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Modules

• Either
• Option
• ReaderT
• 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.

What is the difference between ReadonlyArray and Array modules?

The Array module predates the ReadonlyArray module and will be deprecated in the near future. ReadonlyArray is Array minus all the mutating functions.

Releases

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Roadmap

• Roadmap