[GHC] #10188: prefix type-level cons can't be parsed

24 Mar 2015

      #10188: prefix type-level cons can't be parsed
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              Reporter:  Kinokkory   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Whereas other type operators (whether used as type families or used as
 data constructors) can be safely used in a prefix form, only the type-
 level cons of the built-in list kind can't be used in a prefix form.

 It really is a problem because we don't have any means to express type-
 level cons itself in source code. This for example prevents us from
 writing a generic type-level foldr that takes the built-in list and other
 similar data types alike.

 In a ghci:

 {{{#!hs
...
:set -XDataKinds -XTypeOperators
data Listy a = a ::: Listy a
:kind! (:::)
 (:::) :: k -> Listy k -> Listy k
 = forall (k :: BOX). (':::)
:kind! (:)
<interactive>:1:2: parse error on input ‘:’
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10188
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