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 some code which

will return the value of the argument and an environment

giving the values of its free variables. This evaluationstrategy 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.

(1994-11-29)