@faergeek/monads

Easy to use monads for JavaScript and TypeScript.

Source code itself is pretty small, but it has jsdoc comments, which should serve as a more complete API documentation:

• Maybe

  An abstract box representing either presence or absence of a value.

  A box with a value can be created with Maybe.Some(<value>). Maybe.None represents an absence of a value.

  Read the comments in source file linked above for details.

• Async

  An abstract box representing asynchronous value state. Very similar to Maybe, but has less operations. It exists to make sure that presence/absence of a value and pending/ready state are easy to tell apart. Very useful to convey loading state through any value transformations.

  A box with a value ready to be used can be created with Async.Ready(<value>). Async.Pending represents a pending state meaning there's no value yet.

  Read the comments in source file linked above for details.

• Result

  An abstract box representing success or failure.

  A box representing success can be created with Result.Ok(<value>). A box representing failure can be created with Result.Err(<error>).

  Read the comments in source file linked above for details.

An example of using both Async and Result to represent different states of UI depending on API request result from a hook like SWR or React Query.

First, we need to create a box, representing 3 states: pending, success and failure.

1// let's assume data is something like a number 21
2const { data, error, isLoading } = useSomeApiHook();
3
4const halfTheAnswer = data
5  ? Async.Ready(Result.Ok(data))
6  : isLoading
7    ? Async.Pending
8    : Async.Ready(Result.Err(error));

Then apply transformations with map* or flatMap* functions.

Here we simply multiply the value by 2. But the same approach can be used to extract only some pieces of response data, transform them and pass to child components, all without loosing request state attached to it.

1const theAnswer = halfTheAnswer.mapReady(
2  // this function is only called if `halfTheAnswer` represents ready state
3  result =>
4    result.mapOk(
5      // this function is only called if `result` represents success state
6      x => x * 2,
7    ),
8);

And finally use the value, explicitly handling all cases. In this example, we simply render a piece of UI with something like React.

1return theAnswer.match({
2  Pending: () => <h1>Fetching the answer, please wait...</h1>,
3  Ready: result =>
4    result.match({
5      Err: error => <h1>Could not get the answer: {error.message}</h1>,
6      Ok: data => <h1>The Answer is {data}</h1>,
7    }),
8});