Context

This module provides a data structure called Context that can be used for dependency injection in effectful programs. It is essentially a table mapping Tags to their implementations (called Services), and can be used to manage dependencies in a type-safe way. The Context data structure is essentially a way of providing access to a set of related services that can be passed around as a single unit. This module provides functions to create, modify, and query the contents of a Context, as well as a number of utility types for working with tags and services.

34 exports Since v2.0.0 Source

Constructors

empty

Since v2.0.0

Returns an empty Context.

Signature

declare const empty: () => Context<never>

Example

import * as assert from "node:assert"
import { Context } from "effect"

assert.strictEqual(Context.isContext(Context.empty()), true)

GenericTag

Since v2.0.0

Creates a new Tag instance with the specified key.

Signature

declare const GenericTag: <Identifier, Service = Identifier>(key: string) => Tag<Identifier, Service>

Example

import * as assert from "node:assert"
import { Context } from "effect"

assert.strictEqual(Context.GenericTag("PORT").key === Context.GenericTag("PORT").key, true)

make

Since v2.0.0

Creates a new Context with a single service associated to the tag.

Signature

declare const make: <I, S>(tag: Tag<I, S>, service: Types.NoInfer<S>) => Context<I>

Example

import * as assert from "node:assert"
import { Context } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")

const Services = Context.make(Port, { PORT: 8080 })

assert.deepStrictEqual(Context.get(Services, Port), { PORT: 8080 })

Reference

Since v3.11.0

Creates a context tag with a default value.

Details

Context.Reference allows you to create a tag that can hold a value. You can provide a default value for the service, which will automatically be used when the context is accessed, or override it with a custom implementation when needed.

Signature

declare const Reference: <Self>() => <Id extends string, Service>(id: Id, options: {
  readonly defaultValue: () => Service;
}) => ReferenceClass<Self, Id, Service>

Example Declaring a Tag with a default value

import * as assert from "node:assert"
import { Context, Effect } from "effect"

class SpecialNumber extends Context.Reference<SpecialNumber>()(
  "SpecialNumber",
  { defaultValue: () => 2048 }
) {}

//      ┌─── Effect<void, never, never>
//      ▼
const program = Effect.gen(function* () {
  const specialNumber = yield* SpecialNumber
  console.log(`The special number is ${specialNumber}`)
})

// No need to provide the SpecialNumber implementation
Effect.runPromise(program)
// Output: The special number is 2048

Example Overriding the default value

import { Context, Effect } from "effect"

class SpecialNumber extends Context.Reference<SpecialNumber>()(
  "SpecialNumber",
  { defaultValue: () => 2048 }
) {}

const program = Effect.gen(function* () {
  const specialNumber = yield* SpecialNumber
  console.log(`The special number is ${specialNumber}`)
})

Effect.runPromise(program.pipe(Effect.provideService(SpecialNumber, -1)))
// Output: The special number is -1

Tag

Since v2.0.0

Signature

declare const Tag: <Id extends string>(id: Id) => <Self, Shape>() => TagClass<Self, Id, Shape>

Example

import * as assert from "node:assert"
import { Context, Layer } from "effect"

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

unsafeMake

Since v2.0.0

Signature

declare const unsafeMake: <Services>(unsafeMap: Map<string, any>) => Context<Services>

Getters

get

Since v2.0.0

Get a service from the context that corresponds to the given tag.

Signature

declare const get: {
  <I, S>(tag: Reference<I, S>): <Services>(self: Context<Services>) => S;
  <Services, I, S>(tag: Tag<I, S>): (self: Context<Services>) => S;
  <Services, I, S>(self: Context<Services>, tag: Reference<I, S>): S;
  <Services, I, S>(self: Context<Services>, tag: Tag<I, S>): S;
}

Example

import * as assert from "node:assert"
import { pipe, Context } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")
const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")

const Services = pipe(
  Context.make(Port, { PORT: 8080 }),
  Context.add(Timeout, { TIMEOUT: 5000 })
)

assert.deepStrictEqual(Context.get(Services, Timeout), { TIMEOUT: 5000 })

getOption

Since v2.0.0

Get the value associated with the specified tag from the context wrapped in an Option object. If the tag is not found, the Option object will be None.

