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
Effect, import Context
Context } from "effect"
2

3
class class SomeContext
SomeContext extends import Context
Context.const Tag: <"SomeContext">(id: "SomeContext") => <Self, Shape>() => Context.TagClass<Self, "SomeContext", Shape>
namespace Tag
Tag("SomeContext")<class SomeContext
SomeContext, {}>() {}
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// Assume we have an effect that succeeds with a number,
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// fails with an Error, and requires SomeContext
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declare const const program: Effect.Effect<number, Error, SomeContext>
program: import Effect
Effect.interface Effect<out A, out E = never, out R = never>
namespace EffectThe `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.
Effect<number, interface Error
Error, class SomeContext
SomeContext>
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// Extract the success type, which is number
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type type A = number
A = import Effect
Effect.namespace EffectThe `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.
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
Success<typeof const program: Effect.Effect<number, Error, SomeContext>
program>
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// Extract the error type, which is Error
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type type E = Error
E = import Effect
Effect.namespace EffectThe `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.
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
Error<typeof const program: Effect.Effect<number, Error, SomeContext>
program>
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// Extract the context type, which is SomeContext
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type type R = SomeContext
R = import Effect
Effect.namespace EffectThe `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.
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
Context<typeof const program: Effect.Effect<number, Error, SomeContext>
program>