Switching Modes

Besides using 
IMMEDIATE
and 
POSTPONE
to switch immediacy, you can use the words 
[
and 
]
(OPEN/CLOSE BRACKET) to switch modes in any word's compile time. 
The word 
[
 changes compilation mode into interpretation mode. The word 
]
 changes interpretation mode to compilation mode. An example:

		
		: QQ 1 2 + . [ "hello!" . cr ] ;
		

		will display:
		
		
		hello! 
		ok
		

		as it is being compiled. 
		
		
		see QQ

		[0]	Literal<1>
		[1]	Literal<2>
		[2]	+
		[3]	.

		ok
		


		Clearly, from the results of 
SEE
, the words within 
[
 ... 
]
 are executed at 
QQ
's compile time, but are not compiled into it.
		In other words, the actions within 
[
 ... 
]
 will not be executed when 
QQ
 is called.
	


	
LITERALs

	
Sometimes, you need to compute a value at compilation time and insert the result into the word being compiled. 
	To do this you use the word 
LITERAL
. 
LITERAL
 is an immediate word. It is used only inside a colon definition. At compile time, compiles a value from the stack into the definition as a literal. At run time, the value will be pushed on the stack.
	For example consider the following word 
HEY
:


	
	bp
	: hey [ "Hey " "Jude!" concat ] ;
	bp
	"seeing hey ..." . cr
	see hey

	--breakpoint #1-- 
	<0> 
	
	--breakpoint #2-- 
	<1> Hey Jude! 
	seeing hey ... 
	ok
	

	
	
Clearly, 
HEY
 puts the result of 
concat
, "Hey Jude!", just once, on the stack at compilation time.
			( 
SEE HEY
 therefore shows that the word is empty. ) If we wanted to insert this text onto 
HEY
 itself, here's what we do:
	
 
	
	
	bp
	: hey [ "Hey " "Jude!" concat ] LITERAL ;
	bp
	"seeing hey ..." . cr
	see hey
	
	--breakpoint #1-- 
	<0> 
	
	--breakpoint #2-- 
	<0> 
	seeing hey ... 
	[0]	Literal(Hey Jude!)
	ok
	

						
	
	
Quiz

	
Question 1

	Have you understand clearly how does 
[
 ... 
]
 works? Try out the following:
		
		: test "test1" [ "test2" [ "test3" ] ] ;
		

		What is happening on the stack before and after you run the word 
TEST
? Explain clearly using 
BP
. 
	
	

	
	


	

	
Question 2

	
Write a word called 
ALLOT
 that creates and initializes a data section for any word. The resulting data section should have length N and initilaized with the value 0. 
ALLOT
 should accept just one argument N. For example:
	
	
	: say-hi "jello" . ;  6 allot 
	

	should create a data section for 
SAY-HI
 of length 6, initialized with zeros.
	

	
	

	
	


	

	
Question 3

	
Another way to create the word's data section is to do it within the word's definition like so: 
	
	
	: say-hi 
		[ 0 , 41 , 777 , ] "yellow jello" .  ; 
	
	
	This creates a 
SAY-HI
 with three elements in the data section. Note that we used the Mode Switching words 
[
 and 
]
. Ensure that you understand and test this example thoroughly.

		
	

	
	


	

	
Question 4

	
What would happen if you used 
ALLOT
 after this like so
	
	
	: say-hi 
	 	[ 0 , 41 , 777 , ] "yellow jello" . ; 43 ALLOT
	

	

	
	

	
	


	
	
	
Question 5

	
Would this work? 
	
		
	: say-hi [ 43 ALLOT ] .... ; 
	

	

	
	

	
	


	

	
Question 6

	
The word 
THIS
 is written as follows:
		
		: this latestxt postpone literal ; immediate 	
		

	
THIS
 compiles the XT of the currently defined word as a Literal. Recall the word counting example in Postponing Actions*. Use 
THIS
 to make 
;
 count as before, but using just one re-definition instead of two.
		
	

	
	


	
	
	
Question 7

	Use 
[
 and 
]
 to remove the need to use 
THIS
 as a separate word in your answer to Question 6. Hint: Why is 
POSTPONE
 used in 
THIS
? Is it needed in Question 7?
		
	

	
	


	
 
Next: Handling Errors