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Chain

[Source | Tests]

Concatenates two collections with the same element type, one after another.

This operation is available for any two sequences by calling the chain(_:_:) function.

let numbers = chain([10, 20, 30], 1...5)
// Array(numbers) == [10, 20, 30, 1, 2, 3, 4, 5]

let letters = chain("abcde", "FGHIJ")
// String(letters) == "abcdeFGHIJ"

Unlike placing two collections in an array and calling joined(), chaining permits different collection types, performs no allocations, and can preserve the shared conformances of the two underlying types.

Detailed Design

The chain(_:_:) function takes two sequences as arguments:

public func chain<S1, S2>(_ s1: S1, _ s2: S2) -> Chain2Sequence<S1, S2>
    where S1.Element == S2.Element

The resulting Chain2Sequence type is a sequence, with conditional conformance to Collection, BidirectionalCollection, and RandomAccessCollection when both the first and second arguments conform.

Naming

This function's and type's name match the term of art used in other languages and libraries.

This operation was previously implemented as a Sequence method named chained(with:), and was switched to a free function to align with APIs like zip and product after a lengthy forum discussion. Alternative suggestions for method names include appending(contentsOf:), followed(by:), and concatenated(to:).

Comparison with other languages

Rust: Rust provides a chain function that concatenates two iterators.

Ruby/Python: Ruby and Python’s itertools both define a chain function for concatenating collections of different kinds.