[GHC] #13953: forall

[GHC]

#13953: forall
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        Reporter:  zaoqi             |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:  invalid           |             Keywords:  forall
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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Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 This is completely expected behavior. To recap, the difference between
 your two programs is that the former program has this:

 {{{#!hs
 u :: forall x xs. Usuccs '[x] xs => x -> U xs
 u x = usuccs (UOne x :: U '[x])
 }}}

 Which typechecks. The latter program:

 {{{#!hs
 u :: Usuccs '[x] xs => x -> U xs
 u x = usuccs (UOne x :: U '[x])
 }}}

 Which lacks an explicit `forall`, does not typecheck. The reason is
 because by default, the type variables in a function's type signature do
 not scope over the function body. To enable this behavior, you have to
 turn on `ScopedTypeVariables` and quantify the type variables explicitly
 with a `forall`. Refer to
 [https://downloads.haskell.org/~ghc/8.0.2/docs/html/users_guide/glasgow_exts.html?highlight=scopedtypevariables
 #decl-type-sigs this section] of the users' guide for more information.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13953#comment:1>
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