The Effect Type

The Effect type is an immutable description of a workflow or operation that is lazily executed. This means that when you create an Effect, it doesn’t run immediately, but instead defines a program that can succeed, fail, or require some additional context to complete.

Here is the general form of an Effect:

         ┌─── Represents the success type
         │        ┌─── Represents the error type
         │        │      ┌─── Represents required dependencies
         ▼        ▼      ▼
Effect<Success, Error, Requirements>

This type indicates that an effect:

Succeeds and returns a value of type Success
Fails with an error of type Error
May need certain contextual dependencies of type Requirements to execute

Conceptually, you can think of Effect as an effectful version of the following function type:

type Effect<Success, Error, Requirements> = (
  context: Context<Requirements>
) => Error | Success

However, effects are not actually functions. They can model synchronous, asynchronous, concurrent, and resourceful computations.

Immutability. Effect values are immutable, and every function in the Effect library produces a new Effect value.

Modeling Interactions. These values do not perform any actions themselves, they simply model or describe effectful interactions.

Execution. An Effect can be executed by the Effect Runtime System, which interprets it into actual interactions with the external world. Ideally, this execution happens at a single entry point in your application, such as the main function where effectful operations are initiated.

Type Parameters

The Effect type has three type parameters with the following meanings:

Parameter	Description
Success	Represents the type of value that an effect can succeed with when executed. If this type parameter is `void`, it means the effect produces no useful information, while if it is `never`, it means the effect runs forever (or until failure).
Error	Represents the expected errors that can occur when executing an effect. If this type parameter is `never`, it means the effect cannot fail, because there are no values of type `never`.
Requirements	Represents the contextual data required by the effect to be executed. This data is stored in a collection named `Context`. If this type parameter is `never`, it means the effect has no requirements and the `Context` collection is empty.

Extracting Inferred Types

By using the utility types Effect.Success, Effect.Error, and Effect.Context, you can extract the corresponding types from an effect.

Example (Extracting Success, Error, and Context Types)

1
import { 
import Effect@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect, 
import Context@since ― 2.0.0
@since ― 2.0.0
Context } from "effect"
2

3
class 
class SomeContext
SomeContext extends 
import Context@since ― 2.0.0
@since ― 2.0.0
Context.
const Tag: <"SomeContext">(id: "SomeContext") => <Self, Shape>() => Context.TagClass<Self, "SomeContext", Shape>@example 
import { Context, Layer } from "effect"

class MyTag extends Context.Tag("MyTag")<
 MyTag,
 { readonly myNum: number }
>() {
 static Live = Layer.succeed(this, { myNum: 108 })
}
@since ― 2.0.0
Tag("SomeContext")<
class SomeContext
SomeContext, {}>() {}
4

5
// Assume we have an effect that succeeds with a number,
6
// fails with an Error, and requires SomeContext
7
declare const 
const program: Effect.Effect<number, Error, SomeContext>
program: 
import Effect@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
interface Effect<out A, out E = never, out R = never>
The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
@since ― 2.0.0
@since ― 2.0.0
Effect<number, 
interface Error
Error, 
class SomeContext
SomeContext>
8

9
// Extract the success type, which is number
10
type 
type A = number
A = 
import Effect@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
namespace Effect
The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
@since ― 2.0.0
@since ― 2.0.0
Effect.
type Effect<out A, out E = never, out R = never>.Success<T extends Effect.Effect<any, any, any>> = [T] extends [Effect.Effect<infer _A, infer _E, infer _R>] ? _A : never@since ― 2.0.0
Success<typeof 
const program: Effect.Effect<number, Error, SomeContext>
program>
11

12
// Extract the error type, which is Error
13
type 
type E = Error
E = 
import Effect@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
namespace Effect
The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
@since ― 2.0.0
@since ― 2.0.0
Effect.
type Effect<out A, out E = never, out R = never>.Error<T extends Effect.Effect<any, any, any>> = [T] extends [Effect.Effect<infer _A, infer _E, infer _R>] ? _E : never@since ― 2.0.0
Error<typeof 
const program: Effect.Effect<number, Error, SomeContext>
program>
14

15
// Extract the context type, which is SomeContext
16
type 
type R = SomeContext
R = 
import Effect@since ― 2.0.0
@since ― 2.0.0
@since ― 2.0.0
Effect.
namespace Effect
The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
@since ― 2.0.0
@since ― 2.0.0
Effect.
type Effect<out A, out E = never, out R = never>.Context<T extends Effect.Effect<any, any, any>> = [T] extends [Effect.Effect<infer _A, infer _E, infer _R>] ? _R : never@since ― 2.0.0
Context<typeof 
const program: Effect.Effect<number, Error, SomeContext>
program>