[GHC] #13902: Misleading function arity mismatch error with TypeApplications

[GHC]

#13902: Misleading function arity mismatch error with TypeApplications
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  TypeApplications                   |
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# LANGUAGE TypeApplications #-}

 f :: a -> a
 f x = x

 g :: Int
 g = f @Int 42 5
 }}}
 {{{
 GHCi, version 8.3.20170614: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:7:5: error:
     • Couldn't match expected type ‘Integer -> Int’
                   with actual type ‘Int’
     • The function ‘f’ is applied to three arguments,
       but its type ‘Int -> Int’ has only one
       In the expression: f @Int 42 5
       In an equation for ‘g’: g = f @Int 42 5
   |
 7 | g = f @Int 42 5
   |     ^^^^^^^^^^^
 }}}

 That error message is quite confusing to read, since it reports that:

 * `f` is applied to three arguments, which //includes// a visible type
 application
 * `f` only has one argument, which //excludes// the visible type
 application

 We ought to be able to do better. My suggestion would be to report this
 instead:

 {{{
     • The function ‘f @Int’ is applied to two arguments,
       but its type ‘Int -> Int’ has only one
       In the expression: f @Int 42 5
       In an equation for ‘g’: g = f @Int 42 5
 }}}

 Although I'm sure there are other ways the same information could be
 conveyed (e.g., `The function ‘f’ is applied to two value arguments`).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13902>
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