#12923: MultiParamTypeClasses + ExtendedDefaultRules -------------------------------------+------------------------------------- Reporter: amindfv | Owner: Type: feature request | Status: patch Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2822 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): You have realised that the spec in comment:12 can be further simplified (without changing its meaning): Find all the unsolved constraints. Then: * Find those that are of form `(C a)` where `a` is a type variable, and partition those constraints into groups that share a common type variable `a`. * Keep only the groups in which at least one of the classes is an interactive class. * Now, for each remaining group G, try each type `ty` from the default- type list in turn; if setting `a = ty` would allow the constraints in G to be completely solved. If so, default `a` to `ty`. Note that any multi-parameter constraints `(D a b)` or `(D [a] Int)` do not participate in the process (either to help or to hinder); but they must of course be soluble once the defaulting process is complete. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12923#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler