[GHC] #15898: Promoted type constructors don't print right in HsType

[GHC]

#15898: Promoted type constructors don't print right in HsType
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          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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 Consider this
 {{{
 ghci> import Data.Proxy
 ghci> undefined :: '() -> Int

 <interactive>:11:14: error:
     • Expected a type, but ‘ '()’ has kind ‘()’
     • In an expression type signature:  '() -> Int
 }}}
 What is that strange space doing before the `'()`?

 Similarly
 {{{
 undefined :: Proxy '() Int

 <interactive>:12:14: error:
     • Expected kind ‘* -> *’, but ‘Proxy  '()’ has kind ‘*’
     • In an expression type signature: Proxy  '() Int
 }}}
 Again, the strange space.

 It comes from the `HsType` pretty printer, which is worried
 about printing the type
 {{{
      '['K]
 }}}
 That is, a promoted list with one element `K`.  The trouble is that looks
 like a character literal `'['`.

 So we add an extra space, thus `'[ 'K]`.  ''But we add it before every
 promoded data constructor!"  Hence the spurious spaces.

 In `IfaceType` exactly the same thing happens, but we are more clever,
 and only print the leading space if the promoted data con immediately
 follows `'[` or `'(`.   We should do the same thing for `HsType`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15898>
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