[GHC] #9901: Error message: f is applied to two arguments, but its type has only two (sic)

18 Dec 2014


      #9901: Error message: f is applied to two arguments, but its type has only two
(sic)
-------------------------------------+-------------------------------------
       Reporter:  zardoz             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler (Type     |                 Version:  7.8.3
  checker)                           |        Operating System:
       Keywords:                     |  Unknown/Multiple
   Architecture:  Unknown/Multiple   |         Type of failure:
     Difficulty:  Unknown            |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 I occasionally get type errors where the error message refers to a
 function’s
 arity in a contradictory way:
...
The function ‘X’ is applied to two arguments, […] but its type has only
 two.
Here’s a full error message of this kind:

 lenstest.hs:89:12:
     Couldn't match type ‘Either String Int’ with ‘Maybe b0’
     Expected type: [Char] -> Either String Int
       Actual type: [Char] -> Maybe b0
     The function ‘preview’ is applied to two arguments,
     but its type ‘Getting (Data.Monoid.First b0) (Either b0 c0) b0
                   -> [Char] -> Maybe b0’
     has only two
     In the second argument of ‘($)’, namely
       ‘(preview _Left "abc" :: Either String Int)’
     In a stmt of a 'do' block:
       print $ (preview _Left "abc" :: Either String Int)
 Failed, modules loaded: none.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9901
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