Re: [GHC] #12085: Premature defaulting and variable not in scope

19 May 2016

      #12085: Premature defaulting and variable not in scope
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        Reporter:  Iceland_jack      |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 For the second, a type signature brings into scope ''only'' the type
 variables in the ''leading'' or top-level `forall`.  So in
 {{{
 foo :: forall (a::Type). (Read a, Show a) => Bool -> String
 foo b = ...type variable 'a' is in scope here...
 }}}
 But for ''non-top-level'' foralls, nothing is brought into scope
 {{{
 bar :: Bool -> forall (a::Type). (Read a, Show a) => Bool -> String
 bar b = ...type variable 'a' is NOT NOT in scope here...
 }}}
 This is again by design, and I think it's documented.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12085#comment:2
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