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

5 Nov 2016

      #12811: GHC tells me to use RankNTypes when it's already enabled
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I was stumped for a while today because GHC kept telling me to turn on
 `RankNTypes` even when it was already enabled! I eventually realized this
 was the problem:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 module Bug where

 foo :: foral a. a -> a
 foo x = x
 }}}

 {{{
 $ ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:4:15: error:
     Illegal symbol '.' in type
     Perhaps you intended to use RankNTypes or a similar language
     extension to enable explicit-forall syntax: forall <tvs>. <type>
 }}}

 That is, the real culprit was misspelling `forall` as `foral`.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12811
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