6.1.1700
IF
 
CORE
Interpretation:
Interpretation semantics for this word are undefined.

Compilation:
( C: -- orig )

Put the location of a new unresolved forward reference orig onto the control flow stack. Append the run-time semantics given below to the current definition. The semantics are incomplete until orig is resolved, e.g., by THEN or ELSE.

Run-time:
( x -- )

If all bits of x are zero, continue execution at the location specified by the resolution of orig.

See:
3.2.3.2 Control-flow stack, 6.1.1310 ELSE, 6.1.2270 THEN, A.6.1.1700 IF.

Rationale:
Typical use:

   : X ... test IF ... THEN ... ;

or

   : X ... test IF ... ELSE ... THEN ... ;

Testing:
T{ : GI1 IF 123 THEN ; -> }T
T{ : GI2 IF 123 ELSE 234 THEN ; -> }T
T{  0 GI1 ->     }T
T{  1 GI1 -> 123 }T
T{ -1 GI1 -> 123 }T
T{  0 GI2 -> 234 }T
T{  1 GI2 -> 123 }T
T{ -1 GI1 -> 123 }T

\ Multiple ELSEs in an IF statement
: melse IF 1 ELSE 2 ELSE 3 ELSE 4 ELSE 5 THEN ;
T{ <FALSE> melse -> 2 4 }T
T{ <TRUE>  melse -> 1 3 5 }T