[GHC] #6022: GHC infers over-general types

[GHC]

#6022: GHC infers over-general types
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              Reporter:  simonpj     |            Owner:  simonpj
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.4.1
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by thomie):

 GHC should reject `f` ''without'' type signature, because it also rejects
 `f` ''with'' type signature, unless `FlexibleContexts` is enabled.
 {{{
 Prelude> let f x = x + head  -- GHC should complain here

 Prelude> :t f
 f :: Num ([a] -> a) => ([a] -> a) -> [a] -> a

 Prelude> let f :: Num ([a] -> a) => ([a] -> a) -> [a] -> a; f x = x + head

 <interactive>:4:10:
     Non type-variable argument in the constraint: Num ([a] -> a)
     (Use FlexibleContexts to permit this)
     In the type signature for ‘f’:
       f :: Num ([a] -> a) => ([a] -> a) -> [a] -> a

 Prelude> :set -XFlexibleContexts

 Prelude> let f :: Num ([a] -> a) => ([a] -> a) -> [a] -> a; f x = x + head

 Prelude>
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6022#comment:9>
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