valuewhile

Valuewhile is a conceptual control construct used in discussions of iterative computation. It refers to a process that repeatedly yields or processes a value while a given predicate remains true. The term emphasizes the continual generation of successive values that depend on the preceding ones, rather than just testing a condition in isolation.

Semantics and formal description

Valuewhile can be described as a function or loop that takes an initial value v0, a value-transform function f, and a predicate p. It produces a sequence of values v0, v1, v2, … where vi+1 = f(vi) and the iteration continues as long as p(vi) holds. A common convention defines the flow as: start with v = v0; while p(v) is true, emit v and then update v to f(v). The exact order of emission and predicate evaluation may vary by language or library, but the core idea is that both value generation and continuation depend on the current value.

Relationship to related constructs

Valuewhile is related to unfolds, streams, and generate or iterate operations found in functional programming. It differs from a plain takeWhile or a standard while loop by tying the continuation condition directly to the evolving value rather than only to a separate predicate over emitted results. In stream or lazy evaluation contexts, valuewhile can underpin generators that produce infinite or finite sequences based on a stopping predicate.

Example and use cases

A pseudo-code example:

v = v0

while p(v) is true:

emit v

v = f(v)

Common use cases include generating numerical sequences until a threshold, simulating processes where each state leads to a next state, and producing streams where emission is coupled to the progression of values.

See also

Unfold, iterate, takeWhile, generate, streams.