[GHC] #11765: Allow documentary type signatures

[GHC]

#11765: Allow documentary type signatures
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  7.10.3
  (Type checker)                     |
           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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 In some cases (e.g., `lens`) it's useful to document a binding with a more
 specific type signature than the true one, to show a useful special case.
 Unfortunately, this documentation can potentially be wrong or go out of
 date. I'd like to propose a pragma for this:

 {{{#!hs
 {-# Documentary #-} f :: T -> U
 }}}

 would ask the type checker to verify that `f :: T -> U`. Specifically, it
 would have the effect of creating and type checking, but then discarding,
 a hidden binding

 {{{#!hs
 f_doc :: T -> U
 f_doc t = f t
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11765>
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