#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: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): I like treating the function and visible type application as a whole. The type of `f @Int` is `Int -> Int` and takes a single argument but that is not the type of `f` -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13902#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#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

#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: | -------------------------------------+------------------------------------- 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> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler