I hadn't heard about metagraph. Sounds useful. Where's the source?
As I understand it, the implementation in pypy is lazy by default and only "forces" a result when it's needed. So (again IIRC) it potentially avoids intermediates like numexpr (http://code.google.com/p/numexpr/).
The current code only implements a fairly trivial array structure in python, and it does not seem to implement any expression lazyness. But pypy should obviously make it easier to try this kind of things compared to the current numpy.
It does implement lazy evaluation of array expressions and JIT to compile it on the fly to assembler. Having an assembler generator that's not too bad helps immensely with such efforts
As I understand it, the implementation in pypy is lazy by default and only "forces" a result when it's needed. So (again IIRC) it potentially avoids intermediates like numexpr (http://code.google.com/p/numexpr/).