[GHC] #12811: GHC tells me to use RankNTypes when it's already enabled

Thu Mar 2 00:17:41 UTC 2017

#12811: GHC tells me to use RankNTypes when it's already enabled
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11669            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => newcomer

Comment:

 My suggestion would be that if `RankNTypes` is already on and a user
 incorrectly uses a dot as above, then the error message should say
 something like:

 {{{
 Perhaps you meant to write 'forall <vars> . <body>'?
 }}}

 Not the greatest error message in the world, but still better than the
 status quo.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12811#comment:3>
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