Pretty-printing of the * kind

Before this patch, GHC always printed the * kind unparenthesized.

This led to two issues:

1. Sometimes GHC printed invalid or incorrect code.
   For example, GHC would print:  type F @*   x = x
         when it meant to print:  type F @(*) x = x
   In the former case, instead of a kind application we were getting a
   type operator (@*).

2. Sometimes GHC printed kinds that were correct but hard to read.
   Should  Either * Int  be read as  Either (*) Int
                              or as  (*) Either Int  ?
   This depends on whether -XStarIsType is enabled, but it would be
   easier if we didn't have to check for the flag when reading the code.

We can solve both problems by assigning (*) a different precedence. Note
that Haskell98 kinds are not affected:

  ((* -> *) -> *) -> *  does NOT become  (((*) -> (*)) -> (*)) -> (*)

The parentheses are added when (*) is used in a function argument
position:

   F * * *   becomes  F (*) (*) (*)
   F A * B   becomes  F A (*) B
   Proxy *   becomes  Proxy (*)
   a * -> *  becomes  a (*) -> *

2 files changed, 10 insertions, 10 deletions

diff --git a/testsuite/tests/partial-sigs/should_compile/T15039b.stderr b/testsuite/tests/partial-sigs/should_compile/T15039b.stderr
index 8f9c2c8a45..949995ddda 100644
--- a/testsuite/tests/partial-sigs/should_compile/T15039b.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T15039b.stderr
@@ -37,21 +37,21 @@ T15039b.hs:25:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
         ex3 :: Dict ((a :: *) ~~ (b :: k)) -> () (bound at T15039b.hs:25:1)
 
 T15039b.hs:33:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘Dict (Coercible @* a b)’
+    • Found type wildcard ‘_’ standing for ‘Dict (Coercible @(*) a b)’
       Where: ‘a’, ‘b’ are rigid type variables bound by
                the type signature for:
-                 ex6 :: forall a b. Dict (Coercible @* a b) -> ()
+                 ex6 :: forall a b. Dict (Coercible @(*) a b) -> ()
                at T15039b.hs:32:1-53
     • In a pattern type signature: _
       In the pattern: Dict :: _
       In an equation for ‘ex6’: ex6 (Dict :: _) = ()
     • Relevant bindings include
-        ex6 :: Dict (Coercible @* a b) -> () (bound at T15039b.hs:33:1)
+        ex6 :: Dict (Coercible @(*) a b) -> () (bound at T15039b.hs:33:1)
 
 T15039b.hs:35:8: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘Coercible @* a b’
+    • Found type wildcard ‘_’ standing for ‘Coercible @(*) a b’
       Where: ‘a’, ‘b’ are rigid type variables bound by
-               the inferred type of ex7 :: Coercible @* a b => Coercion @{*} a b
+               the inferred type of ex7 :: Coercible @(*) a b => Coercion @{*} a b
                at T15039b.hs:36:1-14
     • In the type signature:
         ex7 :: _ => Coercion (a :: Type) (b :: Type)
diff --git a/testsuite/tests/partial-sigs/should_compile/T15039d.stderr b/testsuite/tests/partial-sigs/should_compile/T15039d.stderr
index 8548356001..6e26d5a1cb 100644
--- a/testsuite/tests/partial-sigs/should_compile/T15039d.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T15039d.stderr
@@ -38,21 +38,21 @@ T15039d.hs:25:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
         ex3 :: Dict ((a :: *) ~~ (b :: k)) -> () (bound at T15039d.hs:25:1)
 
 T15039d.hs:33:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘Dict (Coercible @* a b)’
+    • Found type wildcard ‘_’ standing for ‘Dict (Coercible @(*) a b)’
       Where: ‘a’, ‘b’ are rigid type variables bound by
                the type signature for:
-                 ex6 :: forall a b. Dict (Coercible @* a b) -> ()
+                 ex6 :: forall a b. Dict (Coercible @(*) a b) -> ()
                at T15039d.hs:32:1-53
     • In a pattern type signature: _
       In the pattern: Dict :: _
       In an equation for ‘ex6’: ex6 (Dict :: _) = ()
     • Relevant bindings include
-        ex6 :: Dict (Coercible @* a b) -> () (bound at T15039d.hs:33:1)
+        ex6 :: Dict (Coercible @(*) a b) -> () (bound at T15039d.hs:33:1)
 
 T15039d.hs:35:8: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘Coercible @* a b’
+    • Found type wildcard ‘_’ standing for ‘Coercible @(*) a b’
       Where: ‘a’, ‘b’ are rigid type variables bound by
-               the inferred type of ex7 :: Coercible @* a b => Coercion @{*} a b
+               the inferred type of ex7 :: Coercible @(*) a b => Coercion @{*} a b
                at T15039d.hs:36:1-14
     • In the type signature:
         ex7 :: _ => Coercion (a :: Type) (b :: Type)