Lambda the Ultimate

There's been a raging battle on comp.lang.scheme for two months and 300+ posts about interpreters, denotational semantics and compositionality. I've skipped most of it, but this post from Will Clinger was instructive to me, particularly the part on full abstraction:

In other words, one of the most un-abstract semantics that could be imagined is fully abstract. Examples like this are part of the reason full abstraction is no longer regarded as a terribly useful concept.

It turns out that full abstraction is more easily achieved by making the operational semantics *less* abstract than by making the denotational semantics *more* abstract. Hence a fully abstract pair of semantics is likely to be *less* abstract than a pair of semantics that are *not* fully abstract.