[GHC] #9569: Tuple constraints don't work right

9 Sep 2014

      #9569: Tuple constraints don't work right
-------------------------------------+-------------------------------------
       Reporter:  simonpj            |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.2
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 Consider this program:
 {{{
 {-# LANGUAGE RankNTypes, ConstraintKinds, KindSignatures, DataKinds,
 TypeFamilies #-}
 module Wrong where

 import GHC.Prim

 data Proxy (c :: Constraint)

 class Deferrable (c :: Constraint) where
   defer :: Proxy c -> (c => a) -> a

 deferPair :: (Deferrable c1, Deferrable c2) =>
                   Proxy (c1,c2) -> ((c1,c2) => a) -> a
 deferPair = undefined

 instance (Deferrable c1, Deferrable c2) => Deferrable (c1,c2) where
     -- defer p f = deferPair p f     -- Succeeds
     defer = deferPair                -- Fails
 }}}
 The first (commented-out) definition of `defer` in the instance
 declaration succeeds; but the second fails with
 an utterly bogus message
 {{{
 ConstraintBug.hs:27:13:
     Could not deduce (c1 ~ (c1, c2))
     from the context (Deferrable c1, Deferrable c2)
 }}}
 The reason is that when type-checking the method defintion we try to unify
 {{{
      ((c1,c2) => a) ~ ((gamma1, gamma2) => alpha)
 }}}
 where
  * the LHS comes from instantiating the signature `(c => a)` (from the
 class decl) with `(c1,c2)/c` from the instance.
  * the RHS comes from instantiating the type of `deferPair` with fresh
 unification variables.

 The difficulty is that in the type of `deferPair`, the concrete syntax
 {{{
   deferPair :: ...((c1,c2) => a)...
 }}}
 is really just syntactic sugar for
 {{{
   deferPair :: ...(c1 => c2 => a)...
 }}}
 i.e. curried.  But the function in the instantiated signature really has
 one constraint argument,
 a pair, not two.

 It's not clear how to fix this.  It would actually be more consistent if
 {{{
 f :: (Eq a, Show a) => blah
 }}}
 really did take a pair of dictionaries, rather than two curried
 dictionaries.  But that
 would be a pretty big change, forced by a corner case.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9569
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