Sourcewhile

Sourcewhile is a control-flow construct found in some programming languages and library designs that blends a stream or source of values with looping behavior. In a sourcewhile loop, the next iteration begins by retrieving a value from a source expression, and the loop continues as long as a value is produced and, optionally, an additional predicate on that value holds. The construct is designed for scenarios in which the loop body consumes a sequence of inputs produced by an ongoing source rather than a locally computed condition.

Semantics of sourcewhile are centered on tying each iteration to an input value. On each pass, the incoming value is bound to a loop variable used by the body. If the source is exhausted, the loop terminates. If a predicate is supplied and evaluates to false, the loop terminates as well. The source can be lazy, asynchronous, or blocking, depending on the language, enabling streaming or coroutine-style workflows. Exceptions raised by the source or inside the body may propagate or be handled by language-level constructs.

Syntax and variants vary by language but share the same core idea. A common form is sourcewhile x from S where P(x) do ... end; other forms may embed the guard into the source or specify it as a separate condition. An example in pseudo-syntax: sourcewhile x from numbers where x < 100 do process(x) end. The exact syntax depends on the language, but the semantics remain consistent: feed values from a source into the loop until the feed stops or the guard fails.

Relation to related constructs includes for-each loops, while loops, and generator-based coroutines. In practice, sourcewhile supports data-driven iteration used in streaming, pipeline stages, and reactive-style programming, with attention to backpressure, laziness, and error handling.