On Mon, 10 Apr 2006, Claus Reinke wrote:
(see the FunctionalDependencies page for background omitted here)
which seems to ignore much of the discussion of confluence we had here some
weeks ago? I thought this list was for communication with the committee, and
since the ticket exists, I had been hoping that the
type class subcommittee would ensure that the wiki would be updated.
but it seems I have been talking to myself..
Yes, I wondered what had happened to your previous argument for the memo
constraint.
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)
CHR-alt:
B a b <=> infer_B a b, memo_B a b.
memo_B a b1, memo_B a b2 ==>b1=b2.
C a b c <=> infer_C a b c, memo_C a b c.
memo_C a b1 c1, memo_C a b2 c2 ==> b1=b2.
infer_C [a] b Bool <=> B a b.
memo_C [a] b' c' ==> b'=b.
What's this last rule for? It mentions a b that does not appear in the head of
the rule. If you mean the rule
memo_C [a] b c, memo_C [a] b' c' ==> b = b'.
then it is already subsumed by the more general
memo_C a b1 c1, memo_C a b2 c2 ==> b1=b2.
Tom
--
Tom Schrijvers
Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium
tel: +32 16 327544
e-mail: [EMAIL PROTECTED]
_______________________________________________
Haskell-prime mailing list
Haskell-prime@haskell.org
http://haskell.org/mailman/listinfo/haskell-prime