Signature

declare const getOption: {
  <S, I>(tag: Tag<I, S>): <Services>(self: Context<Services>) => Option<S>;
  <Services, S, I>(self: Context<Services>, tag: Tag<I, S>): Option<S>;
}

Example

import * as assert from "node:assert"
import { Context, Option } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")
const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")

const Services = Context.make(Port, { PORT: 8080 })

assert.deepStrictEqual(Context.getOption(Services, Port), Option.some({ PORT: 8080 }))
assert.deepStrictEqual(Context.getOption(Services, Timeout), Option.none())

getOrElse

Since v3.7.0

Get a service from the context that corresponds to the given tag, or use the fallback value.

Signature

declare const getOrElse: {
  <S, I, B>(tag: Tag<I, S>, orElse: LazyArg<B>): <Services>(self: Context<Services>) => S | B;
  <Services, S, I, B>(self: Context<Services>, tag: Tag<I, S>, orElse: LazyArg<B>): S | B;
}

Guards

isContext

Since v2.0.0

Checks if the provided argument is a Context.

Signature

declare const isContext: (input: unknown) => input is Context<never>

Example

import * as assert from "node:assert"
import { Context } from "effect"

assert.strictEqual(Context.isContext(Context.empty()), true)

isReference

Since v3.11.0

Checks if the provided argument is a Reference.

Signature

declare const isReference: (u: unknown) => u is Reference<any, any>

isTag

Since v2.0.0

Checks if the provided argument is a Tag.

Signature

declare const isTag: (input: unknown) => input is Tag<any, any>

Example

import * as assert from "node:assert"
import { Context } from "effect"

assert.strictEqual(Context.isTag(Context.GenericTag("Tag")), true)

Models

Context

interface

Since v2.0.0

Signature

interface Context<in Services> extends Equal, Pipeable, Inspectable {
  readonly [TypeId]: {
    readonly _Services: Contravariant<Services>;
  };
  readonly unsafeMap: Map<string, any>;
}

ReadonlyTag

interface

Since v3.5.9

Signature

interface ReadonlyTag<in out Id, out Value> extends Pipeable, Inspectable, Effect<Value, never, Id> {
  readonly _op: "Tag";
  readonly [TagTypeId]: {
    readonly _Identifier: Invariant<Id>;
    readonly _Service: Covariant<Value>;
  };
  readonly Identifier: Id;
  readonly key: string;
  readonly Service: Value;
  readonly stack?: string;
}

Reference

interface

Since v3.11.0

Signature

interface Reference<in out Id, in out Value> extends Pipeable, Inspectable, STM<Value>, Effect<Value> {
  readonly _op: "Tag";
  readonly [ChannelTypeId]: VarianceStruct<never, unknown, never, unknown, Value, unknown, never>;
  readonly [EffectTypeId]: VarianceStruct<Value, never, never>;
  [ignoreSymbol]?: TagUnifyIgnore;
  readonly [ReferenceTypeId]: typeof ReferenceTypeId;
  readonly [SinkTypeId]: VarianceStruct<Value, unknown, never, never, never>;
  readonly [STMTypeId]: {
    readonly _A: Covariant<Value>;
    readonly _E: Covariant<never>;
    readonly _R: Covariant<never>;
  };
  readonly [StreamTypeId]: VarianceStruct<Value, never, never>;
  readonly [TagTypeId]: {
    readonly _Identifier: Invariant<Id>;
    readonly _Service: Invariant<Value>;
  };
  [typeSymbol]?: unknown;
  [unifySymbol]?: TagUnify<Reference<Id, Value>>;
  readonly defaultValue: () => Value;
  readonly Identifier: Id;
  readonly key: string;
  readonly Service: Value;
  readonly stack?: string;
  [iterator](): EffectGenerator<Reference<Id, Value>>;
  context(self: Value): Context<Id>;
  of(self: Value): Value;
}

ReferenceClass

interface

Since v3.11.0

Signature

interface ReferenceClass<Self, Id extends string, Type> extends Reference<Self, Type> {
  constructor(_: never);
  readonly key: Id;
}

