QuickCheck.sk

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

module QuickCheck;

/**
 * Property testing library borrowing ideas from Haskell's
 * [QuickCheck](http://hackage.haskell.org/package/QuickCheck).
 *
 * QuickCheck allows defining "properties" - conditions that should hold
 * for arbitrary values of some type(s) - and verifying that the property
 * does in fact hold given pseudo-randomly generated values of that type
 * Examples of such properties are "Int addition is commutative" or
 * "Vector::sort returns inputs in sorted order".
 *
 * When a property does not hold, QuickCheck attempts to find a minimal
 * failure case (where "minimal" is defined by the type of the input). This
 * can help to hone in on the nature of a bug by finding the exact value
 * that triggers edge-case behavior.
 *
 * ## Example Usage:
 *
 * ```
 * // The general form is to combine check() and forAll():
 * QuickCheck.check(Foo::generator(), foo ~> {
 *   isValid(foo) // => Bool
 * })
 *
 * // Test that converting an Int to/from a String equals the original Int:
 * QuickCheck.check(Int::generator(), x ~> {
 *   x.toString().toInt() == x
 * })
 *
 * // Test that Vector sorts ints:
 * QuickCheck.check(Vector::generator(Int::generator()), vec ~> {
 *   sorted = vec.sorted();
 *   prev = Int::min;
 *   for (x in sorted) {
 *     if (x < prev) { break false }
 *     !prev = x;
 *   } else true
 * })
 * ```
 *
 * ## Built-In Generators
 *
 * QuickCheck.sk supports generating values of the following types by default:
 * - `Bool::generator()`
 * - `Char::generator()`
 * - `Float::generator()`
 * - `Int::generator()`
 * - `String::generator()`
 * - `Order::generator()`
 * - `Vector::generator(<value-generator>)`, ie `Vector::generator(Int::generator())`
 * - `Map::generator(<key-generator>, <value-generator>)`
 * - `Set::generator(<value-generator>)`
 * - `Option::generator(<value-generator>)`
 * - Functions of arity 1 (`A ~> B`) with `QuickCheck.Fun1::generator<A, B>(B::generator())`
 *   Note the two type parameters for the input/output type of the generated function
 *   and that the generator must produce the output type (B, B::generator).
 *
 * ## Custom Generators
 *
 * To create a generator for a custom type, extend `QuickCheck.Generator`:
 *
 * ```
 * // Generator of `Foo`s
 * class FooGenerator extends QuickCheck.Generator<Foo> {
 *   fun generate(rng: mutable Random, sizeBound: Int): Foo {
 *     // create values from rng. for continuous values like Int/Float,
 *     // scale them to within [0, sizeBound)
 *     Foo(...)
 *   }
 * }
 * ```
 *
 * To make your type support shrinking (finding a minimal failure case),
 * extend `QuickCheck.Shrinkable`:
 *
 * ```
 * class Foo uses QuickCheck.Shrinkable {
 *   fun shrink(): mutable Iterator<Foo> {
 *     yield Foo(...)
 *     yield ...
 *   }
 * }
 * ```
 */

// ----------- PUBLIC API -----------

const defaultSeed: Int = 2;
const defaultAttempts: Int = 100;
const defaultMaxSize: Int = 30;

class Config{
  seed: Int = defaultSeed,
  attempts: Int = defaultAttempts,
  maxSize: Int = defaultMaxSize,
}

// Check that a test property holds. The test function should return true
// if the property holds for a given input, false otherwise.
fun check<A: Shrinkable & Show, T: Testable>(
  gen: Generator<A>,
  f: A ~> T,
  config: Config = Config{},
): TestResult {
  run(ForAllProperty(gen, f), config)
}

// Similar to check(), but designed for use in assertion tests where
// exceptions are used to indicate failure. The test function should
// throw if the test property does not hold for a given input. Assert()
// throws an AssertionFailure if the property fails for any input.
fun assert<A: Shrinkable & Show>(
  gen: Generator<A>,
  f: A ~> void,
  config: Config = Config{},
): void {
  test = a ~> {
    try {
      _ = f(a);
      Success(void)
    } catch {
    | exn -> Failure(exn.getMessage())
    }
  };
  assertEqual(run(ForAllProperty(gen, test), config), TestSuccess())
}

