QuickCheck for Swift.

For those already familiar with the Haskell library, check out the source. For everybody else, see the Tutorial Playground for a beginner-level introduction to the major concepts and use-cases of this library.

SwiftCheck is a testing library that automatically generates random data for
testing of program properties. A property is a particular facet of an algorithm
or data structure that must be invariant under a given set of input data,
basically an `XCTAssert`

on steroids. Where before all we could do was define
methods prefixed by `test`

and assert, SwiftCheck allows program properties and
tests to be treated like *data*.

To define a program property the `forAll`

quantifier is used with a type
signature like `(A, B, C, ... Z) -> Testable where A : Arbitrary, B : Arbitrary ... Z : Arbitrary`

. SwiftCheck implements the `Arbitrary`

protocol for most Swift
Standard Library types and implements the `Testable`

protocol for `Bool`

and
several other related types. For example, if we wanted to test the property
that every Integer is equal to itself, we would express it as such:

```
func testAll() {
// 'property' notation allows us to name our tests. This becomes important
// when they fail and SwiftCheck reports it in the console.
property("Integer Equality is Reflexive") <- forAll { (i : Int) in
return i == i
}
}
```

For a less contrived example, here is a program property that tests whether Array identity holds under double reversal:

```
property("The reverse of the reverse of an array is that array") <- forAll { (xs : [Int]) in
// This property is using a number of SwiftCheck's more interesting
// features. `^&&^` is the conjunction operator for properties that turns
// both properties into a larger property that only holds when both sub-properties
// hold. `<?>` is the labelling operator allowing us to name each sub-part
// in output generated by SwiftCheck. For example, this property reports:
//
// *** Passed 100 tests
// (100% , Right identity, Left identity)
return
(xs.reverse().reverse() == xs) <?> "Left identity"
^&&^
(xs == xs.reverse().reverse()) <?> "Right identity"
}
```

Because SwiftCheck doesn't require tests to return `Bool`

, just `Testable`

, we
can produce tests for complex properties with ease:

```
property("Shrunken lists of integers always contain [] or [0]") <- forAll { (l : [Int]) in
// Here we use the Implication Operator `==>` to define a precondition for
// this test. If the precondition fails the test is discarded. If it holds
// the test proceeds.
return (!l.isEmpty && l != [0]) ==> {
let ls = self.shrinkArbitrary(l)
return (ls.filter({ $0 == [] || $0 == [0] }).count >= 1)
}
}
```

Properties can even depend on other properties:

```
property("Gen.one(of:) multiple generators picks only given generators") <- forAll { (n1 : Int, n2 : Int) in
let g1 = Gen.pure(n1)
let g2 = Gen.pure(n2)
// Here we give `forAll` an explicit generator. Before SwiftCheck was using
// the types of variables involved in the property to create an implicit
// Generator behind the scenes.
return forAll(Gen.one(of: [g1, g2])) { $0 == n1 || $0 == n2 }
}
```

All you have to figure out is what to test. SwiftCheck will handle the rest.

What makes QuickCheck unique is the notion of *shrinking* test cases. When fuzz
testing with arbitrary data, rather than simply halt on a failing test, SwiftCheck
will begin whittling the data that causes the test to fail down to a minimal
counterexample.

For example, the following function uses the Sieve of Eratosthenes to generate a list of primes less than some n:

```
/// The Sieve of Eratosthenes:
///
/// To find all the prime numbers less than or equal to a given integer n:
/// - let l = [2...n]
/// - let p = 2
/// - for i in [(2 * p) through n by p] {
/// mark l[i]
/// }
/// - Remaining indices of unmarked numbers are primes
func sieve(_ n : Int) -> [Int] {
if n <= 1 {
return []
}
var marked : [Bool] = (0...n).map { _ in false }
marked[0] = true
marked[1] = true
for p in 2..<n {
for i in stride(from: 2 * p, to: n, by: p) {
marked[i] = true
}
}
var primes : [Int] = []
for (t, i) in zip(marked, 0...n) {
if !t {
primes.append(i)
}
}
return primes
}
/// Short and sweet check if a number is prime by enumerating from 2...⌈√(x)⌉ and checking
/// for a nonzero modulus.
func isPrime(n : Int) -> Bool {
if n == 0 || n == 1 {
return false
} else if n == 2 {
return true
}
let max = Int(ceil(sqrt(Double(n))))
for i in 2...max {
if n % i == 0 {
return false
}
}
return true
}
```

We would like to test whether our sieve works properly, so we run it through SwiftCheck with the following property:

```
import SwiftCheck
property("All Prime") <- forAll { (n : Int) in
return sieve(n).filter(isPrime) == sieve(n)
}
```

Which produces the following in our testing log:

```
Test Case '-[SwiftCheckTests.PrimeSpec testAll]' started.
*** Failed! Falsifiable (after 10 tests):
4
```

Indicating that our sieve has failed on the input number 4. A quick look back at the comments describing the sieve reveals the mistake immediately:

```
- for i in stride(from: 2 * p, to: n, by: p) {
+ for i in stride(from: 2 * p, through: n, by: p) {
```

Running SwiftCheck again reports a successful sieve of all 100 random cases:

```
*** Passed 100 tests
```

SwiftCheck implements random generation for most of the types in the Swift
Standard Library. Any custom types that wish to take part in testing must
conform to the included `Arbitrary`

protocol. For the majority of types, this
means providing a custom means of generating random data and shrinking down to
an empty array.

For example:

```
import SwiftCheck
public struct ArbitraryFoo {
let x : Int
let y : Int
public var description : String {
return "Arbitrary Foo!"
}
}
extension ArbitraryFoo : Arbitrary {
public static var arbitrary : Gen<ArbitraryFoo> {
return Gen<(Int, Int)>.zip(Int.arbitrary, Int.arbitrary).map(ArbitraryFoo.init)
}
}
class SimpleSpec : XCTestCase {
func testAll() {
property("ArbitraryFoo Properties are Reflexive") <- forAll { (i : ArbitraryFoo) in
return i.x == i.x && i.y == i.y
}
}
}
```

There's also a `Gen.compose`

method which allows you to procedurally compose
values from multiple generators to construct instances of a type:

```
public static var arbitrary : Gen<MyClass> {
return Gen<MyClass>.compose { c in
return MyClass(
// Use the nullary method to get an `arbitrary` value.
a: c.generate(),
// or pass a custom generator
b: c.generate(Bool.suchThat { $0 == false }),
// .. and so on, for as many values and types as you need.
c: c.generate(), ...
)
}
}
```

`Gen.compose`

can also be used with types that can only be customized with setters:

```
public struct ArbitraryMutableFoo : Arbitrary {
var a: Int8
var b: Int16
public init() {
a = 0
b = 0
}
public static var arbitrary: Gen<ArbitraryMutableFoo> {
return Gen.compose { c in
var foo = ArbitraryMutableFoo()
foo.a = c.generate()
foo.b = c.generate()
return foo
}
}
}
```

For everything else, SwiftCheck defines a number of combinators to make working with custom generators as simple as possible:

```
let onlyEven = Int.arbitrary.suchThat { $0 % 2 == 0 }
let vowels = Gen.fromElements(of: [ "A", "E", "I", "O", "U" ])
let randomHexValue = Gen<UInt>.choose((0, 15))
let uppers = Gen<Character>.fromElements(in: "A"..."Z")
let lowers = Gen<Character>.fromElements(in: "a"..."z")
let numbers = Gen<Character>.fromElements(in: "0"..."9")
/// This generator will generate `.none` 1/4 of the time and an arbitrary
/// `.some` 3/4 of the time
let weightedOptionals = Gen<Int?>.frequency([
(1, Gen<Int?>.pure(nil)),
(3, Int.arbitrary.map(Optional.some))
])
```

For instances of many complex or "real world" generators, see
`ComplexSpec.swift`

.

SwiftCheck supports OS X 10.9+ and iOS 7.0+.

SwiftCheck can be included one of two ways:

**Using The Swift Package Manager**

- Add SwiftCheck to your
`Package.swift`

file's dependencies section:

`.package(url: "https://github.com/typelift/SwiftCheck.git", from: "0.8.1")`

**Using Carthage**

- Add SwiftCheck to your Cartfile
- Run
`carthage update`

- Drag the relevant copy of SwiftCheck into your project.
- Expand the Link Binary With Libraries phase
- Click the + and add SwiftCheck
- Click the + at the top left corner to add a Copy Files build phase
- Set the directory to
`Frameworks`

- Click the + and add SwiftCheck

**Using CocoaPods**

- Add our Pod to your podfile.
- Run
`$ pod install`

in your project directory.

**Framework**

- Drag SwiftCheck.xcodeproj into your project tree as a subproject
- Under your project's Build Phases, expand Target Dependencies
- Click the + and add SwiftCheck
- Expand the Link Binary With Libraries phase
- Click the + and add SwiftCheck
- Click the + at the top left corner to add a Copy Files build phase
- Set the directory to Frameworks
- Click the + and add SwiftCheck

SwiftCheck is released under the MIT license.