You can create your own deferred words by defining with
DEFER. For example:
creates a deferred word called CAKE. Initially, this
does nothing, which isn't terribly useful. However, you can set the action
of the word at any time with IS, which accepts an
execution token. For example:
: CREAM-CAKE CR ." Yummy!" ;
' CREAM-CAKE IS CAKE
Now, whenever you execute the word CAKE, you'll get
the response "Yummy!". Now try:
: MUD-CAKE CR ." Yuck!" ;
' MUD-CAKE IS CAKE
Now, CAKE provokes the response "Yuck!". The power of
these words is that, unlike simply redefining a word with the same name,
the action of the word changes immediately in every definition it has
been compiled into.
Built-in DEFERred words
CamelForth comes with several important words defined as
deferred words. This means that by substituting your own definitions, you
can easily change the behaviour of the entire system.
The following standard words are currently deferred:
- EMIT ( char -- )
- This word is at the heart of the Forth output system, so changing it
means that every character displayed on the console can be processed by
your own routine. The default word for EMIT is
- KEY ( -- char )
- Often used for getting data from the user. The default word for
KEY is CKEY.
- TYPE ( c-addr u -- )
- The standard word for outputting strings. The default word for
TYPE is CTYPE.
Also set up as deferred words are the following, which are executed
whenever a specific OZ error is detected:
- The action to take when the system receives an RC_QUIT error (this is
a request for the application to terminate itself). The default word is
BYE. This should always be the last word
in the (RC_QUIT) word definition in any case, as who
knows what will happen if OZ asks an application to quit but it refuses!
The main use for this vector is to allow applications to tidy up (by
closing files etc) before quitting.
- The action to take when the system receives an RC_DRAW error (ie the
display has become corrupted). The default word is
CINIT which defines the normal console
window and clears it. Under normal circumstances, this word will not be
called, since CamelForth asks OZ to try and preserve the screen
- The action to take when the system receives an RC_ESC error (ie ESC
has been pressed with escape-detection enabled). The default word for this
is ESCAPE. See next section for more details on
Note that any word you assign to these error handlers must leave all
stacks balanced, and cannot assume anything about the contents of any of
There are some other deferred words in the system; these are dealt with
in the documentation dealing with the facilities they are related to.
More about CamelForth
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