Hello,
I have been trying to follow this discussion, but I cannot quite
understand the problem.
On 4/10/06, Ross Paterson
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 => B a b, C [a] c d (reduce instance) (changed C to B to fix a typo)
It seems to me that the constraint sets {B a b, C [a] b d} and {B a b, C [a] c d} are equivalent in the sense that if we assume the first set we can discharge the constraints in the second, and vice versa. So why are we saying that we have lost confluence? Is there perhaps a different example that illustrates the porblem? -Iavor PS: To show that C [a] b d |- C [a] c d we can apply the improving substitution 'b=c' (using the FD on class C), and then solve the goal by assumption, the proof the other way is symmetric.