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Managing Services

In the context of programming, a service refers to a reusable component or functionality that can be used by different parts of an application. Services are designed to provide specific capabilities and can be shared across multiple modules or components.

Services often encapsulate common tasks or operations that are needed by different parts of an application. They can handle complex operations, interact with external systems or APIs, manage data, or perform other specialized tasks.

Services are typically designed to be modular and decoupled from the rest of the application. This allows them to be easily maintained, tested, and replaced without affecting the overall functionality of the application.

When diving into services and their integration in application development, it helps to start from the basic principles of function management and dependency handling without relying on advanced constructs. Imagine having to manually pass a service around to every function that needs it:

const processData = (data: Data, databaseService: DatabaseService) => {
// Operations using the database service
}

This approach becomes cumbersome and unmanageable as your application grows, with services needing to be passed through multiple layers of functions.

To streamline this, you might consider using an environment object that bundles various services:

type Context = {
databaseService: DatabaseService
loggingService: LoggingService
}
const processData = (data: Data, context: Context) => {
// Using multiple services from the context
}

However, this introduces a new complexity: you must ensure that the environment is correctly set up with all necessary services before it’s used, which can lead to tightly coupled code and makes functional composition and testing more difficult.

The Effect library simplifies managing these dependencies by leveraging the type system. Instead of manually passing services or environment objects around, Effect allows you to declare service dependencies directly in the function’s type signature using the Requirements parameter in the Effect type:

┌─── Represents required dependencies
Effect<Success, Error, Requirements>

This is how it works in practice when using Effect:

Dependency Declaration: You specify what services a function needs directly in its type, pushing the complexity of dependency management into the type system.

Service Provision: Effect.provideService is used to make a service implementation available to the functions that need it. By providing services at the start, you ensure that all parts of your application have consistent access to the required services, thus maintaining a clean and decoupled architecture.

This approach abstracts away manual service handling, letting developers focus on business logic while the compiler ensures all dependencies are correctly managed. It also makes code more maintainable and scalable.

Let’s walk through managing services in Effect step by step:

  1. Creating a Service: Define a service with its unique functionality and interface.
  2. Using the Service: Access and utilize the service within your application’s functions.
  3. Providing a Service Implementation: Supply an actual implementation of the service to fulfill the declared requirements.

Up to this point, our examples with the Effect framework have dealt with effects that operate independently of external services. This means the Requirements parameter in our Effect type signature has been set to never, indicating no dependencies.

However, real-world applications often need effects that rely on specific services to function correctly. These services are managed and accessed through a construct known as Context.

The Context serves as a repository or container for all services an effect may require. It acts like a store that maintains these services, allowing various parts of your application to access and use them as needed.

The services stored within the Context are directly reflected in the Requirements parameter of the Effect type. Each service within the Context is identified by a unique “tag,” which is essentially a unique identifier for the service.

When an effect needs to use a specific service, the service’s tag is included in the Requirements type parameter.

To create a new service, you need two things:

  1. A unique identifier.
  2. A type describing the possible operations of the service.

Example (Defining a Random Number Generator Service)

Let’s create a service for generating random numbers.

  1. Identifier. We’ll use the string "MyRandomService" as the unique identifier.
  2. Type. The service type will have a single operation called next that returns a random number.
1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{ readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number> }
7
>() {}

The exported Random value is known as a tag in Effect. It acts as a representation of the service and allows Effect to locate and use this service at runtime.

The service will be stored in a collection called Context, which can be thought of as a Map where the keys are tags and the values are services:

type Context = Map<Tag, Service>

Let’s summarize the concepts we’ve covered so far:

ConceptDescription
serviceA reusable component providing specific functionality, used across different parts of an application.
tagA unique identifier representing a service, allowing Effect to locate and use it.
contextA collection storing service, functioning like a map with tags as keys and services as values.

Now that we have our service tag defined, let’s see how we can use it by building a simple program.