// Samples a generator - use this to see what values it will produce
// for a given Config.
fun sample<A>(gen: Generator<A>, config: Config = Config{}): Vector<A> {
  rng = Random::mcreate(config.seed);
  Vector::fillBy(config.attempts, index -> {
    size = index % config.maxSize;
    gen.generate(rng, size)
  })
}

// Returns an iterator that can be used to sample how a value will shrink.
// Shrinking a value is not guaranteed to terminate so the resulting iterator
// should be limited via take() or similar.
fun sampleShrink<A: Shrinkable & Show>(a: A): mutable Iterator<A> {
  buildShrinkTree(a).iterator();
}

// ----------- TRAITS/INTERFACES -----------

// A class that can generate values of type T from a pseudo-random source,
// optionally constraining the value based on a size.
mutable base class Generator<+T> {
  fun generate(rng: mutable Random, size: Int): T;

  static fun create(f: (mutable Random, Int) ~> T): Generator<T> {
    CreateComposedGenerator(f)
  }

  fun map<T2>(f: T ~> T2): Generator<T2> {
    MapComposedGenerator(this, f)
  }
}

// A type that has a default generator. There can be multiple generators for
// any given type, for example it may be useful to have a Generator of Ints
// that only yields positive values.
trait Generatable {
  static fun generator(): Generator<inst>;
}

// A type that supports shrinking to smaller values
trait Shrinkable {
  // `firstShrink` may be set to false when shrinking an already
  // shrunk value: the implementation should avoid including values
  // that are guaranteed to have been produced in an earlier shrink.
  // Examples:
  // - Non-zero Ints will always include 0 when firstShrink is true,
  //   and exclude it thereafter.
  // - Non-empty collections will include the empty collection
  //   when firstShrink is true, and exclude it thereafter.
  fun shrink(firstShrink: Bool = true): mutable Iterator<inst>;
}

// A type that can perturb a random value based on its value
trait Perturb {
  fun perturb(rng: mutable Random): void;
}

// A property that can be evaluated against random inputs
base class Property uses Testable {
  fun generator(): Generator<Tree<TestResult>>;

  fun property(): Property {
    this
  }
}

// A type that supports conversion to a Property
trait Testable {
  fun property(): Property;
}

// ----------- INTERNALS -----------

private class ForAllProperty<A: Shrinkable & Show, T: Testable>(
  gen: Generator<A>,
  f: A ~> T,
) extends Property {
  fun generator(): Generator<Tree<TestResult>> {
    ForAllGenerator(this.gen, this.f)
  }
}

private class ForAllGenerator<A: Shrinkable & Show, T: Testable>(
  gen: Generator<A>,
  f: A ~> T,
) extends Generator<Tree<TestResult>> {
  fun generate(rng: mutable Random, size: Int): Tree<TestResult> {
    seed1 = rng.next();
    seed2 = rng.next();
    arg = this.gen.generate(Random::mcreate(seed1), size);
    shrinkTree = buildShrinkTree(arg);
    f = this.f;
    resultTree = shrinkTree.map(x ~> {
      tree = f(x).property().generator().generate(Random::mcreate(seed2), size);
      tree.map(result ~> {
        result.flatMapFailure(failure ~> {
          failure with {
            counterExample => Vector[x.toString()].concat(
              failure.counterExample,
            ),
          }
        })
      })
    });
    resultTree.join()
  }
}

// Check that a test property holds.
private fun run<T: Testable>(test: T, config: Config): TestResult {
  rng = Random::mcreate(config.seed);
  property = test.property();
  generator = property.generator();
  size = -1;
  for (_ in Range(0, config.attempts)) {
    !size = (size + 1) % config.maxSize;
    tree = generator.generate(rng, size);
    visit(tree) match {
    | TestSuccess() -> void
    | failure @ TestFailure _ -> break failure with {seed => config.seed}
    }
  } else {
    TestSuccess()
  }
}

private fun buildShrinkTree<A: Shrinkable & Show>(
  a: A,
  firstShrink: Bool = true,
): Tree<A> {
  Tree(
    a,
    LazySource(() ~> a.shrink(firstShrink)).map(x ~> buildShrinkTree(x, false)),
  )
}

private fun visit(tree: Tree<TestResult>): TestResult {
  tree match {
  | Tree(success @ TestSuccess _, _) -> success
  | Tree(failure @ TestFailure _, descendants) ->
    descendants.find(t ~> t.root.isFailure()).map(visit).default(failure)
  }
}

// ----------- RESULT -----------

base class TestResult uses Testable, Show, Equality {
  children =
  | TestSuccess()
  | TestFailure(seed: Int, counterExample: Vector<String>)

  fun flatMapFailure(f: TestFailure -> TestResult): TestResult
  | TestSuccess() -> TestSuccess()
  | failure @ TestFailure _ -> f(failure)

  fun isFailure(): Bool
  | TestSuccess() -> false
  | TestFailure _ -> true

  fun isSuccess(): Bool
  | TestSuccess() -> true
  | TestFailure _ -> false

  fun fromFailure(): TestFailure
  | TestSuccess _ -> invariant_violation("fromFailure() called on TestSuccess")
  | failure @ TestFailure _ -> failure

  fun property(): Property {
    TestResultProperty(this)
  }

  fun ==(other: TestResult): Bool {
    (this, other) match {
    | (TestSuccess _, TestSuccess _) -> true
    | (
      TestFailure(seed1, counterExample1),
      TestFailure(seed2, counterExample2),
    ) ->
      seed1 == seed2 && counterExample1 == counterExample2
    | _ -> false
    }
  }

  fun toString(): String
  | TestSuccess() -> "TestSuccess"
  | TestFailure(seed, counterExample) ->
    `TestFailure: seed=${seed} counterExample=[${counterExample.join(", ")}]`
}

private class TestResultProperty(result: TestResult) extends Property {
  fun generator(): Generator<Tree<TestResult>> {
    TestResultGenerator(this.result)
  }
}

private class TestResultGenerator(
  result: TestResult,
) extends Generator<Tree<TestResult>> {
  fun generate(_rng: mutable Random, _size: Int): Tree<TestResult> {
    Tree(this.result, Lazy::empty())
  }
}

// ----------- TREE -----------

private class Tree<T>(root: T, descendants: Lazy<Tree<T>>) {
  fun join<T2>[T: Tree<T2>](): Tree<T2> {
    outer = this.descendants;
    root = this.root.root;
    inner = this.root.descendants;
    Tree(root, outer.map(n ~> n.join()).concat(inner))
  }
  fun map<T2>(f: T ~> T2): Tree<T2> {
    Tree(f(this.root), this.descendants.map(node ~> node.map(f)))
  }

  fun iterator(): mutable Iterator<T> {
    yield this.root;
    for (descendant in this.descendants.iterator()) {
      for (x in descendant.iterator()) {
        yield x
      }
    }
  }
}

private base class Lazy<+T>() {
  static fun empty(): Lazy<T> {
    EmptyLazy()
  }

  fun iterator(): mutable Iterator<T>;

  fun map<T2>(f: T ~> T2): Lazy<T2> {
    MapLazy(this, f)
  }

  fun concat<T2>[T: T2](second: Lazy<T2>): Lazy<T2> {
    ConcatLazy(this, second)
  }

  fun find(p: T ~> Bool): ?T {
    this.iterator().find(p)
  }

  fun each(f: T -> void): void {
    this.iterator().each(f)
  }
}

private class LazySource<+T>(
  source: () ~> mutable Iterator<T>,
) extends Lazy<T> {
  fun iterator(): mutable Iterator<T> {
    this.source()
  }
}

private class EmptyLazy<+T>() extends Lazy<T> {
  fun iterator(): mutable Iterator<T> {
    yield break;
  }
}

private class MapLazy<T, T2>(source: Lazy<T>, f: T ~> T2) extends Lazy<T2> {
  fun iterator(): mutable Iterator<T2> {
    this.source.iterator().map(this.f)
  }
}

private class ConcatLazy<T>(source: Lazy<T>, second: Lazy<T>) extends Lazy<T> {
  fun iterator(): mutable Iterator<T> {
    this.source.iterator().concat(this.second.iterator())
  }
}

// ----------- INT -----------

extension class .Int uses Generatable, Perturb, Shrinkable {
  static fun generator(): Generator<Int> {
    IntGenerator()
  }

  fun perturb(rng: mutable Random): void {
    rng.perturb(this)
  }

  fun shrink(firstShrink: Bool = true): mutable Iterator<Int> {
    if (this != 0) {
      absThis = Math.abs(this);
      if (this < 0) {
        yield absThis;
      };
      if (firstShrink) {
        yield 0;
      };
      i = this / 2;
      m = this - i;
      while (Math.abs(m) < absThis) {
        yield m;
        !i = i / 2;
        !m = this - i;
      };
    }
  }
}

class IntGenerator() extends Generator<Int> {
  fun generate(rng: mutable Random, size: Int): Int {
    rng.random(-size, size + 1)
  }
}

