Inside-out evaluation contexts

I've been a big fan of the "inside-out" (i.e., zipper) formulation of evaluation contexts, but almost everyone I talk to has a real problem with it. I think this is primarily due to lack of familiarity, but I do recognize that it requires you to stand on your head.

I'd thought for a while it was the only real way to specify evaluation contexts for a CEK machine semantics, but in fact it's really only an optimized representation for that approach. The outside-in definitions work just fine.

All you need in a CEK machine is to be able to match the innermost frame of a context by decomposing it into E[F]. With the inside-out formulation, frames are stacked up from inside out, so this is tantamount to looking at the head of a list. This is so efficient that the entire history of stack-based language implementations uses it. However, it's still possible to match E[F] with the inside-out outside-in formulation, it just requires recursively traversing the context until you find the innermost frame F.

I think I'll probably stick with outside-in for most of my papers, since it seems to generate so much angst.

Update: whoops! corrected a typo.