Example (Using the Random Service)

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{ readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number> }
7
>() {}
8
9
// Using the service
10
//
11
// ┌─── Effect<void, never, Random>
12
// ▼
13
const
const program: Effect.Effect<void, never, Random>
program
=
import Effect
Effect
.
const gen: <YieldWrap<Context.Tag<Random, { readonly next: Effect.Effect<number>; }>> | YieldWrap<Effect.Effect<number, never, never>>, void>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
gen
(function* () {
14
const
const random: { readonly next: Effect.Effect<number>; }
random
= yield*
class Random
Random
15
const
const randomNumber: number
randomNumber
= yield*
const random: { readonly next: Effect.Effect<number>; }
random
.
(property) next: Effect.Effect<number, never, never>
next
16
namespace console var console: Console

The `console` module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers. The module exports two specific components: * A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream. * A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and [`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module. _**Warning**_: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for more information. Example using the global `console`: ```js console.log('hello world'); // Prints: hello world, to stdout console.log('hello %s', 'world'); // Prints: hello world, to stdout console.error(new Error('Whoops, something bad happened')); // Prints error message and stack trace to stderr: // Error: Whoops, something bad happened // at [eval]:5:15 // at Script.runInThisContext (node:vm:132:18) // at Object.runInThisContext (node:vm:309:38) // at node:internal/process/execution:77:19 // at [eval]-wrapper:6:22 // at evalScript (node:internal/process/execution:76:60) // at node:internal/main/eval_string:23:3 const name = 'Will Robinson'; console.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to stderr ``` Example using the `Console` class: ```js const out = getStreamSomehow(); const err = getStreamSomehow(); const myConsole = new console.Console(out, err); myConsole.log('hello world'); // Prints: hello world, to out myConsole.log('hello %s', 'world'); // Prints: hello world, to out myConsole.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to err const name = 'Will Robinson'; myConsole.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to err ```

console
.
(method) Console.log(message?: any, ...optionalParams: any[]): void

Prints to `stdout` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html) (the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)). ```js const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout ``` See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.

log
(`random number: ${
const randomNumber: number
randomNumber
}`)
17
})

In the code above, we can observe that we are able to yield the Random tag as if it were an effect itself. This allows us to access the next operation of the service.

It’s worth noting that the type of the program variable includes Random in the Requirements type parameter:

const program: Effect<void, never, Random>

This indicates that our program requires the Random service to be provided in order to execute successfully.

If we attempt to execute the effect without providing the necessary service we will encounter a type-checking error:

Example (Type Error Without Service Provision)

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{ readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number> }
7
>() {}
8
9
// Using the service
10
const
const program: Effect.Effect<void, never, Random>
program
=
import Effect
Effect
.
const gen: <YieldWrap<Context.Tag<Random, { readonly next: Effect.Effect<number>; }>> | YieldWrap<Effect.Effect<number, never, never>>, void>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
gen
(function* () {
11
const
const random: { readonly next: Effect.Effect<number>; }
random
= yield*
class Random
Random
12
const
const randomNumber: number
randomNumber
= yield*
const random: { readonly next: Effect.Effect<number>; }
random
.
(property) next: Effect.Effect<number, never, never>
next
13
namespace console var console: Console

The `console` module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers. The module exports two specific components: * A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream. * A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and [`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module. _**Warning**_: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for more information. Example using the global `console`: ```js console.log('hello world'); // Prints: hello world, to stdout console.log('hello %s', 'world'); // Prints: hello world, to stdout console.error(new Error('Whoops, something bad happened')); // Prints error message and stack trace to stderr: // Error: Whoops, something bad happened // at [eval]:5:15 // at Script.runInThisContext (node:vm:132:18) // at Object.runInThisContext (node:vm:309:38) // at node:internal/process/execution:77:19 // at [eval]-wrapper:6:22 // at evalScript (node:internal/process/execution:76:60) // at node:internal/main/eval_string:23:3 const name = 'Will Robinson'; console.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to stderr ``` Example using the `Console` class: ```js const out = getStreamSomehow(); const err = getStreamSomehow(); const myConsole = new console.Console(out, err); myConsole.log('hello world'); // Prints: hello world, to out myConsole.log('hello %s', 'world'); // Prints: hello world, to out myConsole.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to err const name = 'Will Robinson'; myConsole.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to err ```

console
.
(method) Console.log(message?: any, ...optionalParams: any[]): void

Prints to `stdout` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html) (the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)). ```js const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout ``` See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.

log
(`random number: ${
const randomNumber: number
randomNumber
}`)
14
})
15
16
// @ts-expect-error
17
import Effect
Effect
.
const runSync: <void, never>(effect: Effect.Effect<void, never, never>) => void

Executes an effect synchronously and returns its result. Use `runSync` when you are certain that the effect is purely synchronous and will not perform any asynchronous operations. If the effect fails or contains asynchronous tasks, it will throw an error.

runSync
(
const program: Effect.Effect<void, never, Random>
program
)
18
/*
19
Argument of type 'Effect<void, never, Random>' is not assignable to parameter of type 'Effect<void, never, never>'.
20
Type 'Random' is not assignable to type 'never'.ts(2345)
21
*/

To resolve this error and successfully execute the program, we need to provide an actual implementation of the Random service.

In the next section, we will explore how to implement and provide the Random service to our program, enabling us to run it successfully.

In order to provide an actual implementation of the Random service, we can utilize the Effect.provideService function.

Example (Providing a Random Number Implementation)

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{ readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number> }
7
>() {}
8
9
// Using the service
10
const
const program: Effect.Effect<void, never, Random>
program
=
import Effect
Effect
.
const gen: <YieldWrap<Context.Tag<Random, { readonly next: Effect.Effect<number>; }>> | YieldWrap<Effect.Effect<number, never, never>>, void>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
gen
(function* () {
11
const
const random: { readonly next: Effect.Effect<number>; }
random
= yield*
class Random
Random
12
const
const randomNumber: number
randomNumber
= yield*
const random: { readonly next: Effect.Effect<number>; }
random
.
(property) next: Effect.Effect<number, never, never>
next
13
namespace console var console: Console

The `console` module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers. The module exports two specific components: * A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream. * A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and [`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module. _**Warning**_: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for more information. Example using the global `console`: ```js console.log('hello world'); // Prints: hello world, to stdout console.log('hello %s', 'world'); // Prints: hello world, to stdout console.error(new Error('Whoops, something bad happened')); // Prints error message and stack trace to stderr: // Error: Whoops, something bad happened // at [eval]:5:15 // at Script.runInThisContext (node:vm:132:18) // at Object.runInThisContext (node:vm:309:38) // at node:internal/process/execution:77:19 // at [eval]-wrapper:6:22 // at evalScript (node:internal/process/execution:76:60) // at node:internal/main/eval_string:23:3 const name = 'Will Robinson'; console.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to stderr ``` Example using the `Console` class: ```js const out = getStreamSomehow(); const err = getStreamSomehow(); const myConsole = new console.Console(out, err); myConsole.log('hello world'); // Prints: hello world, to out myConsole.log('hello %s', 'world'); // Prints: hello world, to out myConsole.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to err const name = 'Will Robinson'; myConsole.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to err ```

console
.
(method) Console.log(message?: any, ...optionalParams: any[]): void

Prints to `stdout` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html) (the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)). ```js const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout ``` See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.

log
(`random number: ${
const randomNumber: number
randomNumber
}`)
14
})
15
16
// Providing the implementation
17
//
18
// ┌─── Effect<void, never, never>
19
// ▼
20
const
const runnable: Effect.Effect<void, never, never>
runnable
=
import Effect
Effect
.
const provideService: <void, never, Random, typeof Random>(self: Effect.Effect<void, never, Random>, tag: typeof Random, service: { readonly next: Effect.Effect<number>; }) => Effect.Effect<...> (+1 overload)

Provides the effect with the single service it requires. If the effect requires more than one service use `provide` instead.

provideService
(
const program: Effect.Effect<void, never, Random>
program
,
class Random
Random
, {
21
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
const sync: <number>(thunk: LazyArg<number>) => Effect.Effect<number, never, never>

Creates an `Effect` that represents a synchronous side-effectful computation. The provided function (`thunk`) should not throw errors; if it does, the error is treated as a defect. Use `Effect.sync` when you are certain the operation will not fail.

sync
(() =>
var Math: Math

An intrinsic object that provides basic mathematics functionality and constants.

Math
.
(method) Math.random(): number

Returns a pseudorandom number between 0 and 1.

random
())
22
})
23
24
// Run successfully
25
import Effect
Effect
.
const runPromise: <void, never>(effect: Effect.Effect<void, never, never>, options?: { readonly signal?: AbortSignal; } | undefined) => Promise<void>

Executes an effect and returns a `Promise` that resolves with the result. Use `runPromise` when working with asynchronous effects and you need to integrate with code that uses Promises. If the effect fails, the returned Promise will be rejected with the error.

runPromise
(
const runnable: Effect.Effect<void, never, never>
runnable
)
26
/*
27
Example Output:
28
random number: 0.8241872233134417
29
*/

In the code above, we provide the program we defined earlier with an implementation of the Random service.

