#13582: Confusing error message with multiparameter type classes. -------------------------------------+------------------------------------- Reporter: | Owner: (none) facundo.dominguez | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: type-checking | errors multiparameter type classes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
No. It is better because it gives you a clue of how to fix it: Add an instance of First Int b Int.
Alas, all that will happen then is that the missing `Typeable b` constraint will be reported. There really are two missing constraints, but GHC tries to avoid saturating the user by reporting only one per birth site. Reporting the entire path to a constraint is quite painful/voluminous, but I can see the point. Good ideas in here for better error messages. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13582#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler