Shrinker<A>

class Shrinker<A>

Knows how to shrink a value of type A toward a simpler form.

When a property-based test fails, the framework feeds the failing value to the appropriate shrinker and re-runs the predicate on each candidate until no simpler counter-example can be found. Shrinkers are composable: use xmap to derive a shrinker for any type that is isomorphic to A.

Constructors

Shrinker()

Shrinker(ILazyList<A> Function(A) _shrinkF)

Creates a Shrinker from a function that produces shrink candidates.

The function should return candidates in order from simplest to most complex so that the shrinking loop terminates quickly.

Implementation

dart

Shrinker(this._shrinkF);

Properties

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

dart

external int get hashCode;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

dart

external Type get runtimeType;

Methods

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dart

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

dart

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

dart

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

shrink()

ILazyList<A> shrink(A a)

Returns an ILazyList of candidates simpler than a.

Implementation

dart

ILazyList<A> shrink(A a) => _shrinkF(a);

toString() inherited

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Inherited from Object.

Implementation

dart

external String toString();

xmap()

Shrinker<B> xmap<B>(B Function(A) f, A Function(B) g)

Derives a Shrinker for B via an isomorphism between A and B.

f converts an A candidate to B, and g converts the B value under test back to A so that the underlying shrinker can be reused.

Implementation

dart

Shrinker<B> xmap<B>(
  Function1<A, B> f,
  Function1<B, A> g,
) => Shrinker((B b) => shrink(g(b)).map(f));

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

Total: It must return a boolean for all arguments. It should never throw.
Reflexive: For all objects o, o == o must be true.
Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.
Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

dart

external bool operator ==(Object other);

Static Properties

dubble final

final Shrinker<double> dubble

Shrinker for double values.

Returns 0.0 first, then successive values that halve the distance between the candidate and zero, stopping when the delta drops below 1e-12. Returns an empty list for 0.0.

Implementation

dart

static final Shrinker<double> dubble = Shrinker<double>((d) {
  if (d == 0.0) {
    return ILazyList.empty();
  } else {
    return ILazyList.unfold<double, (bool, double)>(
      (false, d.abs() &#47; 2.0),
      (state) {
        final (zeroEmitted, delta) = state;

        if (!zeroEmitted) {
          return Some((0.0, (true, delta)));
        } else if (delta > 1e-12) {
          return Some((d - delta * d.sign, (true, delta &#47; 2.0)));
        } else {
          return const None();
        }
      },
    );
  }
});

integer final

final Shrinker<int> integer

Shrinker for int values.

Returns 0 first, then successive values that halve the absolute difference between the candidate and zero using integer division. Returns an empty list for 0.

Implementation

dart

static final Shrinker<int> integer = Shrinker<int>((i) {
  if (i == 0) {
    return ILazyList.empty();
  } else {
    return ILazyList.unfold<int, (bool, int)>(
      (false, i.abs() ~&#47; 2),
      (state) {
        final (zeroEmitted, delta) = state;

        if (!zeroEmitted) {
          return Some((0, (true, delta)));
        } else if (delta > 0) {
          return Some((i - delta * i.sign, (true, delta ~&#47; 2)));
        } else {
          return const None();
        }
      },
    );
  }
});

string final

final Shrinker<String> string

Shrinker for String values.

Splits the string into a list of single-character strings, shrinks the list length (characters are not individually shrunk), then rejoins.

Implementation

dart

static final Shrinker<String> string = list(
  Shrinker<String>((s) => ILazyList.empty()),
).xmap((l) => l.join(), (s) => s.split(''));

Static Methods

either()

Shrinker<Either<A, B>> either<A, B>(Shrinker<A>? sa, Shrinker<B>? sb)

Shrinker for Either values.

Delegates to sa for Left values and to sb for Right values, preserving the constructor. A null shrinker for the active side yields an empty candidate list.

Implementation

dart

static Shrinker<Either<A, B>> either<A, B>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
) => Shrinker<Either<A, B>>((e) {
  return e.fold(
    (a) => sa?.shrink(a).map((a) => a.asLeft<B>()) ?? ILazyList.empty(),
    (b) => sb?.shrink(b).map((b) => b.asRight<A>()) ?? ILazyList.empty(),
  );
});

ilist()

Shrinker<IList<A>> ilist<A>(Shrinker<A>? sa)

Shrinker for IList values.

