[GHC] #8356: Strangeness with FunDeps

[GHC]

#8356: Strangeness with FunDeps
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        Reporter:  ksf                        |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler                   |          Version:  7.7
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Comment (by rwbarton):

 The error makes sense to me: `sy`, `a`, `b` and `((sy ::: a) ': xs)` do
 not jointly determine `((sy ::: b) ': ys)`, because the whole `ys` part is
 free. (And the same for the other fundep.)

 Perhaps the error message could be more specific, saying what I said
 above? something like
 {{{
 Illegal instance declaration for [...]
       Multiple uses of this instance may be inconsistent
       with the functional dependencies of the class:
           `sy', `a', `b' and `((sy ::: a) ': xs)' do not
           jointly determine `((sy ::: b) ': ys)'
 }}}
 Extra clarity would be good here since there are existing libraries that
 use fundeps incorrectly, that will stop compiling in 7.8, and I anticipate
 a slew of bug reports otherwise. See for example
 http://ghc.haskell.org/trac/ghc/ticket/1241#comment:22.

 > What I'm trying to do is to express "xs is ys and ys is xs with a and b
 interchanged at sy", all in a single predicate because my current type
 family implementation needs two and explodes the inferred types.

 In that case, perhaps you intended
 {{{
 instance Replaced sy a b ((sy ::: a) ': xs) ((sy ::: b) ': xs) -- note xs
 twice
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8356#comment:1>
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