<compile-time if statement>

── ̍IF ──<compile-time Boolean expression>── THEN ─────────────────────►
►─<compile-time statement>─┬───────────────────────────────────┬───────┤
                           └─ ̍ELSE ──<compile-time statement>─┘

<compile-time Boolean expression>

Any <Boolean expression> that can be fully evaluated at compilation time. A compile-time Boolean expression consists of constants and <compile-time variable>s.

The 'IF statement provides conditional compilation of ALGOL source input.

If the value of the compile-time Boolean expression is TRUE, then the compile-time statement following THEN is processed; if it is FALSE, the compile-time statement following 'ELSE is processed, if present. In either case, compilation continues with the statement following the 'IF statement.

A <compile-time Boolean expression> can yield unexpected results if it is accessed when it has no valid value. This condition can be found in an IF statement that is nested in the ELSE part of another IF statement. Even though the outer IF evaluates to TRUE, the nested IF is also evaluated. This evaluation can produce conflicting results. If you have discovered this condition, enclose the IF statement in a 'BEGIN/'END pair. Once you have done this, the expression is evaluated only when the enclosing IF statement evaluates to FALSE.