FunctionSwift

sequence(state:next:)

Returns a sequence formed from repeated lazy applications of next to a mutable state.

func sequence<T, State>(state: State, next: @escaping (inout State) -> T?) -> UnfoldSequence<T, State>

Parameters

state: The initial state that will be passed to the closure.
next: A closure that accepts an inout state and returns the next element of the sequence.

Returns

A sequence that yields each successive value from next.

Overview

The elements of the sequence are obtained by invoking next with a mutable state. The same state is passed to all invocations of next, so subsequent calls will see any mutations made by previous calls. The sequence ends when next returns nil. If next never returns nil, the sequence is infinite.

This function can be used to replace many instances of AnyIterator that wrap a closure.

Example:

// Interleave two sequences that yield the same element type
sequence(state: (false, seq1.makeIterator(), seq2.makeIterator()), next: { iters in
  iters.0 = !iters.0
  return iters.0 ? iters.1.next() : iters.2.next()
})