Short-circuit Evaluation

Short-circuit evaluation, minimal evaluation, or McCarthy evaluation denotes the semantics of some Boolean operators in some programming languages in which the second argument is only executed or evaluated 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 (Java, Ada), both short-circuit and standard Boolean operators are available. For some Boolean operations, like 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. ALGOL 68 used "proceduring" to achieve user defined short-circuit operators & procedures.

In loosely typed languages which have more than the two truth-values True and False, short-circuit operators may return the last evaluated subexpression, so that x Sor y and x Sand y are actually equivalent to if x then x else y and if x then y else x respectively (without actually evaluating x twice). This is called "Last value" in the table below.

In languages that use lazy evaluation by default (like Haskell), all functions are effectively "short-circuit", and special short-circuit operators are unnecessary.