[GHC] #8463: Inferred types problem

[GHC]

#8463: Inferred types problem
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        Reporter:  danilo2           |            Owner:
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:  invalid           |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by kosmikus):

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


Comment:

 I don't see incorrect behaviour here.

 Your type annotation is interpreted as being universally quantified:
 {{{
 f x = method1 x :: a -> b -> (a, b)
 }}}
 This says that the result of this function is of type `forall a. a -> b ->
 (a, b)`
 independently of what the input is, but this isn't true. You need to refer
 to the
 type variables introduced by `x`.

 You can resolve this in various ways:

    1. Just use a type signature:

 {{{
 f :: X (a -> b -> (a, b)) -> a -> b -> (a, b)
 f x = method1 x
 }}}

    2. Enable `ScopedTypeVariables` and annotate the pattern:

 {{{
 f (x :: X (a -> b -> (a, b))) = method1 x :: a -> b -> (a, b)
 }}}

    3. Enable `ScopedTypeVariables` and reuse the type variables from the
 type signature:

 {{{
 f :: forall a b. X (a -> b -> (a, b)) -> a -> b -> (a, b)
 f x = method1 x :: a -> b -> (a, b)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8463#comment:1>
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