// ----------- BOOL -----------

extension class .Bool uses Generatable, Perturb, Shrinkable, Testable {
  static fun generator(): Generator<Bool> {
    BoolGenerator()
  }

  fun perturb(rng: mutable Random): void {
    rng.perturb(if (this) 1 else 0)
  }

  fun shrink(_firstShrink: Bool = true): mutable Iterator<Bool> {
    if (this) {
      yield false;
    }
  }

  fun property(): Property {
    if (this) {
      TestResultProperty(TestSuccess())
    } else {
      TestResultProperty(TestFailure(0, Vector[]))
    }
  }
}

class BoolGenerator() extends Generator<Bool> {
  fun generate(rng: mutable Random, _size: Int): Bool {
    rng.randomBool()
  }
}

// ----------- Char -----------

extension class .Char uses Generatable, Shrinkable {
  static fun generator(): Generator<Char> {
    CharGenerator()
  }

  fun shrink(firstShrink: Bool = true): mutable Iterator<Char> {
    // TODO: Better char shrinking
    this.code().shrink(firstShrink).filter(Char::isValidCharCodePoint).map(x ~>
      x.chr()
    )
  }
}

class CharGenerator() extends Generator<Char> {
  fun generate(rng: mutable Random, size: Int): Char {
    // TODO: Generate a wider range of Chars
    loop {
      code = rng.random(0, size + 1);
      if (Char::isValidCharCodePoint(code)) {
        return code.chr()
      }
    }
  }
}

// ----------- Float -----------

extension class .Float uses Generatable, Shrinkable {
  static fun generator(): Generator<Float> {
    FloatGenerator()
  }

  fun shrink(firstShrink: Bool = true): mutable Iterator<Float> {
    if (this < 0.0) {
      yield -this
    };
    truncated = if (this > 0.0) {
      this.toInt()
    } else {
      this.negate().toInt().negate()
    };
    for (x in truncated.shrink(firstShrink)) {
      yield x.toFloat()
    }
  }
}

class FloatGenerator() extends Generator<Float> {
  fun generate(rng: mutable Random, size: Int): Float {
    rng.randomFloat() * (size * 2).toFloat() - size.toFloat()
  }
}

// ----------- String -----------

extension class .String uses Generatable, Shrinkable {
  static fun generator(): Generator<String> {
    StringGenerator()
  }

  fun shrink(firstShrink: Bool = true): mutable Iterator<String> {
    // TODO: better String shrinking
    chars = this.getIter().collect(Vector);
    chars.shrink(firstShrink).map(x ~> String::fromChars(x.toArray()))
  }
}

class StringGenerator() extends Generator<String> {
  fun generate(rng: mutable Random, sizeBound: Int): String {
    // TODO: better String generation
    size = rng.random(0, sizeBound + 1);
    chars = CharGenerator();
    String::fromChars(Array::fillBy(size, _ -> chars.generate(rng, sizeBound)))
  }
}

// ----------- Order -----------

extension base class .Order uses Perturb, Generatable, Shrinkable {
  static fun generator(): Generator<Order> {
    OrderGenerator()
  }

  fun perturb(rng: mutable Random): void {
    rng.perturb(
      this match {
      | LT() -> 0
      | EQ() -> 1
      | GT() -> 2
      },
    )
  }

  fun shrink(_firstShrink: Bool = true): mutable Iterator<Order>
  | LT() -> yield break
  | EQ() -> yield LT()
  | GT() ->
    yield EQ();
    yield LT()
}

class OrderGenerator() extends Generator<Order> {
  fun generate(rng: mutable Random, _size: Int): Order {
    Array[LT(), EQ(), GT()][rng.random(0, 3)]
  }
}

// ----------- Option -----------

extension base class .Option uses
  Perturb[T: Perturb],
  Shrinkable[T: Shrinkable],
{
  static fun generator<T2>(generator: Generator<T2>): Generator<?T2> {
    OptionGenerator(generator)
  }

  fun perturb[T: Perturb](rng: mutable Random): void
  | None() -> rng.perturb(0)
  | Some(x) ->
    rng.perturb(1);
    x.perturb(rng)

  fun shrink[T: Shrinkable](firstShrink: Bool = true): mutable Iterator<?T>
  | None() -> yield break
  | Some(x) ->
    yield None();
    for (s in x.shrink(firstShrink)) {
      yield Some(s)
    }
}

class OptionGenerator<T>(generator: Generator<T>) extends Generator<?T> {
  fun generate(rng: mutable Random, size: Int): ?T {
    freq = rng.random(0, 4); // weighted random value biasing towards Some
    if (freq == 0) {
      None()
    } else {
      Some(this.generator.generate(rng, size))
    }
  }
}

