#9607: Programs that require AllowAmbiguousTypes in 7.8 -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.3 checker) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | Blocking: Blocked By: | Differential Revisions: Related Tickets: | -------------------------------------+------------------------------------- Comment (by trevorcook): Thanks for the insight. So, this is bad design, and an instance where ambiguity was mistakenly sought in the wild. I was vaguely aware of the mistake, but thought using the ambiguity flag made it all fine. In summary, I was attempting to make a configurable, general purpose function, but giving no way for the user to control the configuration, i.e. which instances of the class constraints containing 'c' to use. Hence the ambiguity. A solution which doesn't require AllowAmbiguousTypes is to let the user specify the configuration by handing forwardingAction some data. Below, I have a similar function which effectively passes the 'ServerAction' method call wrapped in a data ServCfg. {{{#!hs newtype ServCfg st cmd resp c = ServCfg { runService :: st -> cmd ->(resp,c->ComResp cmd)} maybeFwdAction :: forall st cmd trig b c . (ClientAction trig VisCom b) => ServCfg st cmd (Maybe (trig, b->c)) c -> st -> cmd -> (Maybe ([VisCom],[VisResp] -> c), c -> ComResp cmd) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9607#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler