Tuesday, October 26, 2010

equirecursive types

Just a quick status update. As my 'user' program has become larger, the type solver has begun to slow down horribly. I'm on the wrong end of some horrible O(x) where x is probably n^4 or worse.

So I decided to try an experiment, going back to the classic algorithm W. This only took a couple of days! However. One thing I didn't realize I was getting for 'free' from the constraint-based solver was recursive types. Now I'm forced to actually understand the concept.

I see two potential outcomes: 1) I grok recursive types well enough to fix my unifier, and the compiler gets much much faster; or 2) I rewrite the constraint solver to be more efficient. It's possible that I understand things well enough now to be able to do so.

If anyone out there can explain how to do unification with equirecursive types, I'd love to have the help!

2 comments:

  1. What I'm trying today: detect cycles in apply_subst(), and have it return a 'μ' encoding of the resultant type. I'm hoping this will then magically make the problem go away, in the same way that isorecursive types do.

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  2. Ok, that approach seems to work. At least, it compiles a relatively large source file successfully. And interestingly, the types are more detailed than they were with the constraint solver (because I had to actually detect recursion in types).

    The new code is about 20X faster than the constraint solver, and less than half the size. Fingers crossed that it's correct.

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