maybe_just_nothing 0.1.7

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

Maybe Just Nothing

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

import 'package:maybe_just_nothing/maybe_just_nothing.dart';

Maybe<int> addTwoIntegers(dynamic a, dynamic b) =>
    Maybe(a).cast<int>().merge(Maybe(b).cast<int>(), (a, b) => a + b);

void main() {
  // Prints 42
  addTwoIntegers(40, 2).ifPresent(print);

  // Does nothing since one of the values is null.
  addTwoIntegers(40, null).ifPresent(print);

  // Does nothing since one of the values is not an integer.
  addTwoIntegers('Oops', 2).ifPresent(print);
}

Classes:

• Maybe(nullableValue) wraps a possibly null value
• Just(42) wraps a non null value (throws ArgumentError on null)
• Nothing<T>() represents a missing value of type T

Methods

map

Maps the value.

flatMap

Maps the value.

filter or where

Filters the value by the given predicate.

or

Returns the wrapped value (if present), or the default value (otherwise).

orAsync

Returns a Future of the wrapped value (if present), or the default value (otherwise).

orGet

Returns the wrapped value (if present), or the result of the producer function (otherwise).

orGetAsync

Returns a Future of the wrapped value (if present), or the result of the producer function (otherwise).

orThrow

Returns the wrapped value (if present), or throws the result of the producer function (otherwise).

ifPresent

Calls the consumer function if the wrapped value is present.

ifNothing

Calls the callback function if the wrapped value is not present.

cast

Narrows the type to P if the value is present and has actually the type of P.

merge

Merges the other Maybe using the merger function.

chain

A sort of "Chain of Responsibility" pattern. Returns the next Maybe if this is empty.

Q&A

Where is isPresent?

It is not there by design. Most of the times using ifPresent() and ifNothing() should be sufficient. If there is no choice but inline logic, use the type checks:

void fun(Maybe<int> score) {
  if (score is Just<int>) {
    doSomething(score.value); // Bonus! The score.value is available and guaranteed to be non-null.
  } else {
    // do something else
  }
}