// ----------- TestResult -----------

extension base class .Result uses
  Testable[E: readonly Show],
  Perturb[T: Perturb, E: Perturb],
  Shrinkable[T: Shrinkable, E: Shrinkable],
{
  static fun generator<T2, E2>(
    successGenerator: Generator<T2>,
    failureGenerator: Generator<E2>,
  ): Generator<Result<T2, E2>> {
    ResultGenerator(successGenerator, failureGenerator)
  }

  fun perturb[T: Perturb, E: Perturb](rng: mutable Random): void
  | .Success(s) ->
    rng.perturb(0);
    s.perturb(rng)
  | .Failure(f) ->
    rng.perturb(1);
    f.perturb(rng)

  fun shrink[T: Shrinkable, E: Shrinkable](
    firstShrink: Bool = true,
  ): mutable Iterator<Result<T, E>>
  | .Success(s) ->
    s.shrink(firstShrink).map(s2 ~> (.Success(s2) : Result<T, E>))
  | .Failure(f) ->
    f.shrink(firstShrink).map(f2 ~> (.Failure(f2) : Result<T, E>))

  fun property[E: readonly Show](): Property
  | .Success _ -> TestResultProperty(TestSuccess())
  | .Failure(f) -> TestResultProperty(TestFailure(0, Vector[f.toString()]))
}

class ResultGenerator<T, E>(
  successGenerator: Generator<T>,
  failureGenerator: Generator<E>,
) extends Generator<Result<T, E>> {
  fun generate(rng: mutable Random, size: Int): Result<T, E> {
    freq = rng.random(0, 3); // weighted random value biasing towards Success
    if (freq == 0) {
      .Failure(this.failureGenerator.generate(rng, size))
    } else {
      .Success(this.successGenerator.generate(rng, size))
    }
  }
}

// ----------- Vector -----------

extension class .Vector uses Perturb[T: Perturb], Shrinkable[T: Shrinkable] {
  static fun generator<U>(generator: Generator<U>): Generator<Vector<U>> {
    VectorGenerator(generator)
  }

  fun perturb[T: Perturb](rng: mutable Random): void {
    for (x in this) {
      x.perturb(rng)
    }
  }

  fun shrink[T: Shrinkable](
    firstShrink: Bool = true,
  ): mutable Iterator<Vector<T>> {
    // TODO: better Vector shrinking
    if (!this.isEmpty()) {
      for (last in this.last().shrink(firstShrink)) {
        yield Vector[last]
      };
      for (size in this.size().shrink(firstShrink)) {
        yield this.slice(0, size);
      };
    };
  }
}

class VectorGenerator<T>(generator: Generator<T>) extends Generator<Vector<T>> {
  fun generate(rng: mutable Random, sizeBound: Int): Vector<T> {
    size = rng.random(0, sizeBound + 1);
    Vector::fillBy(size, _ -> this.generator.generate(rng, sizeBound))
  }
}

// ----------- Map -----------

extension class .Map uses
  Perturb[K: Perturb, V: Perturb],
  Shrinkable[K: Shrinkable, V: Shrinkable],
{
  static fun generator<K2: Hashable & Equality, V2>(
    keyGenerator: Generator<K2>,
    valueGenerator: Generator<V2>,
  ): Generator<Map<K2, V2>> {
    MapGenerator(keyGenerator, valueGenerator)
  }

  fun perturb[K: Perturb, V: Perturb](rng: mutable Random): void {
    for (k => v in this) {
      k.perturb(rng);
      v.perturb(rng)
    }
  }

  fun shrink[K: Shrinkable, V: Shrinkable](
    _firstShrink: Bool = true,
  ): mutable Iterator<Map<K, V>> {
    // TODO: Map shrinking
    if (!this.isEmpty()) {
      yield Map[]
    };
  }
}

class MapGenerator<K: Hashable & Equality, V>(
  keyGenerator: Generator<K>,
  valueGenerator: Generator<V>,
) extends Generator<Map<K, V>> {
  fun generate(rng: mutable Random, sizeBound: Int): Map<K, V> {
    size = rng.random(0, sizeBound + 1);
    map = Map::mcreate(size);
    for (_ in Range(0, size)) {
      map.set(
        this.keyGenerator.generate(rng, sizeBound),
        this.valueGenerator.generate(rng, sizeBound),
      );
    };
    map.chill()
  }
}

