call-by-name

(reduction)

call-by-name - (CBN) (Normal order reduction, leftmost, outermost reduction). An argument passing convention (first provided by ALGOL 60?) where argument expressions are passed unevaluated. This is usually implemented by passing a pointer to a thunk - some code which will return the value of the argument and an environment giving the values of its free variables. This evaluation strategy is guaranteed to reach a normal form if one exists. When used to implement functional programming languages, call-by-name is usually combined with graph reduction to avoid repeated evaluation of the same expression. This is then known as call-by-need. The opposite of call-by-name is call-by-value where arguments are evaluated before they are passed to a function. This is more efficient but is less likely to terminate in the presence of infinite data structures and recursive functions. Arguments to macros are usually passed using call-by-name.