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call-by-name

call-by-name in Technology

reduction
(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.
(2006-05-27)

The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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