Overhaul -fprint-explicit-kinds to use VKA

This patch changes the behavior of `-fprint-explicit-kinds`
so that it displays kind argument using visible kind application.
In other words, the flag now:

1. Prints instantiations of specified variables with `@(...)`.
2. Prints instantiations of inferred variables with `@{...}`.

In addition, this patch removes the `Use -fprint-explicit-kinds to
see the kind arguments` error message that often arises when a type
mismatch occurs due to different kinds. Instead, whenever there is a
kind mismatch, we now enable the `-fprint-explicit-kinds` flag
locally to help cue to the programmer where the error lies.
(See `Note [Kind arguments in error messages]` in `TcErrors`.)
As a result, these funny `@{...}` things can now appear to the user
even without turning on the `-fprint-explicit-kinds` flag explicitly,
so I took the liberty of documenting them in the users' guide.

Test Plan: ./validate

Reviewers: goldfire, simonpj, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, carter

GHC Trac Issues: #15871

Differential Revision: https://phabricator.haskell.org/D5314

1 files changed, 5 insertions, 5 deletions

diff --git a/testsuite/tests/partial-sigs/should_compile/T15039b.stderr b/testsuite/tests/partial-sigs/should_compile/T15039b.stderr
index 21ec20ae40..c28b94879b 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)