Re: the MPTC Dilemma (please solve)

On Sat, Feb 18, 2006 at 12:26:36AM +0000, Ross Paterson wrote:

Martin Sulzmann writes:

...

Result2: Assuming we can guarantee termination, then type inference is complete if we can satisfy - the Bound Variable Condition, - the Weak Coverage Condition, - the Consistency Condition, and - and FDs are full. Effectively, the above says that type inference is sound, complete but semi-decidable. That is, we're complete if each each inference goal terminates.

I think that this is a little stronger than Theorem 2 from the paper, which assumes that the CHR derived from the instances is terminating. If termination is obtained via a depth limit (as in hugs -98 and ghc -fallow-undecidable-instances), it is conceivable that for a particular goal, one strategy might run into the limit and fail, while a different strategy might reach success in fewer steps.

Rereading, I see you mentioned dynamic termination checks, but not depth limits. Can you say a bit more about termination? It seems to be crucial for your proofs of confluence.