56 lines
1.5 KiB
Text
56 lines
1.5 KiB
Text
# 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 . . .
|
|
```
|