#9613: when giving an error "No instance for C (a -> b)", suggest that a function may be underapplied -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: (Type checker) | Architecture: Unknown/Multiple Resolution: | Difficulty: Unknown Operating System: | Blocked By: Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by MikeIzbicki): This is roughly equivalent to the patch I suggest in: https://phabricator.haskell.org/D201 (TcErrors.lhs, line 1094ish). The difference is that my patch doesn't suggest the underapplied function name or the instances that do exist. It seems pretty straightforward to mention the existing instance, so I'll try to add that. I'm pretty sure it's in general impossible to name the argument. For example, in `show (length . head . tail)` which function do you say is underapplied? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9613#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler