---
title: "sequence(state:next:)"
framework: swift
role: symbol
role_heading: Function
path: "swift/sequence(state:next:)"
---

# sequence(state:next:)

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

## Declaration

```swift
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.

## Return Value

Return Value A sequence that yields each successive value from next.

## Discussion

Discussion 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() })

## See Also

### Dynamic Sequences

- [sequence(first:next:)](swift/sequence(first:next:).md)