mapswhile

Mapswhile is a functional programming concept describing an operation that maps elements of a collection to new values while a predicate remains true for each element. It applies a function to each item in order and stops when the predicate fails for the current item; the resulting sequence contains the mapped values for all preceding items and excludes the rest.

Formally, given a sequence xs, a predicate p: X -> Bool, and a mapping f: X -> Y, mapswhile p f xs yields the prefix [f x1, f x2, ..., f xk], where p xi is true for i ≤ k and either k equals the length of xs or p x_{k+1} is false. In other words, processing ends at the first element that does not satisfy p, and no further elements are transformed.

Implementation can be eager or lazy depending on the language. A simple recursive definition is: mapswhile p f (x:xs) = if p x then f x : mapswhile p f xs else []; mapswhile p f [] = []. Edge cases include empty input yielding an empty result, and an all-true predicate producing a full mapped sequence. For infinite streams, mapswhile yields a finite prefix when the predicate eventually fails or the input ends.

Example: mapswhile (>0) sqrt [4, 9, 16, 25, -1] -> [2, 3, 4, 5]. This stops before applying sqrt to the negative value.

Usage and variants: mapswhile is useful in data processing pipelines where later work depends on a validity condition being met for each item. It is related to takeWhile and map, but differs from chaining map and takeWhile, which may require buffering or additional passes. See also map, takeWhile, and filter.