Short-circuit evaluation

Short-circuit evaluation, minimal evaluation, or McCarthy evaluation (after John McCarthy) is the semantics of some Boolean operators in some programming languages in which the second argument is executed or evaluated only if the first argument does not suffice to determine the value of the expression: when the first argument of the AND function evaluates to false, the overall value must be false; and when the first argument of the OR function evaluates to true, the overall value must be true. In some programming languages (Lisp), the usual Boolean operators are short-circuit. In others (Ada, Java), both short-circuit and standard Boolean operators are available. For some Boolean operations, like exclusive or (XOR), it is not possible to short-circuit, because both operands are always required to determine the result.

The short-circuit expression x Sand y (using Sand to denote the short-circuit variety) is equivalent to the conditional expression if x then y else false; the expression x Sor y is equivalent to if x then true else y.

Short-circuit operators are, in effect, control structures rather than simple arithmetic operators, as they are not strict. In imperative language terms (notably C and C++), where side effects are important, short-circuit operators introduce a sequence point – they completely evaluate the first argument, including any side effects, before (optionally) processing the second argument. ALGOL 68 used proceduring to achieve user defined short-circuit operators & procedures.

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