Eager evaluation

Evaluation strategies
Strict evaluation Applicative order Call by value Call by reference Call by sharing Call by copy-restore Non-strict evaluation Normal order Call by name Call by need/Lazy evaluation Call by macro expansion Nondeterministic strategies Full β-reduction Call by future Optimistic evaluation Other Partial evaluation Remote evaluation Short-circuit evaluation

In computer programming, eager evaluation or greedy evaluation is the evaluation strategy in most traditional programming languages. In eager evaluation an expression is evaluated as soon as it gets bound to a variable. The term is typically used to contrast lazy evaluation, where expressions are only evaluated when evaluating a dependent expression. Eager evaluation is almost exclusively used in imperative programming languages where the order of execution is implicitly defined by the source code organization.

One advantage of eager evaluation is that it eliminates the need to track and schedule the evaluation of expressions. It also allows the programmer to dictate the order of execution, making it easier to determine when sub-expressions (including functions) within the expression will be evaluated, as these sub-expressions may have side-effects that will affect the evaluation of other expressions.

A disadvantage of eager evaluation is that it forces the evaluation of expressions that may not be necessary at run time, or it may delay the evaluation of expressions that have a more immediate need. It also forces the programmer to organize the source code for optimal order of execution.

Note that many modern compilers are capable of scheduling execution to better optimize processor resources and can often eliminate unnecessary expressions from being executed entirely. Therefore, the notions of purely eager or purely lazy evaluation may not be applicable in practice.

Eager evaluation

See also