[GHC] #12978: User guide section on AllowAmbiguousTypes should mention TypeApplications

[GHC]

#12978: User guide section on AllowAmbiguousTypes should mention TypeApplications
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           Reporter:  mpickering     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  newcomer       |  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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 https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html?highlight=allowambiguoustypes
 #ghc-flag--XAllowAmbiguousTypes

 With type applications it is tempting to write

 {{{
 class C x a where
     meth :: a
 }}}

 which will fail the ambiguity check as `x` is not used in any methods. One
 way to get around this is to use `TypeApplications` in order to specify
 what `x` should be at each usage of `meth`. A small section in the user
 guide explaining this and linking to the section on `TypeApplications`
 would be useful.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12978>
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