value-task

Library allows describing asynchronous operations that may fail, like HTTP requests or writing to a database. Tasks also provide abstraction similar to light-weight threads, so you can have a bunch of tasks running at the same time and the runtime will hop between them if they are blocked.

Library allows one to model side-effects explicitly, in form of plain data structures. Task themselves just describe operations, performing them is a whole another story, typically on would use scheduler to do that which provides a handy separation. Think of tasks as a items on the todo list - they just describe things to do, who will do those tasks and when is a separate concern, concern of scheduler.

API

Basics

In nutshell Task <error, value> type represents asynchronous effect that may fail with a specific error type or succeed with a specific value type.

For example, maybe we have a task with the type Task<string, User>. This means that when we perform this task, it will either fail with a string message or succeed with a value of User (instance). So this could represent a task that is asking a server for a certain user.

Note: Thinking about task types is useful as that gives very specific insight what the result of performing that task will look like. Library is also makes use of flow type checker and provides all the necessary annotations so that you could catch all the possible errors associated with invalid uses of the task results early on.

succeed(a)

A task that succeeds immediately when run with a value provided:

Task.succeed(42)
    .fork(value => console.log(value), error => console.error(error))

// => 42

fail

A task that fails immediately when run with a value provided:

Task.fail("file not found")

Mapping

map

map2

chaining

chain

Chain together a task and a callback. The first task will run, and if it is successful, you give the result to the callback resulting in another task. This task then gets run.

Task
  .succeed(2)
  .chain(n => succeed(n + 2)) // Succeed(4)
  .fork(console.error, console.log)

// 4

sequence

Start with a list of tasks, and turn them into a single task that returns a list. The tasks will be run in order one-by-one and if any task fails the whole sequence fails.

Errors

capture

Recover from a failure in a task. If the given task fails, we use the callback to recover.

format

Transform the error value. This can be useful if you need a bunch of error types to match up.

Threads

spawn

sleep

Types

Task<error, value>

ThreadID

Time

Install

npm install value-task

Prior art

• Task in Elm.