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

14 Jul 2017

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

Old description:
...
{{{#!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`).
New description:

 {{{#!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 expression ‘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#comment:3
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler