#10971: ExtendedDefaultRules defaults to [] for Foldable/Traversable -------------------------------------+------------------------------------- Reporter: kanetw | Owner: kanetw Type: feature request | Status: patch Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10963 | Differential Rev(s): phab:D1329 Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Replying to [comment:6 simonpj]:
Also, this implementation would seem to accept `default Either`, which is utterly meaningless, as `Either` has a non-defaultable kind. This is also utterly harmless, but it makes me suspicious of the design.
It would not be utterly meaningless if we added some new type class of kind `(* -> * -> *) -> Constraint`, and wanted defaulting to work on it.
Simon
That's true. But we have no such classes with defaulting yet, and it would take a change to the compiler to make it so. But, I've heard a loud enough chorus of voices asking for just one list (including some on IRC). And I think KaneTW's argument in comment:4 is convincing. I stand down. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10971#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler