[GHC] #10447: DeriveFoldable rejects instances with constraints in last argument of data type

[GHC]

#10447: DeriveFoldable rejects instances with constraints in last argument of data
type
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        Reporter:  RyanGlScott       |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #8678             |  Differential Revisions:
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Comment (by dolio):

 Yeah, I'm not a big fan (at the moment) of having equality constraints and
 the like pick up fields that aren't syntactically identical. For instance:

 {{{
 data Foo b a where
   MkFoo :: (T b ~ a) => a -> b -> Int -> Foo b a
  deriving Foldable
 }}}

 Now if `T b` reduces to `Int`, we fold over the first ''and'' third
 argument, but if it doesn't, we only fold over the first (because we can't
 do anything else)? I could probably agree with having it succeed and
 always fold over the first field, because it's syntactically an `a`. But I
 feel like that should be the default behavior, not trying to deduce things
 from the constraints.

 Also consider:

 {{{
 class (b ~ Int) => Foo a b where ...
 class Foo a b => Bar a b where ...

 data Quux a b where
   MkQ :: Bar a b => Int -> a -> b -> Quux a b
 }}}

 Does this pick up the `Int` because if we trace back from `Bar a b` we get
 `b ~ Int`? What about more complicated examples?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10447#comment:16>
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