Graphomata – Forth

Forth Control Flow

Primitive Keywords

BEGIN          L = new label;  push(L);  emit "L:"
AGAIN          L = pop();  emit "jump to L"
UNTIL          L = pop();  emit "jump conditionally to L"
AHEAD          L = new label;  push(L);  emit "jump to L"
IF             L = new label;  push(L);  emit "jump conditionally to L"
THEN           L = pop();  emit "L:"
[ n CS-ROLL ]  stack manipulation: move the n-th label (where top is n=0) to the top

* Conditionally means "if false".

Additional Keywords

ELSE           equivalent to AHEAD  [ 1 CS-ROLL ]  THEN
WHILE          equivalent to IF  [ 1 CS-ROLL ]
REPEAT         equivalent to AGAIN  THEN
EXIT           emit "return from function"
0=             (before UNTIL, IF and WHILE): emit "negate logically"

Primitive Patterns

AHEAD  -->  THEN           jump forward
... IF  -->  THEN          jump forward conditionally
BEGIN  <--  AGAIN          jump backward
BEGIN  <--  ... UNTIL      jump backward conditionally

Structured Patterns

... IF ... THEN                 if-then
... IF ... ELSE ... THEN        if-then-else
BEGIN ... AGAIN                 infinite loop
BEGIN ... UNTIL                 do-while loop (with the condition negated)
BEGIN ... WHILE ... REPEAT      while loop

Example:

BEGIN ... UNTIL

Keyword	Code	Stack (top on the right)	C++ equivalent
BEGIN	A:	A	do { } while (!...);
...	...	A
UNTIL	jump conditionally to A

Example:

... IF ... ELSE ... THEN

Keyword	Primitive	Code	Stack (top on the right)	C++ equivalent
...		...		if (...) { ... } else { ... }
IF		jump conditionally to A	A
...		...	A
ELSE	AHEAD	jump to B	A B
	[ 1 CS-ROLL ]		B A
	THEN	A:	B
...		...	B
THEN		B:

Example:

BEGIN ... WHILE ... REPEAT

Keyword	Primitive	Code	Stack (top on the right)	C++ equivalent
BEGIN		A:	A	while (true) { if (!...) break; ... }
...		...	A
WHILE	IF	jump conditionally to B	A B
WHILE	[ 1 CS-ROLL ]		B A
...		...	B A
REPEAT	AGAIN	jump to A	B
REPEAT	THEN	B:

Example:

BEGIN BEGIN ... WHILE ... UNTIL ... [ 1 CS-ROLL ]  REPEAT

Keyword	Primitive	Code	Stack (top on the right)	C++ equivalent
BEGIN		A:	A	while (true) { if (!...) break; if (!...) continue; ... }
BEGIN		B:	A B
...		...	A B
WHILE	IF	jump conditionally to C	A B C
WHILE	[ 1 CS-ROLL ]		A C B
...		...	A C B
UNTIL		jump conditionally to B	A C
...		...	A C
[ 1 CS-ROLL ]			C A
REPEAT	AGAIN	jump to A	C
REPEAT	THEN	C:

Forth Control Flow

Primitive Keywords

Additional Keywords

Primitive Patterns

Structured Patterns

Example:
BEGIN ... UNTIL

Example:
... IF ... ELSE ... THEN

Example:
BEGIN ... WHILE ... REPEAT

Example:
BEGIN BEGIN ... WHILE ... UNTIL ... [ 1 CS-ROLL ] REPEAT