[GHC] #15871: Revamp -fprint-explicit-kinds

[GHC]

#15871: Revamp -fprint-explicit-kinds
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 With the imminent #12045, we will have type application in kinds. This
 ticket tracks mirroring this new behavior in the pretty-printer.

 To wit, when `-fprint-explicit-kinds` is enabled, we should

 1. Print instantiations of specified variables like `@...`.

 2. Print instantiations of inferred variables like `@{...}`.

 Examples:

 {{{#!hs
 data Proxy (a :: k) = Proxy
  -- Then, `Proxy Int` would be printed as `Proxy @Type Int` with -fprint-
 explicit-kinds

 data Proxy2 a = Proxy2
  -- Then, `Proxy2 Int` would be printed as `Proxy2 @{Type} Int` with
 -fprint-explicit-kinds
 }}}

 In addition, sometimes error messages suggest enabling `-fprint-explicit-
 kinds`. Instead of doing this, we should just locally enable the flag. The
 existence of the `@` prefixes will alert the reader that these are not
 type arguments. (Without the `@` syntax, locally and silently enabling
 `-fprint-explicit-kinds` would be very confusing.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15871>
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