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if

Use if to execute one statement only when a Boolean expression evaluates to True.

Syntax​

if BooleanExpression then
  Statement;

For several statements, make the branch a compound statement:

if BooleanExpression then
begin
  Statement1;
  Statement2;
end;

How it works​

TPSPascalCompiler.ProcessIf compiles the expression between if and then, writes its result to the compiler's default Boolean temporary, and emits a conditional jump over the following statement when the result is false. The condition is evaluated once each time execution reaches the if.

The expression must be assignable to the script compiler's Boolean type. Numeric, string, object and Variant values are not a substitute for an explicit test merely because some host-language contexts might treat them as truthy. Compare values directly or call an appropriate registered test such as VarIsNull.

Velox enables Boolean short-circuit evaluation. In a Boolean A and B, B is skipped when A is false. In A or B, B is skipped when A is true. This applies to Boolean and and or; integer bitwise operations evaluate both operands.

Example​

procedure ScriptEvent(var Value: Variant);
begin
  if VarIsNull(Value) then
    Value := 'No value supplied';
end;

The assignment runs only for a Null Variant. Empty strings, numeric zero and False are distinct values and do not satisfy VarIsNull.

Common mistakes​

  • Writing if Value then when Value is a Variant whose runtime content is not reliably Boolean. Use an explicit type/value test.
  • Expecting an indented second line to belong to the if. Indentation is for readers; only one statement follows then unless begin..end groups more.
  • Using assignment (:=) instead of equality (=) in the condition. Assignment is a statement, not a relational expression.
  • Depending on a function call in the right operand of short-circuited and or or for a required side effect. The call may not occur.

Edge cases and quirks​

  • A semicolon immediately after then creates an empty controlled statement, so the following statement runs unconditionally.
  • The compiler's short-circuit option is fixed by the Velox host setup. Do not rely on Delphi compiler directives to change it from an individual script.
  • Variant operations inside the condition can still raise runtime conversion, Null or type errors. Short-circuit guards are useful for preventing an unsafe right-hand access, but only when the left operand is itself safe.
  • A bare if has no value result. It controls statement execution; it is not Delphi's newer conditional expression syntax.

Errors and side effects​

A malformed expression or non-Boolean-compatible result produces a compile-time syntax/type error. Runtime errors from evaluated operands propagate through the current script exception path. Any function calls made while evaluating the condition keep their normal side effects, except when short-circuiting prevents their evaluation.

Performance and concurrency​

The branch itself is constant-time. Cost and thread-safety are determined by the condition and the selected statement, particularly calls into datasets or shared module objects.

  • if..else — adds an alternative branch.
  • Boolean — documents and, or and xor operand rules.
  • Relational (Comparison) — documents the comparisons normally used to form conditions.
  • Testing values — distinguishes Null, Empty, empty text, zero and False.

External references​