Computer Dictionary/call-by-name

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(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 [[Computer Dictionary/free variables|free variables]].

This evaluation strategy is guaranteed to reach a [[Computer Dictionary/normal form|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.

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