lazyevaluation

Lazy evaluation, sometimes written as lazyevaluation in programming discussions, is an evaluation strategy in which expressions are not evaluated when bound to variables, but are deferred until their values are actually required. This deferral can avoid unnecessary computations and permit the construction of infinite data structures and complex control flows.

In practice, lazy evaluation is usually implemented with thunks—representations of suspended computations. When a value is needed, the thunk is evaluated, and the result is typically cached (memoized) so subsequent uses reuse the same value. There is a distinction between call-by-name, where re-evaluation can occur each time the value is demanded, and call-by-need, where the computed result is stored for future use. The latter is the common form of lazy evaluation in modern languages.

Benefits include potential efficiency gains when computations can be skipped, the ability to represent infinite or very large data structures, and natural support for short-circuiting in conditional expressions. It also often aligns with referential transparency, supporting reasoning about programs without side effects.

Drawbacks include variability in execution time, possible memory growth due to unevaluated thunks, and the risk of space leaks if not carefully managed. Debugging can be harder because the order of evaluation is not explicit, and interactions with side effects can be subtle.

Languages and implementations: languages such as Haskell use lazy evaluation by default, while others offer laziness through constructs or libraries that delay evaluation (for example, streams or promises). Lazy evaluation can be combined with strict evaluation in controlled regions.