Candidates are produced in three phases:

Subsequences obtained by removing progressively smaller contiguous blocks (halving strategy).
The empty list.
Lists with a single element replaced by a shrunk version, when sa is provided.

Returns an empty list for an empty input.

Implementation

dart

static Shrinker<IList<A>> ilist<A>(
  Shrinker<A>? sa,
) => Shrinker<IList<A>>((l) {
  if (l.isEmpty) {
    return ILazyList.empty();
  } else {
    final halves =
        ILazyList.unfold<ILazyList<IList<A>>, int>(
          l.length ~&#47; 2,
          (k) =>
              k > 0
                  ? Some((
                    ILazyList.tabulate(
                      l.length - k + 1,
                      (i) => i,
                    ).map((i) => l.take(i).concat(l.drop(i + k))),
                    k ~&#47; 2,
                  ))
                  : const None(),
        ).flatten();

    final empty = ILazyList.from(IList.fromDart([nil<A>()]));

    final elements =
        sa == null
            ? ILazyList.empty<IList<A>>()
            : ILazyList.tabulate(l.length, (i) => i).flatMap(
              (i) => sa.shrink(l[i]).map((shrunk) => l.updated(i, shrunk)),
            );

    return halves.concat(empty).concat(elements);
  }
});

imap()

Shrinker<IMap<A, B>> imap<A, B>(Shrinker<A>? sa, Shrinker<B>? sb)

Shrinker for IMap values.

Converts the map to an IList of key-value pairs, shrinks that list with ilist using a tuple2 shrinker, then converts back.

Implementation

dart

static Shrinker<IMap<A, B>> imap<A, B>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
) => ilist(tuple2(sa, sb)).xmap(
  (l) => IMap.from(l),
  (m) => m.toIList(),
);

list()

Shrinker<List<A>> list<A>(Shrinker<A>? elementShrinker)

Shrinker for List values.

Delegates to ilist and converts between IList and List.

Implementation

dart

static Shrinker<List<A>> list<A>(
  Shrinker<A>? elementShrinker,
) => ilist(elementShrinker).xmap((l) => l.toList(), (l) => IList.fromDart(l));

map()

Shrinker<Map<A, B>> map<A, B>(Shrinker<A>? sa, Shrinker<B>? sb)

Shrinker for Map values.

Converts the map to an IList of key-value pairs, shrinks that list with ilist using a tuple2 shrinker, then converts back.

Implementation

dart

static Shrinker<Map<A, B>> map<A, B>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
) => ilist(tuple2(sa, sb)).xmap(
  (l) => IMap.from(l).toMap(),
  (m) => IMap.fromDart(m).toIList(),
);

option()

Shrinker<Option<A>> option<A>(Shrinker<A>? sa)

Shrinker for Option values.

None is always a candidate for any Some value. When sa is provided the inner value is also shrunk and each result is wrapped back in Some. Returns an empty list for None.

Implementation

dart

static Shrinker<Option<A>> option<A>(
  Shrinker<A>? sa,
) => Shrinker<Option<A>>((o) {
  return o.fold(
    () => ILazyList.empty(),
    (value) => (sa?.shrink(value).map((a) => a.some) ?? ILazyList.empty()).prepended(none<A>()),
  );
});

tuple10()

Shrinker<Record> tuple10<A, B, C, D, E, F, G, H, I, J>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
  Shrinker<H>? sh,
  Shrinker<I>? si,
  Shrinker<J>? sj,
)

Shrinker for 10-tuples.

Shrinks the first nine elements via tuple9, then shrinks the tenth.

Implementation

dart

static Shrinker<(A, B, C, D, E, F, G, H, I, J)> tuple10<A, B, C, D, E, F, G, H, I, J>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
  Shrinker<H>? sh,
  Shrinker<I>? si,
  Shrinker<J>? sj,
) {
  final s9 = tuple9(sa, sb, sc, sd, se, sf, sg, sh, si);

  return Shrinker(
    (t) => s9
        .shrink((t.$1, t.$2, t.$3, t.$4, t.$5, t.$6, t.$7, t.$8, t.$9))
        .map((t9) => (t9.$1, t9.$2, t9.$3, t9.$4, t9.$5, t9.$6, t9.$7, t9.$8, t9.$9, t.$10))
        .concat(
          sj
                  ?.shrink(t.$10)
                  .map((j) => (t.$1, t.$2, t.$3, t.$4, t.$5, t.$6, t.$7, t.$8, t.$9, j)) ??
              ILazyList.empty(),
        ),
  );
}

tuple2()

Shrinker<Record> tuple2<A, B>(Shrinker<A>? sa, Shrinker<B>? sb)

Shrinker for 2-tuples.

