FDs and confluence

[Simon Peyton-Jones]

Interesting!  It'd be great if you've found a simpler more uniform rule.
(Which you seem to be getting rather good at.)    Let's see if you can
convince Martin, first, and then articulate the proposed rules.  I'll
look fwd to that.

Simon

| -----Original Message-----
| From: haskell-prime-bounces at haskell.org
[mailto:haskell-prime-bounces at haskell.org] On Behalf Of
| Ross Paterson
| Sent: 10 April 2006 09:53
| To: haskell-prime at haskell.org
| Subject: FDs and confluence
| 
| (see the FunctionalDependencies page for background omitted here)
| 
| One of the problems with the relaxed coverage condition implemented
| by GHC and Hugs is a loss of confluence.  Here is a slightly cut-down
| version of Ex. 18 from the FD-CHR paper:
| 
| 	class B a b | a -> b
| 	class C a b c | a -> b
| 
| 	instance B a b => C [a] b Bool
| 
| Starting from a constraint set C [a] b Bool, C [a] c d, we have two
| possible reductions:
| 
| 1) C [a] b Bool, C [a] c d
| 	=> c = b, C [a] b Bool, C [a] b d	(use FD on C)
| 	=> c = b, B a b, C [a] b d		(reduce instance)
| 
| 2) C [a] b Bool, C [a] c d
| 	=> C a b, C [a] c d			(reduce instance)
| 
| The proposed solution was to tighten the restrictions on instances to
| forbid those like the above one for C.  However there may be another
| way out.
| 
| The consistency condition implies that there cannot be another
| instance C [t1] t2 t3: a substitution unifying a and t1 need not
| unify b and t2.  Thus we could either
| 
| 1) consider the two constraint sets equivalent, since they describe
|    the same set of ground instances, or
| 
| 2) enhance the instance improvement rule: in the above example, we
|    must have d = Bool in both cases, so both reduce to
| 
| 	c = b, d = Bool, B a b
| 
|    More precisely, given a dependency X -> Y and an instance C t, if
|    tY is not covered by tX, then for any constraint C s with sX = S tX
|    for some substitution S, we can unify s with S t.
| 
|    We would need a restriction on instances to guarantee termination:
|    each argument of the instance must either be covered by tX or be
|    a single variable.  That is less restrictive (and simpler) than
|    the previous proposal, however.
| 
| Underlying this is an imbalance between the two restrictions on
instances.
| In the original version, neither took any account of the context of
the
| instance declaration.  The implementations change this for the
coverage
| condition but not the consistency condition.  Indeed the original form
of
| the consistency condition is necessary for the instance improvement
rule.
| 
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