bcquality/microsoft/knowledge/performance/boolean-operators-do-not-short-circuit.md

bc-version	domain	keywords	technologies	countries	application-area
all	performance	short-circuit lazy-evaluation boolean-operators nested-if guard and-operator or-operator xor-operator early-exit	al	w1	all

AL boolean operators do not short-circuit

Description

AL gives no short-circuit (lazy) evaluation guarantee for and, or, and xor: every operand of a boolean expression is evaluated, even when the leftmost operand already determines the result. Neither the AL operators documentation nor the boolean operators documentation defines a lazy evaluation order, so code must not depend on one. Developers arriving from C#, JavaScript, or SQL routinely assume the left operand guards the right; in AL it does not. xor is not actually a short-circuit candidate in any language — its result depends on both operands regardless of their values, so there is nothing to skip — but AL still evaluates both operands unconditionally, so neither should carry a cost or a risk the developer assumed the other would guard against. For and and or, the right operand still runs even when the left already decides the result, so its cost is paid on every evaluation, and a check intended to protect an unsafe expression — an array subscript, a division, a field read that is only valid after a successful Get — does not protect it.

Best Practice

For an and-shaped guard — a condition that must hold before the next operand is safe or worth evaluating — split into nested if statements: the guarding or cheapest condition in the outer if, the dependent or expensive one in the inner if. This preserves the result, since if A then if B then Action matches if A and B then Action exactly. Where there is no else branch, nesting is a pure win; where there is one, extract the conditions into a helper procedure that exits early instead.

For an or-shaped condition, do not nest: nesting if A then if B then Action drops the case where A is true and B is false, silently changing the result of A or B. Exit as soon as the cheap or safe operand already decides the outcome, and reach the other operand only on the path where it can still change the result — if A then exit(true); exit(B); for a boolean return, or if A then Action else if B then Action; when both branches share one action.

xor has no equivalent rewrite, because its result always depends on both operands; the only actionable guidance is to keep both operands of an xor cheap and free of side effects, since AL evaluates both unconditionally.

Where a chain of and-guards runs past about three conditions, stop nesting and use a case statement instead — see case-true-of-for-long-condition-chains.md. Keep and and or for operands that are independently safe and cheap — in-memory field comparisons, enum tests, bound checks — where combining them reads better and costs nothing.

See sample: boolean-operators-do-not-short-circuit.good.al.

Anti Pattern

A single condition that joins a guard with an operand depending on that guard, or with an expensive operand, using and or or. The consequence is either wasted work on every evaluation — a database call or validation procedure invoked even when the outcome is already decided — or a runtime error or silently wrong result that the guard was written to prevent. Applying the and fix to an or condition is a distinct mistake: rewriting A or B as nested ifs drops the A-true/B-false case instead of preserving it. Detection signals: an operand that indexes an array or list with a variable whose bounds are checked in a sibling operand; Record.Get(...) or a Find/IsEmpty call as one operand of and with a field read of the same record as another; an expensive or unsafe operand combined with or next to a condition that alone already makes the result true; a boolean-returning procedure call combined with a cheap field test. The pattern is common in code ported from a language that does short-circuit, and in conditions grown by appending a clause to an existing if.

See sample: boolean-operators-do-not-short-circuit.bad.al.

See also

case-true-of-for-long-condition-chains.md covers what to do when nesting an and-guard chain would go more than about three levels deep. microsoft/knowledge/performance/apply-guards-before-get.md covers the related ordering rule for statements rather than operands.