Monday, September 17, 2007

Code-as-Data, Encapsulation and the Lisp Dogma

Raganwald talks about code/data separation and encapsulation. Here he quotes Steve Yegge from one of his drunken rants, where the latter points out the virtues of using Lisp s-expressions as executable XML.

Just could not resist ruminating Douglas Hofstadter in his Metamagical Themas on the same subject in his essay Lisp: Recursion and Generality. He talks about Lisp as the medium that unifies the *inert* data (which he calls declarative knowledge) with *active* code (or procedural knowledge). He mentions ..
The main idea is that in Lisp, one has the ability to "elevate" an inert, information-containing data structure to the level of "animate agent", where it becomes a manipulator of inert structures itself. This program-data cycle, or loop, can continue on and on, with structures reaching out, twisting back, and indirectly modifying themselves or related structures.

Talking about this data-code duality he goes on ..
Moreover, Lisp's loop of program and data should remind biologists of the way that genes dictate the form of enzymes, and enzymes manipulate genes (among other things). Thus Lisp's procedural-declarative program-data loop provides a primitive, but very useful and tangible example of one of the most fundamental patterns at the base of life: the ability of passive structures to control their own destiny, by creating and regulating active structures whose form they dictate.

Steve Yegge has also expressed it succinctly in the same post (mentioned above) ..
But Lisp is directly executable, so you could simply make the tag names functions that automatically transform themselves. It'd be a lot easier than using XSLT, and less than a tenth the size.

When you have code-as-data and data-as-code, you have encapsulated the data structures in a form where they can transform themselves. While Lisp allows you to do this, is it too unnatural for a programming language that forces you to program in its native abstract syntax tree format?


Anonymous said...

Interesting post, thanks for sharing. I should check out this book by Hofstadter - his GEB also has references to Lisp.

Unknown said...

@Eli: Surely you should! Hofstadter's Metamagical Thema has some wonderful essays on Lisp and Recursion.

Unknown said...

The post is being discussed in reddit. Here ..

Anonymous said...

Lua, anyone?