Custom Nimble matcher for use with the Promissum library

PromiseMatchers.swift

import Promissum
import Nimble

private func stringifyPromise<T, U>(promise: Promise<T, U>) -> String {
    switch promise.result {
    case nil: return "<pending>"
    case let .Value(value)?: return "<resolved: \(value)>"
    case let .Error(error)?: return "<rejected: \(error)>"
    }
}

private func promiseMatcher<T, U>(
    postfixMessage postfixMessage: String,
    checker: Result<T, U>? -> Bool) -> NonNilMatcherFunc<Promise<T, U>> {
    return NonNilMatcherFunc { actualExpression, failureMessage in
        failureMessage.postfixMessage = postfixMessage

        if let actualValue = try actualExpression.evaluate() {
            failureMessage.actualValue = stringifyPromise(actualValue)
            return checker(actualValue.result)
        }

        return false
    }
}

func beResolved<T, U>() -> NonNilMatcherFunc<Promise<T, U>> {
    return promiseMatcher(postfixMessage: "be resolved", checker: { result in
        switch result {
        case .Value?: return true
        default: return false
        }
    })
}

func beRejected<T, U>() -> NonNilMatcherFunc<Promise<T, U>> {
    return promiseMatcher(postfixMessage: "be rejected", checker: { result in
        switch result {
        case .Error?: return true
        default: return false
        }
    })
}

func bePending<T, U>() -> NonNilMatcherFunc<Promise<T, U>> {
    return promiseMatcher(postfixMessage: "be pending", checker: { result in
        switch result {
        case nil: return true
        default: return false
        }
    })
}

/////
struct Handler: AssertionHandler {
    func assert(assertion: Bool, message: FailureMessage, location: SourceLocation) {
        if !assertion {
            print("\(location): \(message.stringValue)")
        }
    }
}
NimbleAssertionHandler = Handler()
/////

let resolvedPromise: Promise<Int, NoError> = Promise(value: 42)
expect(resolvedPromise).to(beResolved())
expect(resolvedPromise).to(beRejected())
expect(resolvedPromise).to(bePending())

let rejectedPromise : Promise<(), NSError> = Promise(error: NSError(domain: "", code: 0, userInfo: nil))
expect(rejectedPromise).to(beResolved())
expect(rejectedPromise).to(beRejected())
expect(rejectedPromise).to(bePending())

let pendingPromise: Promise<Int, NoError> = PromiseSource().promise
expect(pendingPromise).to(beResolved())
expect(pendingPromise).to(beRejected())
expect(pendingPromise).to(bePending())