Ribs

Appearance

ribs_core›Classes›Eval<A>

Eval<A> sealed ​

sealed class Eval<A>

A data type for controlling evaluation semantics: eager, lazy-memoized, or always-recomputed.

Eval has three primary subtypes, analogous to final, late, and a function:

  • Eval.now — value computed immediately (eager), like final.
  • Eval.later — value computed once on first access and memoized, like late.
  • Eval.always — value recomputed on every access, like a function invocation.

flatMap chains are evaluated using a trampoline, making them stack-safe even for deeply recursive computations.

Example:

dart
final result = Eval.always(() => expensiveComputation())
    .map((n) => n * 2)
    .flatMap((n) => Eval.now(n + 1));
print(result.value);

Available Extensions

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;

value no setter ​

A get value

Extracts the value, triggering any pending computation.

  • now: returns immediately.
  • later: evaluates once and memoizes.
  • always: evaluates every time.
  • flatMap / defer chains: evaluated with a stack-safe trampoline.
Implementation
dart
A get value => _run(this) as A;

Methods ​

flatMap() ​

Eval<B> flatMap<B>(Eval<B> Function(A) f)

Returns an Eval that evaluates this, then passes the result to f.

Chains are evaluated stack-safely via trampolining.

Implementation
dart
Eval<B> flatMap<B>(Function1<A, Eval<B>> f) => _Bind<A, B>(this, Fn1(f));

map() ​

Eval<B> map<B>(B Function(A) f)

Returns an Eval whose value is the result of applying f to this value.

Implementation
dart
Eval<B> map<B>(Function1<A, B> f) => flatMap((a) => _Now(f(a)));

memoize() ​

Eval<A> memoize()

Returns a memoized version of this Eval.

If this is already memoized (now or later), returns this. Otherwise wraps in a later so f / the chain is evaluated at most once.

Implementation
dart
Eval<A> memoize();

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);

productL() ​

Eval<A> productL<B>(Eval<B> other)

Evaluates this and then other, discarding the result of other.

Implementation
dart
Eval<A> productL<B>(Eval<B> other) => flatMap((a) => other.map((_) => a));

productR() ​

Eval<B> productR<B>(Eval<B> other)

Evaluates this and then other, discarding the result of this.

Implementation
dart
Eval<B> productR<B>(Eval<B> other) => flatMap((_) => other);

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();

Extension Methods ​

flatten() extension ​

Eval<A> flatten()

Flattens a nested Eval into a single Eval.

Available on Eval<A>, provided by the EvalNestedOps<A> extension

Implementation
dart
Eval<A> flatten() => flatMap(identity);

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 ​

unit final ​

final Eval<Unit> unit

An already-evaluated Eval of Unit.

Implementation
dart
static final Eval<Unit> unit = now(Unit());

Static Methods ​

always() ​

Eval<A> always<A>(A Function() f)

Creates an Eval that calls f on every access.

Analogous to function call — f is called every time value is read.

Implementation
dart
static Eval<A> always<A>(Function0<A> f) => _Always(f);

defer() ​

Eval<A> defer<A>(Eval<A> Function() f)

Creates an Eval whose evaluation is deferred to another Eval produced by f.

Useful for building recursive Eval values without stack overflow:

dart
Eval<int> countdown(int n) =>
    n <= 0 ? Eval.now(0) : Eval.defer(() => countdown(n - 1));
Implementation
dart
static Eval<A> defer<A>(Function0<Eval<A>> f) => _Defer(Fn0(f));

later() ​

Eval<A> later<A>(A Function() f)

Creates an Eval that computes f on first access and memoizes the result.

Analogous to late — f is called at most once.

Implementation
dart
static Eval<A> later<A>(Function0<A> f) => _Later(f);

now() ​

Eval<A> now<A>(A a)

Creates an Eval whose value is already computed.

Analogous to final — no deferred work, value returns immediately.

Implementation
dart
static Eval<A> now<A>(A a) => _Now(a);

pure() ​

Eval<A> pure<A>(A a)

Creates an Eval whose value is already computed. Alias for now.

Implementation
dart
static Eval<A> pure<A>(A a) => now(a);