A very small TDD DSL in Swift

Given+Expectation.swift

class Given<T> {
    var sut: T

    init(_ sut: T) {
        self.sut = sut
    }

    func the<U>(_ keyPath: WritableKeyPath<T, U>, is u: U) -> Given {
        sut[keyPath: keyPath] = u
        return self
    }

    func apply(_ c: (T) -> Void) -> Given {
        c(sut)
        return self
    }

    func map<U>(_ c: (T) -> U) -> Given<U> {
        return Given<U>(c(sut))
    }

    var expect: Expectation<T> {
        return Expectation(sut: sut)
    }
}

struct Expectation<T> {
    let sut: T

    @discardableResult
    func the<U: Equatable>(_ keyPath: KeyPath<T, U>,
                           toBe value: U) -> Expectation {
        XCTAssertEqual(sut[keyPath: keyPath], value)
        return self
    }

    @discardableResult
    func the<U: Comparable>(_ keyPath: KeyPath<T, U>,
                            toBeGreaterThan value: U) -> Expectation {
        XCTAssertGreaterThan(sut[keyPath: keyPath], value)
        return self
    }

    @discardableResult
    func the<U: Comparable>(_ keyPath: KeyPath<T, U>,
                            toBeLessThan value: U) -> Expectation {
        XCTAssertLessThan(sut[keyPath: keyPath], value)
        return self
    }

    @discardableResult
    func the<U: Equatable>(_ keyPath: KeyPath<T, U>,
                           toNotBe value: U) -> Expectation {
        XCTAssertNotEqual(sut[keyPath: keyPath], value)
        return self
    }

    @discardableResult
    func nilValueFor<U: Equatable>(_ keyPath: KeyPath<T, U?>) -> Expectation {
        XCTAssertNil(sut[keyPath: keyPath])
        return self
    }

    @discardableResult
    func nonNilValueFor<U: Equatable>(_ keyPath: KeyPath<T, U?>) -> Expectation {
        XCTAssertNotNil(sut[keyPath: keyPath])
        return self
    }
}