Callbyname

Call-by-name is a parameter passing mechanism used in some programming languages in which the actual argument expressions are not evaluated at the time of the call. Instead, each reference to a formal parameter inside the callee is replaced with the corresponding actual expression and evaluated in the caller’s environment every time the parameter is used. This makes the parameter behave like a symbolic alias to the expression rather than a stored value.

Origin and usage

Call-by-name was popularized by the Algol 60 language and is often described in contrast to call-by-value (evaluate arguments once before the call) and call-by-reference (pass an alias to the actual variable). Implementations historically used textual substitution or a mechanism called a thunk, which delays and re-evaluates the expression in the caller’s context whenever the parameter is accessed.

Semantics and implications

Under call-by-name, the argument expression can be evaluated multiple times, potentially with different results if the surrounding state changes between uses. Side effects in the argument expression are therefore observable each time the parameter is evaluated. If the formal parameter is assigned to inside the callee, the effect can propagate back to the actual parameter only when the actual parameter denotes a variable in the caller; otherwise, assignments may be illegal or have surprising effects. This evaluation strategy can lead to powerful abstractions but also to subtle bugs and inefficiencies due to repeated evaluation.

Relation to other strategies

Call-by-name is conceptually between substitution-based evaluation and reference-based binding. It contrasts with call-by-value, which fixes a computed value for the duration of the call, and with call-by-reference, which permits direct modification of the caller’s variables through the callee.

See also

Call-by-value, call-by-reference, thunks, Algol 60.