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Modules

• Either
• Option
• ReadonlyArray
• Task
• TaskEither

FAQ

How to choose an effect for my app?

Here's a table containing the basic effects

Type constructor	Effect (interpretation)
ReadonlyArray<A>	a non deterministic computation
Option<A>	a computation that may fail
Either<E, A>	a computation that may fail and yield an error
IO<A>	a synchronous computation that never fails
Task<A>	an asynchronous computation that never fails
Reader<R, A>	reading from an environment (dependency injection)
State<S, A>	stateful computation

These basic effects are like ingredients for a recipe.

In order to get your recipe (the effect of your app) you can add all the ingredients you need by nesting the corresponding type constructors.

Example

Let's say your app

• will make use of dependency injection
• will run in an asynchronous context
• will handle operations which may fail

Here's the corresponding ingredients

• Reader
• Task
• Either

in order to mix the ingredients you just nest the type constructors

import { Reader } from 'fp-ts/Reader'
import { Task } from 'fp-ts/Task'
import { Either } from 'fp-ts/Either'

type MyEffect<R, E, A> = Reader<R, Task<Either<E, A>>>

These three ingredients are so common that fp-ts exports a ready-to-use module, the ReaderTaskEither module.