First exhausts shrink candidates for the first element (keeping the second element fixed), then exhausts candidates for the second element.

Implementation

dart

static Shrinker<(A, B)> tuple2<A, B>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
) => Shrinker<(A, B)>(
  (t) => ILazyList.unfold<(A, B), (RIterator<A>?, RIterator<B>?)>(
    (sa?.shrink(t.$1).iterator, sb?.shrink(t.$2).iterator),
    (state) {
      final (itA, itB) = state;

      if (itA != null && itA.hasNext) {
        return Some(((itA.next(), t.$2), (itA, itB)));
      } else if (itB != null && itB.hasNext) {
        return Some(((t.$1, itB.next()), (null, itB)));
      } else {
        return none();
      }
    },
  ),
);

tuple3()

Shrinker<Record> tuple3<A, B, C>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
)

Shrinker for 3-tuples.

Shrinks the first two elements via tuple2, then shrinks the third.

Implementation

dart

static Shrinker<(A, B, C)> tuple3<A, B, C>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
) {
  final s2 = tuple2(sa, sb);

  return Shrinker(
    (t) => s2
        .shrink((t.$1, t.$2))
        .map((ab) => (ab.$1, ab.$2, t.$3))
        .concat(sc?.shrink(t.$3).map((c) => (t.$1, t.$2, c)) ?? ILazyList.empty()),
  );
}

tuple4()

Shrinker<Record> tuple4<A, B, C, D>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
)

Shrinker for 4-tuples.

Shrinks the first three elements via tuple3, then shrinks the fourth.

Implementation

dart

static Shrinker<(A, B, C, D)> tuple4<A, B, C, D>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
) {
  final s3 = tuple3(sa, sb, sc);

  return Shrinker(
    (t) => s3
        .shrink((t.$1, t.$2, t.$3))
        .map((abc) => (abc.$1, abc.$2, abc.$3, t.$4))
        .concat(sd?.shrink(t.$4).map((d) => (t.$1, t.$2, t.$3, d)) ?? ILazyList.empty()),
  );
}

tuple5()

Shrinker<Record> tuple5<A, B, C, D, E>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
)

Shrinker for 5-tuples.

Shrinks the first four elements via tuple4, then shrinks the fifth.

Implementation

dart

static Shrinker<(A, B, C, D, E)> tuple5<A, B, C, D, E>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
) {
  final s4 = tuple4(sa, sb, sc, sd);

  return Shrinker(
    (t) => s4
        .shrink((t.$1, t.$2, t.$3, t.$4))
        .map((abcd) => (abcd.$1, abcd.$2, abcd.$3, abcd.$4, t.$5))
        .concat(se?.shrink(t.$5).map((e) => (t.$1, t.$2, t.$3, t.$4, e)) ?? ILazyList.empty()),
  );
}

tuple6()

Shrinker<Record> tuple6<A, B, C, D, E, F>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
)

Shrinker for 6-tuples.

Shrinks the first five elements via tuple5, then shrinks the sixth.

Implementation

dart

static Shrinker<(A, B, C, D, E, F)> tuple6<A, B, C, D, E, F>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
) {
  final s5 = tuple5(sa, sb, sc, sd, se);

  return Shrinker(
    (t) => s5
        .shrink((t.$1, t.$2, t.$3, t.$4, t.$5))
        .map((abcde) => (abcde.$1, abcde.$2, abcde.$3, abcde.$4, abcde.$5, t.$6))
        .concat(
          sf?.shrink(t.$6).map((f) => (t.$1, t.$2, t.$3, t.$4, t.$5, f)) ?? ILazyList.empty(),
        ),
  );
}

tuple7()

Shrinker<Record> tuple7<A, B, C, D, E, F, G>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
)

Shrinker for 7-tuples.

Shrinks the first six elements via tuple6, then shrinks the seventh.

