WHILE is the solution we hinted at last time.
This is an
IF that swaps its administration packet
with the administration packet of the enclosing structure.
That's why
WHILE can only be used
inside other control structures.
The other structures should have been closed before
the accompanying
THEN follows.
WHILE nests crosswise, and serves to jump
out of a structure,
to
THEN:
: WHILE ( -- )
POSTPONE IF
1 CS-ROLL ; IMMEDIATE
.. BEGIN .. WHILE .. AGAIN THEN ..
1 2 1 2
The numbers indicate how the words are linked.
WHILE can be used
inside all other control structures. Examples:
a) .. BEGIN .. WHILE .. UNTIL .. THEN ..
1 2 1 2
b) .. BEGIN .. WHILE .. AGAIN ELSE .. THEN ..
1 2 1 2 2
c) .. BEGIN .. WHILE .. WHILE .. AGAIN THEN .. THEN ...
1 2 3 1 3 2
In c)
WHILE-2 shoves its compiler administration under
BEGIN-1,
WHILE-3 shoves its compiler administration also under
BEGIN-1,
where it lands on top of
WHILE-2.
Internally, both WHILE's are nested normally.
Trick question:
What's the difference between
OIL and
VINEGAR ?
: OIL aaa IF bbb IF ccc THEN THEN ddd ;
: VINEGAR aaa IF bbb WHILE ccc THEN THEN ddd ;
Remark:
REPEAT (a standard word) is actually the same as
AGAIN THEN.
.. BEGIN .. WHILE .. REPEAT ..
