Maybe

folktale.maybe

• Models either a value or nothing.
• Allows distinction between an explicit null value (null was returned). and an implicit null value (nothing was returned).
• It is a Monad as it has a chain property.
• It will be either a value: Maybe.Just, or no value: Maybe.Nothing.
• If a Maybe.Just value will be stored in the value property.
• Will have either an isJust or isNothing property, but these are deprecated.
• Allows distinction between an explicit null value (null was returned). and an implicit null value (nothing was returned).

Creation

Maybe.Just(value);
Maybe.Nothing();

Extracting Result

getOrElse

It also offers a getOrElse method which can be used for extracting a value with a default if the result is an error:

const value = maybe.getOrElse(DEFAULT_VALUE);

Transformation

matchWith

To map the result to a different function depending on whether it is a Result.Ok or a Result.Error:

maybe.matchWith({
  Just: ({value}) => `Value: ${value}`,
  Nothing: () => `No value`,
})

map

To perform an action on the result only if it is OK, map can be used. If the result is a Maybe.Nothing, it will just return that Maybe.Nothing, but if the result is Maybe.Just, its value will be passed in to the function supplied, and it will return whatever the function returns as the value of a Maybe.Just.

const value = maybe.map(example);

chain

However, if the function supplied to map also returns a Maybe, chain should be used instead to avoid a nested Maybe. In this case it is assumed that the value returned will be a Maybe, so whatever is returned from the function is returned as is, without being wrapped in a Maybe.

const value = maybe.chain(example);

orElse

To perform an action on the result only if it is Nothing, orElse_ can be used. _If the result is a Maybe.Just, it will just return that Maybe.Just, but if the result is Maybe.Nothing, its value will be passed in to the function supplied, and it will return whatever the function returns. For consistency, it should return a Maybe unless it throws.

const value = result.orElse(example);Checking Type

Introspection

To devine whether a Maybe is a Just or a Nothing , hasInstance is used:

Maybe.Just.hasInstance(result);
Maybe.Nothing.hasInstance(result);

Example

const a = range(1,3);

const alpha = (value) => {
  const result = contains(value, a) ?
        Maybe.Just(value) :
        Maybe.Nothing();
  return result;
}

const hasValue = alpha(2);
const noValue = alpha(3);

console.log('Has Value: ', hasValue.value); // 2
console.log('Has Value - is Maybe.Just: ', Maybe.Just.hasInstance(hasValue)); // true
console.log('Has Value - is Maybe.Nothing: ', Maybe.Nothing.hasInstance(hasValue)); // false
console.log('No Value: ', noValue); // Nothing
console.log('No Value - is Maybe.Just: ', Maybe.Just.hasInstance(noValue)); // false
console.log('No Value - is Maybe.Nothing: ', Maybe.Nothing.hasInstance(noValue)); // true

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