[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:  patch
        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):  Phab:D3868
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"8476097609a2044e965157380aeb5d4882a71248/ghc"
 84760976/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8476097609a2044e965157380aeb5d4882a71248"
 Revise function arity mismatch errors involving TypeApplications

 Summary:
 Currently, whenever you apply a function to too many arguments and
 some of those arguments happen to be visible type applications, the error
 message that GHC gives is rather confusing. Consider the message you
 receive
 when typechecking `id @Int 1 2`:

 ```
 The function `id` is applied to three arguments,
 but its type `Int -> Int` has only one
 ```

 This is baffling, since the two lines treat the visible type argument
 `@Int`
 differently. The top line ("applied to three arguments") includes `@Int`,
 whereas the bottom line ("has only one") excludes `@Int` from
 consideration.

 There are multiple ways one could fix this, which I explain in an addendum
 to
 `Note [Herald for matchExpectedFunTys]`. The approach adopted here is to
 change
 the herald of this error message to include visible type arguments, and to
 avoid counting them in the "applied to n arguments" part of the error. The
 end
 result is that the new error message for `id @Int 1 2` is now:

 ```
 The expression `id @Int` is applied to two arguments,
 but its type `Int -> Int` has only one
 ```

 Test Plan: make test TEST=T13902

 Reviewers: goldfire, austin, bgamari

 Reviewed By: goldfire

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #13902

 Differential Revision: https://phabricator.haskell.org/D3868
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13902#comment:5>
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