[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 RyanGlScott):

 The code that I would expect {{{DeriveFoldable}}} to derive for {{{T1}}},
 {{{T2}}}, or {{{T3}}} would be:

 {{{#!hs
 instance Foldable T where
     foldr f z (MkT a) = f a z
     foldMap f (MkT a) = f a
 }}}

 which is the same code that you would get for {{{data T a = MkT a deriving
 Foldable}}}. I don't believe this would require any change to the
 {{{deriving Foldable}}} algorithm (but I'm not intimately familiar with
 the implementation details).

 I agree that it would be nice to have more documentation on the algorithms
 themselves. I gained a better intuition for the
 [http://git.haskell.org/ghc.git/blob/9f968e97a0de9c2509da00f6337b612dd72a0389:/compiler/typecheck/TcGenDeriv.hs#l1476
 Functor],
 [http://git.haskell.org/ghc.git/blob/9f968e97a0de9c2509da00f6337b612dd72a0389:/compiler/typecheck/TcGenDeriv.hs#l1725
 Foldable], and
 [http://git.haskell.org/ghc.git/blob/9f968e97a0de9c2509da00f6337b612dd72a0389:/compiler/typecheck/TcGenDeriv.hs#l1800
 Traversable] algorithms by reading the comments in {{{TcGenDeriv.hs}}}.
 For example, here's a somewhat formal description of how {{{deriving
 Foldable}}} works:

 {{{
 The cases are:

   $(foldr 'a 'b)         =  \x z -> z     -- when b does not contain a
   $(foldr 'a 'a)         =  f
   $(foldr 'a '(b1,b2))   =  \x z -> case x of (x1,x2) -> $(foldr 'a 'b1)
 x1 ( $(foldr 'a 'b2) x2 z )
   $(foldr 'a '(T b1 b2)) =  \x z -> foldr $(foldr 'a 'b2) z x  -- when a
 only occurs in the last parameter, b2
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10447#comment:6>
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