How does Lisp let you redefine the language itself?

Question

I\'ve heard that Lisp lets you redefine the language itself, and I have tried to research it, but there is no clear explanation anywhere. Does anyone have a simple example?<

Answer 1

This answer is specifically concerning Common Lisp (CL hereafter), although parts of the answer may be applicable to other languages in the lisp family.

Since CL uses S-expressions and (mostly) looks like a sequence of function applications, there's no obvious difference between built-ins and user code. The main difference is that "things the language provides" is available in a specific package within the coding environment.

With a bit of care, it is not hard to code replacements and use those instead.

Now, the "normal" reader (the part that reads source code and turns it into internal notation) expects the source code to be in a rather specific format (parenthesised S-expressions) but as the reader is driven by something called "read-tables" and these can be created and modified by the developer, it is also possible to change how the source code is supposed to look.

These two things should at least provide some rationale as to why Common Lisp can be considered a re-programmable programming language. I don't have a simple example at hand, but I do have a partial implementation of a translation of Common Lisp to Swedish (created for April 1st, a few years back).