[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
       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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 {{{
 {-# LANGUAGE UndecidableInstances #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTSyntax #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE FunctionalDependencies #-}
 import GHC.TypeLits
 data (:::) :: Symbol -> * -> * where
     Field :: sy ::: t

 class Replaced (sy :: Symbol) a b (xs :: [*]) (ys :: [*]) | sy a b xs ->
 ys, sy a b ys -> xs

 instance Replaced sy a b ((sy ::: a) ': xs) ((sy ::: b) ': ys)

 }}}

 results in

 {{{

 Illegal instance declaration for [...]
       Multiple uses of this instance may be inconsistent
       with the functional dependencies of the class

 }}}

 The guess is that the FunDep Checker chokes on [*], as that error message
 doesn't make sense in this context.

 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.

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