[GHC] #9607: Programs that require AllowAmbiguousTypes in 7.8

[GHC]

#9607: Programs that require AllowAmbiguousTypes in 7.8
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        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:                    |
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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)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9607#comment:9>
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