#15986: Poor error message source location reporting with unsaturated type family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.2 Keywords: TypeFamilies, | Operating System: Unknown/Multiple TypeErrors | Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If you compile this program: {{{#!hs {-# LANGUAGE TypeFamilies #-} module Bug where newtype WrapChar f = MkWrapChar (f Char) type family F a type family T a type instance T Int = WrapChar F }}} Then you'll get this error message: {{{ $ /opt/ghc/8.6.2/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:7:15: error: • The type family ‘F’ should have 1 argument, but has been given none • In the type instance declaration for ‘T’ | 7 | type instance T Int = WrapChar F | ^ }}} The thing is, the location of that caret is rather unfocused. It's pointing at `T`, but the real culprit is `F`! I know that GHC can do better here because other error messages actually get this right. For instance, if you change the `T Int` instance to this: {{{#!hs type instance T Int = F }}} Then GHC will actually point to `F` in the resulting error message: {{{ $ /opt/ghc/8.6.2/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:7:23: error: • Expecting one more argument to ‘F’ Expected a type, but ‘F’ has kind ‘* -> *’ • In the type ‘F’ In the type instance declaration for ‘T’ | 7 | type instance T Int = F | ^ }}} If GHC can point to the right place in the source code in this situation, then it ought to get the earlier situation right as well. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15986 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler