Postponing Actions

The word

POSTPONE

suppresses the immediacy of any immediate word.

For example, the word

THEN

is immediate:

		' THEN immediate? . 
		true ok	
		

Suppose we wanted to re-define

THEN

to be

ENDIF

. Since

THEN

is immediate, we have to suppress its immediacy with

POSTPONE

:

		:  ENDIF
			POSTPONE THEN 
		; IMMEDIATE
		

We have to mark

ENDIF

as "immediate" because we need to replicate the immediacy of

THEN

. You can then use

ENDIF

instead of

THEN

:

		:  xx ( n -- )
			0 > IF 
				"positive!"
			ELSE
				"negative or zero!"
			ENDIF
			.
		;
		

While this use of

POSTPONE

is trivial, more useful applications often involve adding functionality to old words.

An Example

Suppose you wanted to count the number of words defined in your program. Instead of doing this each time by hand, one way is to amend the way the

;

word works. Here's how we can do it step by step:

Step 1: Redefine ; to include a data section

		: ;  
			postpone ; 
		; immediate 0 ,  	
		

Like

THEN

,

;

is an immediate word, we need to

POSTPONE

its action for later. Notice that there are three distinct

;

in this redefinition:

1. The first is the name of the new redefinition. Since it hasn't been completed, it doesn't interfere with the original definition.
2. The second is fed into postpone, which compiles the XT of the old

   ;

   into the re-defined/new

   ;

3. The final

   ;

   completes the redefinition. Its behaviour is the old

   ;

   . This is the last old

   ;

   is used. All subsequent uses of

   ;

   adopt the new redefinition.

Step 2: Define a new word COUNT? to retrieve the contents of ;'s data section

		: ;  
			postpone ; 
		; immediate 0 , 

		: count? 
			['] ; @ . ;	
		

Note that

[']

is like

'

but for compilation mode. So far, the behaviour of

;

hasn't changed.

Step 3: Re-Define ; again this time to increment the content of ;'s data section

		: ;  
			postpone ; 
		; immediate 0 , 

		: count? 
			['] ; @ . ;
			
		: ; 
			postpone ;
			1 ['] ; +! 
		; immediate		
		

This new redefinition contains additional behaviour -- it increments the data section entry by 1 each time it is executed. This way, we can count the number of words that have been defined.

Note that we had to define

COUNT?

before this second redefinition because the latest

;

doesn't have a data section.

After this last definition, the word

;

will count the times its is used:

	: hey "Hey Jude!" . ; 
	: double dup + ; 
	
	count?
	2 ok
	

Quiz

Question 1

Because we use it so often,

POSTPONE

has a short form,

`

(called BACKTICK). Re-write the example above using

`

. Should

`

be immediate?

Show / Hide Ans of Q1

Question 2

Could we have re-defined

:

instead of

;

to accomplish the same purpose of counting words? Prove your answer with a program.

Show / Hide Ans of Q2

Next: Switching Modes