Maybe Just Nothing

Yet another variation of the good old Maybe monad with eager execution written in Dart. Or maybe it's just nothing?

Creating maybe-values

The most common scenario is to create a Maybe from a nullable value:

int nullableValue;
final val = Maybe(nullableValue); // Creates an instance of Maybe<int>
final doubled = val.map((x) => x * 2);
doubled.ifPresent(print); // would print the doubled value of nullableValue if it's not null

Internally, Maybe is an abstract class with two implementations: Just and Nothing. Both can be instantiated directly. Direct instantiation gives more precise type inference in certain cases.

final two = Just(2);
final four = two.map((x) => x * 2); // four is inferred as Just<int>
four.ifPresent(print); // prints "4"

final nothing = Nothing<int>(); // the type parameter is optional
final alsoNothing = nothing.map((x) => x * 2); // alsoNothing is inferred as Nothing<int>
alsoNothing.ifPresent(print); // does nothing since the value is not present
alsoNothing.ifNothing(() => print('Empty value')); // prints "Empty value"

Just does not accept null values:

Just(null); // throws ArgumentError

Mapping values

Mapping means transformation of the wrapped value by applying a function. Since Maybe itself is immutable, mapping operations do not actually modify the value. Instead, they always return another Maybe.

Maybe(2).map((x) => x * 2).ifPresent(print); // prints "4"

If the mapping function also returns a Maybe, use flatMap():

Maybe<int> triple(int x) => Maybe(x).map((x) => x * 3);

Maybe(2).flatMap(triple).ifPresent(print); // prints "6"

An operation on two maybe-values can be performed using merge():

final two = Maybe(2);
final three = Maybe(3);

two.merge(three, (x, y) => x + y).ifPresent(print); // prints "5"

Filtering values

Filtering is checking whether the maybe-value satisfies a certain condition. If it does, the value remains intact, otherwise Nothing is returned.

To filter bu the value itself, use the where():

Maybe(2).where((x) => x.isEven).ifPresent(print); // prints "2"
Maybe(3).where((x) => x.isEven).ifPresent(print); // 3 is odd, so nothing happens

To filter by the type, use type<T>():

final maybeInt = Maybe(2).type<int>(); // Just<int>
final maybeString = Maybe(2).type<String>(); // Nothing<String>

Fallback chain

The chain() method implements the Chain of Responsibility design pattern. It accepts another maybe-value of the same type. If the current value is empty, the next value in the chain gets returned.

Another way to implement the same idea is to use the fallback() method. It accepts a "fallback" function which returns another maybe-value of the same type. If the current value is nothing, this fallback function will be called and its result will be returned. You can provide several fallback functions. They will be called in sequence until a non-empty value is received.

int a;
int b;
int c;
b = 2;
c = 3;

Maybe(a) // this one if empty
  .chain(Maybe(b)) // this value is not empty, so it will be used
  .chain(Maybe(c)) // this value will NOT be used
  .ifPresent(print); // prints "2"

// Same with fallback()
Maybe(a) // this one if empty
  .fallback(() => Maybe(b)) // this function returns a non-empty value
  .fallback(() => Maybe(c)) // this function will NOT be called
  .ifPresent(print); // prints "2"

Consuming the value

The intention of Maybe is to give it the consumer function instead of retrieving the value. This is the most concise and clear way if using it.

int a;
final value = Maybe(a);
final value.ifPresent(print);
final value.ifPresent(print, otherwise: () {/* do something else*/});
final value.ifNothing(() {/* do something else*/});

Reading the value

Sometimes, however, you need the actual value. In such cases you'll have to provide the default value as well.

In the simplest scenario, use or():

int a;
final value = Maybe(a).or(0); // value is 0
final valueFromFuture = await Maybe(a).orAsync(Future.value(0)); // value is 0

A provider function can be specified instead of the default value:

int a;
final value = Maybe(a).orGet(() => 0); // value is 0
final valueFromFuture = await Maybe(a).orGetAsync(() async => 0); // value is 0

If there is no default value, an exception can be thrown:

int a;
final value = Maybe(a).orThrow(() => 'Oops!');

In some rare cases, it can be convenient to check for emptiness directly:

int a;
final myInt = Maybe(a);

if (myInt is Just<int>) {
  print(myInt.value); // .value is guaranteed to be non-null
}

if (myInt is Nothing) {
  print('The value is missing');
}

Libraries

maybe_just_nothing