if
IF tf instructionlist
(IF tf instructionlist1 instructionlist2)
command. If the first input has the value TRUE, then IF runs
the second input. If the first input has the value FALSE, then
IF does nothing. (If given a third input, IF acts like IFELSE,
as described below.) It is an error if the first input is not
either TRUE or FALSE.
For compatibility with earlier versions of Logo, if an IF
instruction is not enclosed in parentheses, but the first thing
on the instruction line after the second input expression is a
literal list (i.e., a list in square brackets), the IF is
treated as if it were IFELSE, but a warning message is given.
If this aberrant IF appears in a procedure body, the warning is
given only the first time the procedure is invoked in each Logo
session.