retroforth/doc/book/Programming-Techniques-Word-Classes

# Word Classes

Word classes are one of the two elements at the heart of
RETRO's interpreter.

There are different types of words in a Forth system. At a
minimum there are data words, regular words, and immediate
words. There are numerous approaches to dealing with this.

In RETRO I define special words which receive a pointer and
decide how to deal with it. These are grouped into a `class:`
namespace.

## How It Works

When a word is found in the dictionary, RETRO will push a
pointer to the definition (the `d:xt` field) to the stack
and then call the word specified by the `d:class` field.

The word called is responsible for processing the pointer
passed to it.

As a simple case, let's look at `immediate` words. These are
words which will always be called when encountered. A common
strategy is to have an immediacy bit which the interpreter
will look at, but RETRO uses a class for this. The class is
defined:

```
:class:immediate (a-)  call ;
```

Or a normal word. These should be called at interpret time
or compiled into definitions. The handler for this can look
like:

```
:class:word (a-) compiling? [ compile:call ] [ call ] choose ;
```

## Using Classes

The ability to add new classes is useful. If I wanted to add
a category of word that preserves an input value, I could do
it with a class:

```
:class:duplicating (a-)
  compiling? [ &dup compile:call ] [ &dup dip ] choose
  class:word ; 

:duplicating &class:duplicating reclass ;

:. n:put nl ; duplicating
#100 . . .
```