Implementation

dart

static Shrinker<(A, B, C, D, E, F, G)> tuple7<A, B, C, D, E, F, G>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
) {
  final s6 = tuple6(sa, sb, sc, sd, se, sf);

  return Shrinker(
    (t) => s6
        .shrink((t.$1, t.$2, t.$3, t.$4, t.$5, t.$6))
        .map((abcdef) => (abcdef.$1, abcdef.$2, abcdef.$3, abcdef.$4, abcdef.$5, abcdef.$6, t.$7))
        .concat(
          sg?.shrink(t.$7).map((g) => (t.$1, t.$2, t.$3, t.$4, t.$5, t.$6, g)) ??
              ILazyList.empty(),
        ),
  );
}

tuple8()

Shrinker<Record> tuple8<A, B, C, D, E, F, G, H>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
  Shrinker<H>? sh,
)

Shrinker for 8-tuples.

Shrinks the first seven elements via tuple7, then shrinks the eighth.

Implementation

dart

static Shrinker<(A, B, C, D, E, F, G, H)> tuple8<A, B, C, D, E, F, G, H>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
  Shrinker<H>? sh,
) {
  final s7 = tuple7(sa, sb, sc, sd, se, sf, sg);

  return Shrinker(
    (t) => s7
        .shrink((t.$1, t.$2, t.$3, t.$4, t.$5, t.$6, t.$7))
        .map((t7) => (t7.$1, t7.$2, t7.$3, t7.$4, t7.$5, t7.$6, t7.$7, t.$8))
        .concat(
          sh?.shrink(t.$8).map((h) => (t.$1, t.$2, t.$3, t.$4, t.$5, t.$6, t.$7, h)) ??
              ILazyList.empty(),
        ),
  );
}

tuple9()

Shrinker<Record> tuple9<A, B, C, D, E, F, G, H, I>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
  Shrinker<H>? sh,
  Shrinker<I>? si,
)

Shrinker for 9-tuples.

Shrinks the first eight elements via tuple8, then shrinks the ninth.

Implementation

dart

static Shrinker<(A, B, C, D, E, F, G, H, I)> tuple9<A, B, C, D, E, F, G, H, I>(
  Shrinker<A>? sa,
  Shrinker<B>? sb,
  Shrinker<C>? sc,
  Shrinker<D>? sd,
  Shrinker<E>? se,
  Shrinker<F>? sf,
  Shrinker<G>? sg,
  Shrinker<H>? sh,
  Shrinker<I>? si,
) {
  final s8 = tuple8(sa, sb, sc, sd, se, sf, sg, sh);

  return Shrinker(
    (t) => s8
        .shrink((t.$1, t.$2, t.$3, t.$4, t.$5, t.$6, t.$7, t.$8))
        .map((t8) => (t8.$1, t8.$2, t8.$3, t8.$4, t8.$5, t8.$6, t8.$7, t8.$8, t.$9))
        .concat(
          si?.shrink(t.$9).map((i) => (t.$1, t.$2, t.$3, t.$4, t.$5, t.$6, t.$7, t.$8, i)) ??
              ILazyList.empty(),
        ),
  );
}

Shrinker<A> ​

Constructors ​

Shrinker() ​

Properties ​

hashCode no setter inherited ​

runtimeType no setter inherited ​

Methods ​

noSuchMethod() inherited ​

shrink() ​

toString() inherited ​

xmap() ​

Operators ​

operator ==() inherited ​

Static Properties ​

dubble final ​

integer final ​

string final ​

Static Methods ​

either() ​

ilist() ​

imap() ​

list() ​

map() ​

option() ​

tuple10() ​

tuple2() ​

tuple3() ​

tuple4() ​

tuple5() ​

tuple6() ​

tuple7() ​

tuple8() ​

tuple9() ​

Shrinker<A>

Constructors

Shrinker()

Properties

hashCode no setter inherited

runtimeType no setter inherited

Methods

noSuchMethod() inherited

shrink()

toString() inherited

xmap()

Operators

operator ==() inherited

Static Properties

dubble final

integer final

string final

Static Methods

either()

ilist()

imap()

list()

map()

option()

tuple10()

tuple2()

tuple3()

tuple4()

tuple5()

tuple6()

tuple7()

tuple8()

tuple9()