Aside from metaprogramming, from reading the "four reports" document that is the first Google link, it seems PIE also addresses another hard problem. In any hierarchically organized program, there are always related pieces of code that we would like to maintain together, but which get ripped apart and spread out because the hierarchy was split according to a different set of aspects. You can't get around this problem because if you change what criteria the hierarchy is split on in order to put these pieces near each other, now you've ripped apart code that was related on the original aspect. I've come to the conclusion that hierarchical code organization itself is the problem, and we would be better served by a way to assemble programs relationally (in the sense of an RDBMS). It seems like PIE was in that same conceptual space. Could you comment on that or elaborate more on the PIE system? Thanks.
Good insights -- and check out Alex Warth's "Worlds" paper on the Viewpoints site -- this goes beyond what PIE could do with "possible worlds" reasoning and computing ...
This is a very interesting paper. Its invocation of state space over time as a model of program side effects reminds me of an idea I had a couple years ago: if you think of a program as an entity in state-space where one dimension is time, then "private" object members in OO-programming and immutable values in functional programming are actually manifestations of the same underlying concept. Both are ways to create fences in the state-space-time of a program. Private members create fences along a "space" axis and functional programming creates fences along the "time" axis.
And you get to use "relational" and "relativity" side by side in a discussion.
A lot of interesting things tend to happen when you introduce invariants, including "everything-is-a" invariants. Everything is a file, everything is an object, everything is a function, everything is a relation, etc.
