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The Effect Type

The Effect type is a 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)

import { Effect, Context } from "effect"


class SomeContext extends Context.Tag("SomeContext")<SomeContext, {}>() {}


// Assume we have an effect that succeeds with a number,
// fails with an Error, and requires SomeContext
declare const program: Effect.Effect<number, Error, SomeContext>


// Extract the success type, which is number
type A = Effect.Effect.Success<typeof program>


// Extract the error type, which is Error
type E = Effect.Effect.Error<typeof program>


// Extract the context type, which is SomeContext
type R = Effect.Effect.Context<typeof program>