In Lisp, code is data. What benefit does that provide?

Question

In Lisp, any program\'s code is actually a valid data structure. For example, this adds one and two together, but it\'s also a list of three items.

(+ 1 2)
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Answer 1

Lisp has been developed for the manipulation of symbolic data of all kinds. It turns out that one can also see any Lisp program as symbolic data. So you can apply the manipulation capabilities of Lisp to itself.

If you want to compute with programs (to create new programs, to compile programs to machine code, to translate programs from Lisp to other languages) one has basically three choices:

• use strings. This gets tedious parsing and unparsing strings all the time.

• use parse trees. Useful but gets complex.

• use symbolic expressions like in Lisp. The programs are preparsed into familiar datastructures (lists, symbols, strings, numbers, ...) and the manipulation routines are written in the usual language provided functionality.

So what you get with Lisp? A relatively simple way to write programs that manipulate other programs. From Macros to Compilers there are many examples of this. You also get that you can embed languages into Lisp easily.