[GHC] #11588: haddock documents misleading type signature for TypeError in GHC.TypeLits

[GHC]

#11588: haddock documents misleading type signature for TypeError in GHC.TypeLits
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           Reporter:  carter         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Documentation  |           Version:  8.0.1-rc2
           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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 {{{
 type family TypeError a :: b
 }}}

 is what is shown in the haddocks, which is misleading, at least to those
 accustomed to reading vanilla type families.

    1) its impossible from the haddock info to know that the actual type
 signature is `type family TypeError (a :: ErrorMessage) :: b` , that
 requires reading the source of the module (a generally useful thing
 always, but still, disappointing that its needed to understand this)

    2) i also need  read the source to notice that the definition of the
 type family is a closed with no cases! It seems odd (at least to me) that
 haddock doesn't disclose the closedness. (this might be an artifact of it
 being an empty closed type family, which wasn't possible prior to ghc 8.0
 anyways)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11588>
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