compactMap

compactMap is a higher-order function available on Swift’s Sequence and Collection types. It applies a transformation to each element and returns an array containing only the non-nil results. In effect, it combines mapping and filtering out nils in a single operation.

The usual form takes a closure that returns an optional value. For example, given a sequence of strings, strings.compactMap { Int($0) } yields [Int] by converting each string to an integer and discarding any nil results. There is also a throwing variant that rethrows, allowing the transform to throw and propagating errors to the caller.

Compared with map, which returns an array of the transformed values (possibly including nils if the transform returns an optional), compactMap unwraps non-nil results and omits nils. This makes it a common choice for operations like parsing or filtering while transforming elements.

In practice, compactMap is often used to combine conversion and validation steps. Example: ["1","2","x","3"].compactMap { Int($0) } results in [1, 2, 3]. It is a preferred approach when you want to skip invalid or missing results without requiring a separate filter step.

Relation to other operators: in modern Swift, compactMap is the dedicated method for removing nils produced by a transform. map can produce an array of optionals, while compactMap yields a compacted array of non-nil results.