PBT-SwiftCheck.swift

func testConcatLengthIsEqualToSumOfLengths() {
    property("Concatenation length is equal to sum of input lengths") <- forAll { (a : String, b : String) in
        let output = concat(first: a, second: b)
        return output.characters.count == a.characters.count + b.characters.count
    }
}

func testConcatIsAssociative() {
    property("Concatenation is associative") <- forAll { (a : String, b : String, c : String) in
        let output1 = concat(first: concat(first: a, second: b), second: c)
        let output2 = concat(first: a, second: concat(first: b, second: c))
        return output1 == output2
    }
}

func testConcatHasEmptyStringAsNeutralElement() {
    property("Concatenation has empty String as neutral element") <- forAll { (a : String) in
        let output = concat(first: a, second: "")
        return output == a
    }
}