Thanks for the pull request! While this should be useful, I think the name would confused with the existing beginWith matcher. How about making it beginWith overload?

I could, however, this case:

let y = [[Int]]()
expect(y).to(beginWith([]))

won't work unless you specify the type explicitly:

expect(y).to(beginWith([] as [[Int]]))

without explicit type cast it calls this:

also this case won't work for the same reason:

let y = [Int]()
expect([]).to(beginWith(y))
// or hypothetically
expect([]).to(beginWith([] as [Int]))

unless:

expect([] as [Int]).to(beginWith(y))
expect([] as [Int]).to(beginWith([]))

So to avoid confusion and add more convenience at call site, in these and maybe some other cases, I have named it startWith.

Fair point, thanks for pointing it out. But I'm still uncomfortable about beginWith/startWith coexistence.

Let's see Sequence.starts(with:) signature:

func starts<PossiblePrefix>(with possiblePrefix: PossiblePrefix) -> Bool where PossiblePrefix : Sequence, Self.Element == PossiblePrefix.Element

I propose beginWith(prefix:) for this matcher. The "prefix" word would be suitable in connection with PossiblePrefix/possiblePrefix names and Collection.prefix(_:) API.