We use the Effect.provideService function to associate the Random tag with its implementation, an object with a next operation that generates a random number.

Notice that the Requirements type parameter of the runnable effect is now never. This indicates that the effect no longer requires any service to be provided.

With the implementation of the Random service in place, we are able to run the program without any further requirements.

To retrieve the service type from a tag, use the Context.Tag.Service utility type.

Example (Extracting Service Type)

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
3
// Declaring a tag
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{ readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number> }
7
>() {}
8
9
// Extracting the type
10
type
type RandomShape = { readonly next: Effect.Effect<number>; }
RandomShape
=
import Context
Context
.
namespace Tag
Tag
.
type Tag<in out Id, in out Value>.Service<T extends Context.Tag<any, any> | Context.TagClassShape<any, any>> = T extends Context.Tag<any, any> ? T["Service"] : T extends Context.TagClassShape<any, infer A> ? A : never
Service
<
class Random
Random
>
11
/*
12
This is equivalent to:
13
type RandomShape = {
14
readonly next: Effect.Effect<number>;
15
}
16
*/

When we require the usage of more than one service, the process remains similar to what we’ve learned in defining a service, repeated for each service needed.

Example (Using Random and Logger Services)

Let’s examine an example where we need two services, namely Random and Logger:

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{
7
readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number>
8
}
9
>() {}
10
11
// Declaring a tag for the logging service
12
class
class Logger
Logger
extends
import Context
Context
.
const Tag: <"MyLoggerService">(id: "MyLoggerService") => <Self, Shape>() => Context.TagClass<Self, "MyLoggerService", Shape> namespace Tag
Tag
("MyLoggerService")<
13
class Logger
Logger
,
14
{
15
readonly
(property) log: (message: string) => Effect.Effect<void>
log
: (
(parameter) message: string
message
: string) =>
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<void>
16
}
17
>() {}
18
19
const
const program: Effect.Effect<void, never, Random | Logger>
program
=
import Effect
Effect
.
const gen: <YieldWrap<Context.Tag<Random, { readonly next: Effect.Effect<number>; }>> | YieldWrap<Context.Tag<Logger, { readonly log: (message: string) => Effect.Effect<void>; }>> | YieldWrap<...>, void>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
gen
(function* () {
20
// Acquire instances of the 'Random' and 'Logger' services
21
const
const random: { readonly next: Effect.Effect<number>; }
random
= yield*
class Random
Random
22
const
const logger: { readonly log: (message: string) => Effect.Effect<void>; }
logger
= yield*
class Logger
Logger
23
24
const
const randomNumber: number
randomNumber
= yield*
const random: { readonly next: Effect.Effect<number>; }
random
.
(property) next: Effect.Effect<number, never, never>
next
25
26
yield*
const logger: { readonly log: (message: string) => Effect.Effect<void>; }
logger
.
(property) log: (message: string) => Effect.Effect<void>
log
(
var String: StringConstructor (value?: any) => string

Allows manipulation and formatting of text strings and determination and location of substrings within strings.

String
(
const randomNumber: number
randomNumber
))
27
})

The program effect now has a Requirements type parameter of Random | Logger:

const program: Effect<void, never, Random | Logger>

indicating that it requires both the Random and Logger services to be provided.

To execute the program, we need to provide implementations for both services:

Example (Providing Multiple Services)

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
22 collapsed lines
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{
7
readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number>
8
}
9
>() {}
10
11
// Declaring a tag for the logging service
12
class
class Logger
Logger
extends
import Context
Context
.
const Tag: <"MyLoggerService">(id: "MyLoggerService") => <Self, Shape>() => Context.TagClass<Self, "MyLoggerService", Shape> namespace Tag
Tag
("MyLoggerService")<
13
class Logger
Logger
,
14
{
15
readonly
(property) log: (message: string) => Effect.Effect<void>
log
: (
(parameter) message: string
message
: string) =>
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<void>
16
}
17
>() {}
18
19
const
const program: Effect.Effect<void, never, Random | Logger>
program
=
import Effect
Effect
.
const gen: <YieldWrap<Context.Tag<Logger, { readonly log: (message: string) => Effect.Effect<void>; }>> | YieldWrap<Context.Tag<Random, { readonly next: Effect.Effect<number>; }>> | YieldWrap<...>, void>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
gen
(function* () {
20
const
const random: { readonly next: Effect.Effect<number>; }
random
= yield*
class Random
Random
21
const
const logger: { readonly log: (message: string) => Effect.Effect<void>; }
logger
= yield*
class Logger
Logger
22
const
const randomNumber: number
randomNumber
= yield*
const random: { readonly next: Effect.Effect<number>; }
random
.
(property) next: Effect.Effect<number, never, never>
next
23
return yield*
const logger: { readonly log: (message: string) => Effect.Effect<void>; }
logger
.
(property) log: (message: string) => Effect.Effect<void>
log
(
var String: StringConstructor (value?: any) => string

Allows manipulation and formatting of text strings and determination and location of substrings within strings.

String
(
const randomNumber: number
randomNumber
))
24
})
25
26
// Provide service implementations for 'Random' and 'Logger'
27
const
const runnable: Effect.Effect<void, never, never>
runnable
=
const program: Effect.Effect<void, never, Random | Logger>
program
.
(method) Pipeable.pipe<Effect.Effect<void, never, Random | Logger>, Effect.Effect<void, never, Logger>, Effect.Effect<void, never, never>>(this: Effect.Effect<...>, ab: (_: Effect.Effect<...>) => Effect.Effect<...>, bc: (_: Effect.Effect<...>) => Effect.Effect<...>): Effect.Effect<...> (+21 overloads)
pipe
(
28
import Effect
Effect
.
const provideService: <typeof Random>(tag: typeof Random, service: { readonly next: Effect.Effect<number>; }) => <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, Exclude<R, Random>> (+1 overload)

Provides the effect with the single service it requires. If the effect requires more than one service use `provide` instead.

provideService
(
class Random
Random
, {
29
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
const sync: <number>(thunk: LazyArg<number>) => Effect.Effect<number, never, never>

Creates an `Effect` that represents a synchronous side-effectful computation. The provided function (`thunk`) should not throw errors; if it does, the error is treated as a defect. Use `Effect.sync` when you are certain the operation will not fail.

sync
(() =>
var Math: Math

An intrinsic object that provides basic mathematics functionality and constants.

Math
.
(method) Math.random(): number

Returns a pseudorandom number between 0 and 1.

random
())
30
}),
31
import Effect
Effect
.
const provideService: <typeof Logger>(tag: typeof Logger, service: { readonly log: (message: string) => Effect.Effect<void>; }) => <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<...> (+1 overload)

Provides the effect with the single service it requires. If the effect requires more than one service use `provide` instead.

provideService
(
class Logger
Logger
, {
32
(property) log: (message: string) => Effect.Effect<void>
log
: (
(parameter) message: string
message
) =>
import Effect
Effect
.
const sync: <void>(thunk: LazyArg<void>) => Effect.Effect<void, never, never>

Creates an `Effect` that represents a synchronous side-effectful computation. The provided function (`thunk`) should not throw errors; if it does, the error is treated as a defect. Use `Effect.sync` when you are certain the operation will not fail.

sync
(() =>
namespace console var console: Console

The `console` module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers. The module exports two specific components: * A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream. * A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and [`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module. _**Warning**_: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for more information. Example using the global `console`: ```js console.log('hello world'); // Prints: hello world, to stdout console.log('hello %s', 'world'); // Prints: hello world, to stdout console.error(new Error('Whoops, something bad happened')); // Prints error message and stack trace to stderr: // Error: Whoops, something bad happened // at [eval]:5:15 // at Script.runInThisContext (node:vm:132:18) // at Object.runInThisContext (node:vm:309:38) // at node:internal/process/execution:77:19 // at [eval]-wrapper:6:22 // at evalScript (node:internal/process/execution:76:60) // at node:internal/main/eval_string:23:3 const name = 'Will Robinson'; console.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to stderr ``` Example using the `Console` class: ```js const out = getStreamSomehow(); const err = getStreamSomehow(); const myConsole = new console.Console(out, err); myConsole.log('hello world'); // Prints: hello world, to out myConsole.log('hello %s', 'world'); // Prints: hello world, to out myConsole.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to err const name = 'Will Robinson'; myConsole.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to err ```

console
.
(method) Console.log(message?: any, ...optionalParams: any[]): void

Prints to `stdout` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html) (the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)). ```js const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout ``` See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.

log
(
(parameter) message: string
message
))
33
})
34
)

Alternatively, instead of calling provideService multiple times, we can combine the service implementations into a single Context and then provide the entire context using the Effect.provide function:

Example (Combining Service Implementations)

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
22 collapsed lines
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{
7
readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number>
8
}
9
>() {}
10
11
// Declaring a tag for the logging service
12
class
class Logger
Logger
extends
import Context
Context
.
const Tag: <"MyLoggerService">(id: "MyLoggerService") => <Self, Shape>() => Context.TagClass<Self, "MyLoggerService", Shape> namespace Tag
Tag
("MyLoggerService")<
13
class Logger
Logger
,
14
{
15
readonly
(property) log: (message: string) => Effect.Effect<void>
log
: (
(parameter) message: string
message
: string) =>
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<void>
16
}
17
>() {}
18
19
const
const program: Effect.Effect<void, never, Random | Logger>
program
=
import Effect
Effect
.
const gen: <YieldWrap<Context.Tag<Logger, { readonly log: (message: string) => Effect.Effect<void>; }>> | YieldWrap<Context.Tag<Random, { readonly next: Effect.Effect<number>; }>> | YieldWrap<...>, void>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
gen
(function* () {
20
const
const random: { readonly next: Effect.Effect<number>; }
random
= yield*
class Random
Random
21
const
const logger: { readonly log: (message: string) => Effect.Effect<void>; }
logger
= yield*
class Logger
Logger
22
const
const randomNumber: number
randomNumber
= yield*
const random: { readonly next: Effect.Effect<number>; }
random
.
(property) next: Effect.Effect<number, never, never>
next
23
return yield*
const logger: { readonly log: (message: string) => Effect.Effect<void>; }
logger
.
(property) log: (message: string) => Effect.Effect<void>
log
(
var String: StringConstructor (value?: any) => string

Allows manipulation and formatting of text strings and determination and location of substrings within strings.

String
(
const randomNumber: number
randomNumber
))
24
})
25
26
// Combine service implementations into a single 'Context'
27
const
const context: Context.Context<Random | Logger>
context
=
import Context
Context
.
const empty: () => Context.Context<never>

Returns an empty `Context`.

empty
().
(method) Pipeable.pipe<Context.Context<never>, Context.Context<Random>, Context.Context<Random | Logger>>(this: Context.Context<...>, ab: (_: Context.Context<never>) => Context.Context<...>, bc: (_: Context.Context<...>) => Context.Context<...>): Context.Context<...> (+21 overloads)
pipe
(
28
import Context
Context
.
const add: <typeof Random>(tag: typeof Random, service: { readonly next: Effect.Effect<number>; }) => <Services>(self: Context.Context<Services>) => Context.Context<Random | Services> (+1 overload)

Adds a service to a given `Context`.

add
(
class Random
Random
, {
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
const sync: <number>(thunk: LazyArg<number>) => Effect.Effect<number, never, never>

Creates an `Effect` that represents a synchronous side-effectful computation. The provided function (`thunk`) should not throw errors; if it does, the error is treated as a defect. Use `Effect.sync` when you are certain the operation will not fail.

sync
(() =>
var Math: Math

An intrinsic object that provides basic mathematics functionality and constants.

Math
.
(method) Math.random(): number

Returns a pseudorandom number between 0 and 1.

random
()) }),
29
import Context
Context
.
const add: <typeof Logger>(tag: typeof Logger, service: { readonly log: (message: string) => Effect.Effect<void>; }) => <Services>(self: Context.Context<Services>) => Context.Context<...> (+1 overload)

Adds a service to a given `Context`.

add
(
class Logger
Logger
, {
30
(property) log: (message: string) => Effect.Effect<void>
log
: (
(parameter) message: string
message
) =>
import Effect
Effect
.
const sync: <void>(thunk: LazyArg<void>) => Effect.Effect<void, never, never>

Creates an `Effect` that represents a synchronous side-effectful computation. The provided function (`thunk`) should not throw errors; if it does, the error is treated as a defect. Use `Effect.sync` when you are certain the operation will not fail.

sync
(() =>
namespace console var console: Console

The `console` module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers. The module exports two specific components: * A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream. * A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and [`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module. _**Warning**_: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for more information. Example using the global `console`: ```js console.log('hello world'); // Prints: hello world, to stdout console.log('hello %s', 'world'); // Prints: hello world, to stdout console.error(new Error('Whoops, something bad happened')); // Prints error message and stack trace to stderr: // Error: Whoops, something bad happened // at [eval]:5:15 // at Script.runInThisContext (node:vm:132:18) // at Object.runInThisContext (node:vm:309:38) // at node:internal/process/execution:77:19 // at [eval]-wrapper:6:22 // at evalScript (node:internal/process/execution:76:60) // at node:internal/main/eval_string:23:3 const name = 'Will Robinson'; console.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to stderr ``` Example using the `Console` class: ```js const out = getStreamSomehow(); const err = getStreamSomehow(); const myConsole = new console.Console(out, err); myConsole.log('hello world'); // Prints: hello world, to out myConsole.log('hello %s', 'world'); // Prints: hello world, to out myConsole.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to err const name = 'Will Robinson'; myConsole.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to err ```

console
.
(method) Console.log(message?: any, ...optionalParams: any[]): void

Prints to `stdout` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html) (the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)). ```js const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout ``` See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.

log
(
(parameter) message: string
message
))
31
})
32
)
33
34
// Provide the entire context
35
const
const runnable: Effect.Effect<void, never, never>
runnable
=
import Effect
Effect
.
const provide: <void, never, Random | Logger, Random | Logger>(self: Effect.Effect<void, never, Random | Logger>, context: Context.Context<Random | Logger>) => Effect.Effect<...> (+9 overloads)

Splits the context into two parts, providing one part using the specified layer/context/runtime and leaving the remainder `R0`

provide
(
const program: Effect.Effect<void, never, Random | Logger>
program
,
const context: Context.Context<Random | Logger>
context
)

There are situations where we may want to access a service implementation only if it is available. In such cases, we can use the Effect.serviceOption function to handle this scenario.

The Effect.serviceOption function returns an implementation that is available only if it is actually provided before executing this effect. To represent this optionality it returns an Option of the implementation.

Example (Handling Optional Services)

To determine what action to take, we can use the Option.isNone function provided by the Option module. This function allows us to check if the service is available or not by returning true when the service is not available.

1
import {
import Effect
Effect
,
import Context
Context
,
import Option
Option
} from "effect"
2
3
// Declaring a tag for a service that generates random numbers
4
class
class Random
Random
extends
import Context
Context
.
const Tag: <"MyRandomService">(id: "MyRandomService") => <Self, Shape>() => Context.TagClass<Self, "MyRandomService", Shape> namespace Tag
Tag
("MyRandomService")<
5
class Random
Random
,
6
{ readonly
(property) next: Effect.Effect<number, never, never>
next
:
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<number> }
7
>() {}
8
9
const
const program: Effect.Effect<void, never, never>
program
=
import Effect
Effect
.
const gen: <YieldWrap<Effect.Effect<Option.Option<{ readonly next: Effect.Effect<number>; }>, never, never>> | YieldWrap<Effect.Effect<number, never, never>>, void>(f: (resume: Effect.Adapter) => Generator<...>) => Effect.Effect<...> (+1 overload)
gen
(function* () {
10
const
const maybeRandom: Option.Option<{ readonly next: Effect.Effect<number>; }>
maybeRandom
= yield*
import Effect
Effect
.
const serviceOption: <Random, { readonly next: Effect.Effect<number>; }>(tag: Context.Tag<Random, { readonly next: Effect.Effect<number>; }>) => Effect.Effect<Option.Option<{ readonly next: Effect.Effect<number>; }>, never, never>
serviceOption
(
class Random
Random
)
11
const
const randomNumber: number
randomNumber
=
import Option
Option
.
const isNone: <{ readonly next: Effect.Effect<number>; }>(self: Option.Option<{ readonly next: Effect.Effect<number>; }>) => self is Option.None<{ readonly next: Effect.Effect<number>; }>

Determine if a `Option` is a `None`.

isNone
(
const maybeRandom: Option.Option<{ readonly next: Effect.Effect<number>; }>
maybeRandom
)
12
? // the service is not available, return a default value
13
-1
14
: // the service is available
15
yield*
const maybeRandom: Option.Some<{ readonly next: Effect.Effect<number>; }>
maybeRandom
.
(property) Some<{ readonly next: Effect.Effect<number>; }>.value: { readonly next: Effect.Effect<number>; }
value
.
(property) next: Effect.Effect<number, never, never>
next
16
namespace console var console: Console

The `console` module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers. The module exports two specific components: * A `Console` class with methods such as `console.log()`, `console.error()` and `console.warn()` that can be used to write to any Node.js stream. * A global `console` instance configured to write to [`process.stdout`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstdout) and [`process.stderr`](https://nodejs.org/docs/latest-v22.x/api/process.html#processstderr). The global `console` can be used without importing the `node:console` module. _**Warning**_: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the [`note on process I/O`](https://nodejs.org/docs/latest-v22.x/api/process.html#a-note-on-process-io) for more information. Example using the global `console`: ```js console.log('hello world'); // Prints: hello world, to stdout console.log('hello %s', 'world'); // Prints: hello world, to stdout console.error(new Error('Whoops, something bad happened')); // Prints error message and stack trace to stderr: // Error: Whoops, something bad happened // at [eval]:5:15 // at Script.runInThisContext (node:vm:132:18) // at Object.runInThisContext (node:vm:309:38) // at node:internal/process/execution:77:19 // at [eval]-wrapper:6:22 // at evalScript (node:internal/process/execution:76:60) // at node:internal/main/eval_string:23:3 const name = 'Will Robinson'; console.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to stderr ``` Example using the `Console` class: ```js const out = getStreamSomehow(); const err = getStreamSomehow(); const myConsole = new console.Console(out, err); myConsole.log('hello world'); // Prints: hello world, to out myConsole.log('hello %s', 'world'); // Prints: hello world, to out myConsole.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to err const name = 'Will Robinson'; myConsole.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to err ```

console
.
(method) Console.log(message?: any, ...optionalParams: any[]): void

Prints to `stdout` with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to [`printf(3)`](http://man7.org/linux/man-pages/man3/printf.3.html) (the arguments are all passed to [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args)). ```js const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout ``` See [`util.format()`](https://nodejs.org/docs/latest-v22.x/api/util.html#utilformatformat-args) for more information.

log
(
const randomNumber: number
randomNumber
)
17
})

In the code above, we can observe that the Requirements type parameter of the program effect is never, even though we are working with a service. This allows us to access something from the context only if it is actually provided before executing this effect.

When we run the program effect without providing the Random service:

Effect.runPromise(program).then(console.log)
// Output: -1

We see that the log message contains -1, which is the default value we provided when the service was not available.

However, if we provide the Random service implementation:

Effect.runPromise(
Effect.provideService(program, Random, {
next: Effect.sync(() => Math.random())
})
).then(console.log)
// Example Output: 0.9957979486841035

We can observe that the log message now contains a random number generated by the next operation of the Random service.

Sometimes a service in your application may depend on other services. To maintain a clean architecture, it’s important to manage these dependencies without making them explicit in the service interface. Instead, you can use layers to handle these dependencies during the service construction phase.

Example (Defining a Logger Service with a Configuration Dependency)

Consider a scenario where multiple services depend on each other. In this case, the Logger service requires access to a configuration service (Config).

1
import {
import Effect
Effect
,
import Context
Context
} from "effect"
2
3
// Declaring a tag for the Config service
4
class
class Config
Config
extends
import Context
Context
.
const Tag: <"Config">(id: "Config") => <Self, Shape>() => Context.TagClass<Self, "Config", Shape> namespace Tag
Tag
("Config")<
class Config
Config
, {}>() {}
5
6
// Declaring a tag for the logging service
7
class
class Logger
Logger
extends
import Context
Context
.
const Tag: <"MyLoggerService">(id: "MyLoggerService") => <Self, Shape>() => Context.TagClass<Self, "MyLoggerService", Shape> namespace Tag
Tag
("MyLoggerService")<
8
class Logger
Logger
,
9
{
10
// ❌ Avoid exposing Config as a requirement
11
readonly
(property) log: (message: string) => Effect.Effect<void, never, Config>
log
: (
(parameter) message: string
message
: string) =>
import Effect
Effect
.
interface Effect<out A, out E = never, out R = never> 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.

Effect
<void, never,
class Config
Config
>
12
}
13
>() {}

To handle these dependencies in a structured way and prevent them from leaking into the service interfaces, you can use the Layer abstraction. For more details on managing dependencies with layers, refer to the Managing Layers page.