (a :: k). F1 a = Proxy a
+  	-- Defined at T15827.hs:10:34
+type family F2 (a :: k) :: * where
+  forall k (a :: k). F2 a = Proxy a
+  	-- Defined at T15827.hs:12:1
+data family D (a :: k) 	-- Defined at T15827.hs:15:1
+data instance forall k (a :: k). D a = MkD (Proxy a)
+  	-- Defined at T15827.hs:16:34
diff --git a/testsuite/tests/ghci/scripts/T4175.stdout b/testsuite/tests/ghci/scripts/T4175.stdout
index e95d9c76af..de4e28c46f 100644
--- a/testsuite/tests/ghci/scripts/T4175.stdout
+++ b/testsuite/tests/ghci/scripts/T4175.stdout
@@ -11,10 +11,9 @@ class C a where
   	-- Defined at T4175.hs:16:5
 type instance D () () = Bool 	-- Defined at T4175.hs:22:10
 type instance D Int () = String 	-- Defined at T4175.hs:19:10
-type family E a :: *
-  where
-      E () = Bool
-      E Int = String
+type family E a :: * where
+    E () = Bool
+    E Int = String
   	-- Defined at T4175.hs:24:1
 data () = () 	-- Defined in ‘GHC.Tuple’
 instance [safe] C () -- Defined at T4175.hs:21:10
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
diff --git a/testsuite/tests/ghci/scripts/T8674.stdout b/testsuite/tests/ghci/scripts/T8674.stdout
index 45d4f0af0e..d938f95692 100644
--- a/testsuite/tests/ghci/scripts/T8674.stdout
+++ b/testsuite/tests/ghci/scripts/T8674.stdout
@@ -1,3 +1,4 @@
 data family Sing (a :: k) 	-- Defined at T8674.hs:4:1
 data instance Sing Bool = SBool 	-- Defined at T8674.hs:6:15
-data instance Sing a = SNil 	-- Defined at T8674.hs:5:15
+data instance forall k (a :: [k]). Sing a = SNil
+  	-- Defined at T8674.hs:5:15
diff --git a/testsuite/tests/ghci/scripts/T9181.stdout b/testsuite/tests/ghci/scripts/T9181.stdout
index 5e520fb0a2..a30879c316 100644
--- a/testsuite/tests/ghci/scripts/T9181.stdout
+++ b/testsuite/tests/ghci/scripts/T9181.stdout
@@ -21,7 +21,7 @@ data GHC.TypeLits.SomeSymbol
     GHC.TypeLits.KnownSymbol n =>
     GHC.TypeLits.SomeSymbol (Data.Proxy.Proxy n)
 type family GHC.TypeLits.TypeError (a :: GHC.TypeLits.ErrorMessage)
-            :: b
+            :: b where
 GHC.TypeLits.natVal ::
   GHC.TypeNats.KnownNat n => proxy n -> Integer
 GHC.TypeLits.natVal' ::
diff --git a/testsuite/tests/ghci/scripts/all.T b/testsuite/tests/ghci/scripts/all.T
index 493daa4412..82197072c7 100755
--- a/testsuite/tests/ghci/scripts/all.T
+++ b/testsuite/tests/ghci/scripts/all.T
@@ -288,4 +288,5 @@ test('T15568', normal, ghci_script, ['T15568.script'])
 test('T15325', normal, ghci_script, ['T15325.script'])
 test('T15591', normal, ghci_script, ['T15591.script'])
 test('T15743b', normal, ghci_script, ['T15743b.script'])
+test('T15827', normal, ghci_script, ['T15827.script'])
 test('T15898', normal, ghci_script, ['T15898.script'])