adding beginWith matcher for sequence prefixes

[Skoti]

Hi I would like to add a convenience matcher for sequence prefixes.
So that we can expect a particular sequence to start with provided elements.

Checklist - While not every PR needs it, new features should consider this list:

• Does this have tests?
• Does this have documentation?
• Does this break the public API (Requires major version bump)?
• Is this a new feature (Requires minor version bump)?

[ikesyo]

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?

[Skoti]

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:

Nimble/Sources/Nimble/Matchers/BeginWith.swift

Lines 5 to 6 in bfcdaca

	public func beginWith<S: Sequence, T: Equatable>(_ startingElement: T) -> Predicate<S>
	where S.Iterator.Element == T {

and fails with a hard to understand error (at least at first glimpse):
failed - expected to begin with <[]>, got <[]>.

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.

[ikesyo]

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.

[ikesyo]

Just a few minor comments but entirely looks good 👍

[ikesyo]

Thanks for your contribution!