FunctionSwift

sequence(first:next:)

Returns a sequence formed from first and repeated lazy applications of next.

func sequence<T>(first: T, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T>

Parameters

first

The first element to be returned from the sequence.

next

A closure that accepts the previous sequence element and returns the next element.

Returns

A sequence that starts with first and continues with every value returned by passing the previous element to next.

Overview

The first element in the sequence is always first, and each successive element is the result of invoking next with the previous element. The sequence ends when next returns nil. If next never returns nil, the sequence is infinite.

This function can be used to replace many cases that were previously handled using C-style for loops.

Example:

// Walk the elements of a tree from a node up to the root
for node in sequence(first: leaf, next: { $0.parent }) {
  // node is leaf, then leaf.parent, then leaf.parent.parent, etc.
}

// Iterate over all powers of two (ignoring overflow)
for value in sequence(first: 1, next: { $0 * 2 }) {
  // value is 1, then 2, then 4, then 8, etc.
}