// ----------- Set -----------

extension class .Set uses Perturb[T: Perturb], Shrinkable[T: Shrinkable] {
  static fun generator<T2: Hashable & Equality>(
    generator: Generator<T2>,
  ): Generator<Set<T2>> {
    SetGenerator(generator)
  }

  fun perturb[T: Perturb](rng: mutable Random): void {
    for (x in this) {
      x.perturb(rng);
    }
  }

  fun shrink[T: Shrinkable](
    _firstShrink: Bool = true,
  ): mutable Iterator<Set<T>> {
    // TODO: Set shrinking
    if (!this.isEmpty()) {
      yield Set[]
    };
  }
}

class SetGenerator<T: Hashable & Equality>(
  generator: Generator<T>,
) extends Generator<Set<T>> {
  fun generate(rng: mutable Random, sizeBound: Int): Set<T> {
    size = rng.random(0, sizeBound + 1);
    set = Set::mcreate(size);
    for (_ in Range(0, size)) {
      set.insert(this.generator.generate(rng, sizeBound));
    };
    set.chill()
  }
}

// ----------- Tuple2 -----------

extension class .Tuple2 uses
  Perturb[T0: Perturb, T1: Perturb],
  Shrinkable[T0: Shrinkable, T1: Shrinkable],
{
  static fun generator(
    g0: Generator<T0>,
    g1: Generator<T1>,
  ): Generator<Tuple2<T0, T1>> {
    Tuple2Generator(g0, g1)
  }

  fun perturb[T0: Perturb, T1: Perturb](rng: mutable Random): void {
    this.i0.perturb(rng);
    this.i1.perturb(rng);
  }

  fun shrink[T0: Shrinkable, T1: Shrinkable](
    firstShrink: Bool = true,
  ): mutable Iterator<Tuple2<T0, T1>> {
    for (i0 in this.i0.shrink(firstShrink)) {
      yield (i0, this.i1);
    };
    for (i1 in this.i1.shrink(firstShrink)) {
      yield (this.i0, i1)
    }
  }
}

class Tuple2Generator<T0, T1>(
  g0: Generator<T0>,
  g1: Generator<T1>,
) extends Generator<Tuple2<T0, T1>> {
  fun generate(rng: mutable Random, sizeBound: Int): Tuple2<T0, T1> {
    i0 = this.g0.generate(rng, sizeBound);
    i1 = this.g1.generate(rng, sizeBound);
    (i0, i1)
  }
}

// ----------- Function (arity 1) -----------

class Fun1<I1, O: frozen>(f: I1 ~> O) uses Show, Shrinkable[O: Shrinkable] {
  static fun generator<I1: Perturb, O: frozen>(
    generator: Generator<O>,
  ): Generator<Fun1<I1, O>> {
    Fun1Generator<I1, O>(generator)
  }

  fun toString(): String {
    "<<function>>"
  }
  fun shrink[O: Shrinkable](_firstShrink: Bool = true): mutable Iterator<this> {
    yield break;
  }
}

private class Fun1Generator<I1: Perturb, O: frozen>(
  generator: Generator<O>,
) extends Generator<Fun1<I1, O>> {
  fun generate(rng: mutable Random, size: Int): Fun1<I1, O> {
    seed = {
      seed = rng.next();
      while (seed == 0) {
        !seed = rng.next()
      };
      seed
    };
    generator = this.generator;
    Fun1(i1 ~> {
      rng1 = Random::mcreate(seed);
      i1.perturb(rng1);
      generator.generate(rng1, size);
    })
  }
}

// ----------- Generator Composition -----------

class CreateComposedGenerator<T>(
  f: (mutable Random, Int) ~> T,
) extends Generator<T> {
  fun generate(rng: mutable Random, sizeBound: Int): T {
    this.f(rng, sizeBound)
  }
}

class MapComposedGenerator<T, T2>(
  generator: Generator<T>,
  f: T ~> T2,
) extends Generator<T2> {
  fun generate(rng: mutable Random, sizeBound: Int): T2 {
    this.f(this.generator.generate(rng, sizeBound))
  }
}

module end;