#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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler