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, 7 insertions, 10 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T6018fail.stderr b/testsuite/tests/typecheck/should_fail/T6018fail.stderr
index 7f7dadd499..0e230e62c8 100644
--- a/testsuite/tests/typecheck/should_fail/T6018fail.stderr
+++ b/testsuite/tests/typecheck/should_fail/T6018fail.stderr
@@ -60,33 +60,31 @@ T6018fail.hs:53:15: error:
 T6018fail.hs:61:10: error:
     Type family equation violates injectivity annotation.
     Kind variable ‘k2’ cannot be inferred from the right-hand side.
-    Use -fprint-explicit-kinds to see the kind arguments
     In the type family equation:
-      PolyKindVarsF '[] = '[] -- Defined at T6018fail.hs:61:10
+      PolyKindVarsF @{[k2]} @[k1] ('[] @k2) = '[] @k1
+        -- Defined at T6018fail.hs:61:10
 
 T6018fail.hs:64:15: error:
     Type family equation violates injectivity annotation.
     Kind variable ‘k1’ cannot be inferred from the right-hand side.
-    Use -fprint-explicit-kinds to see the kind arguments
     In the type family equation:
-      PolyKindVars '[] = '[] -- Defined at T6018fail.hs:64:15
+      PolyKindVars @[k1] @[k2] ('[] @k1) = '[] @k2
+        -- Defined at T6018fail.hs:64:15
 
 T6018fail.hs:68:15: error:
     Type family equation violates injectivity annotation.
     Kind variable ‘k’ cannot be inferred from the right-hand side.
-    Use -fprint-explicit-kinds to see the kind arguments
     In the type family equation:
       forall k (a :: k) (b :: k).
-        Fc a b = Int -- Defined at T6018fail.hs:68:15
+        Fc @k a b = Int -- Defined at T6018fail.hs:68:15
 
 T6018fail.hs:72:15: error:
     Type family equation violates injectivity annotation.
     Type and kind variables ‘k’, ‘a’, ‘b’
     cannot be inferred from the right-hand side.
-    Use -fprint-explicit-kinds to see the kind arguments
     In the type family equation:
       forall k (a :: k) (b :: k).
-        Gc a b = Int -- Defined at T6018fail.hs:72:15
+        Gc @k a b = Int -- Defined at T6018fail.hs:72:15
 
 T6018fail.hs:76:15: error:
     Type family equations violate injectivity annotation:
@@ -140,10 +138,9 @@ T6018fail.hs:120:15: error:
     Type family equation violates injectivity annotation.
     Type and kind variables ‘k’, ‘c’
     cannot be inferred from the right-hand side.
-    Use -fprint-explicit-kinds to see the kind arguments
     In the type family equation:
       forall k a b (c :: k).
-        G7 a b c = [G7a a b c] -- Defined at T6018fail.hs:120:15
+        G7 @k a b c = [G7a @k a b c] -- Defined at T6018fail.hs:120:15
 
 T6018fail.hs:131:1: error:
     Type family equations violate injectivity annotation: