Red's built-in functions can be categorized as follows. The type is indicated within ():

• operators (op!), which can be used with infix notation, such as a + b, a / b, and so on.
• native (native!) functions, such as if, either, while, throw, all, wait, and so on; you can see a complete list by using ? op! in the console.
• routine (routine!) functions, such as exists?, write-clipboard, and so on; you can see a list using ? routine! It is also used in Red to define a function that calls a Red/System function (see Chapter 10, Advanced Red).

• action (action!) functions, such as copy, move, clear, to, form, and so on; you can see a complete list using ? action!

The native, routine, and action functions are written in Red/System. Action functions are special in that they are polymorphic—they are defined for more than one datatype. Which action code is executed depends on the type of its first argument.

• mezzanine (function!) functions, which are higher-level functions written in Red; you can see a complete list using ? function!

In this section, we'll cover the variety of ways that exist to define your own functions.