Stephen Wolfram: Something Very Big Is Coming

Whereas in Wolfram Language, If is just an ordinary symbol that has attribute HoldRest to ensure that it can prevent evaluation of its two clauses until it knows which one to evaluate.

In other words, code is just data (expression) that happens to have specific rules that will cause it to evaluate in some way, yielding a new expression (which could be more code, or partially evaluated code, or data, or whatever).

But you can also have rules attached to several symbols together. So for example, when you see a Foo containing a Bar, replace it with a Baz:

   Foo[___, Bar, ___] := Baz;

There's one more twist, which is that everything is not just serializable, but in some sense it is its serialization (called FullForm). It's hard to explain what this means without playing around with stuff.

Also, the language has actual m-expressions, thank god, unlike most LISPs.

(... at least in the Northern Hemisphere)

A New Science Of Kinds.