Tag

interface

Since v3.5.9

Signature

interface Tag<in out Id, in out Value> extends Pipeable, Inspectable, ReadonlyTag<Id, Value>, STM<Value, never, Id>, Effect<Value, never, Id> {
  readonly _op: "Tag";
  [ignoreSymbol]?: TagUnifyIgnore;
  readonly [STMTypeId]: {
    readonly _A: Covariant<Value>;
    readonly _E: Covariant<never>;
    readonly _R: Covariant<Id>;
  };
  readonly [TagTypeId]: {
    readonly _Identifier: Invariant<Id>;
    readonly _Service: Invariant<Value>;
  };
  [typeSymbol]?: unknown;
  [unifySymbol]?: TagUnify<Tag<Id, Value>>;
  readonly Identifier: Id;
  readonly key: string;
  readonly Service: Value;
  readonly stack?: string;
  context(self: Value): Context<Id>;
  of(self: Value): Value;
}

TagClass

interface

Since v2.0.0

Signature

interface TagClass<Self, Id extends string, Type> extends Tag<Self, Type> {
  constructor(_: never);
  readonly key: Id;
}

TagClassShape

interface

Since v2.0.0

Signature

interface TagClassShape<Id, Shape> {
  readonly [TagTypeId]: typeof TagTypeId;
  Id: Id;
  readonly Type: Shape;
}

TagUnify

interface

Since v2.0.0

Signature

interface TagUnify<A extends {
  [typeSymbol]?: any;
}> {
  Tag?: () => Extract<A[typeof typeSymbol], Tag<any, any>>;
}

TagUnifyIgnore

interface

Since v2.0.0

Signature

interface TagUnifyIgnore {
  Effect?: true;
  Either?: true;
  Option?: true;
}

ValidTagsById

type

Since v2.0.0

Signature

type ValidTagsById<R> = R extends infer S ? Tag<S, any> : never

Other

add

Since v2.0.0

Adds a service to a given Context.

Signature

declare const add: {
  <I, S>(tag: Tag<I, S>, service: NoInfer<S>): <Services>(self: Context<Services>) => Context<I | Services>;
  <Services, I, S>(self: Context<Services>, tag: Tag<I, S>, service: NoInfer<S>): Context<Services | I>;
}

Example

import * as assert from "node:assert"
import { Context, pipe } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")
const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")

const someContext = Context.make(Port, { PORT: 8080 })

const Services = pipe(
  someContext,
  Context.add(Timeout, { TIMEOUT: 5000 })
)

assert.deepStrictEqual(Context.get(Services, Port), { PORT: 8080 })
assert.deepStrictEqual(Context.get(Services, Timeout), { TIMEOUT: 5000 })

merge

Since v2.0.0

Merges two Contexts, returning a new Context containing the services of both.

Signature

declare const merge: {
  <R1>(that: Context<R1>): <Services>(self: Context<Services>) => Context<R1 | Services>;
  <Services, R1>(self: Context<Services>, that: Context<R1>): Context<Services | R1>;
}

Example

import * as assert from "node:assert"
import { Context } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")
const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")

const firstContext = Context.make(Port, { PORT: 8080 })
const secondContext = Context.make(Timeout, { TIMEOUT: 5000 })

const Services = Context.merge(firstContext, secondContext)

assert.deepStrictEqual(Context.get(Services, Port), { PORT: 8080 })
assert.deepStrictEqual(Context.get(Services, Timeout), { TIMEOUT: 5000 })

mergeAll

Since v3.12.0

Merges any number of Contexts, returning a new Context containing the services of all.

Signature

declare const mergeAll: <T extends Array<unknown>>(...ctxs: [...{ [K in keyof T]: Context<T[K]> }]) => Context<T[number]>

Example

import * as assert from "node:assert"
import { Context } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")
const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")
const Host = Context.GenericTag<{ HOST: string }>("Host")

const firstContext = Context.make(Port, { PORT: 8080 })
const secondContext = Context.make(Timeout, { TIMEOUT: 5000 })
const thirdContext = Context.make(Host, { HOST: "localhost" })

const Services = Context.mergeAll(firstContext, secondContext, thirdContext)

assert.deepStrictEqual(Context.get(Services, Port), { PORT: 8080 })
assert.deepStrictEqual(Context.get(Services, Timeout), { TIMEOUT: 5000 })
assert.deepStrictEqual(Context.get(Services, Host), { HOST: "localhost" })

omit

Since v2.0.0

Signature

declare const omit: <Tags extends ReadonlyArray<Tag<any, any>>>(...tags: Tags) => <Services>(self: Context<Services>) => Context<Exclude<Services, Tag.Identifier<Tags[number]>>>

pick

Since v2.0.0

Returns a new Context that contains only the specified services.

Signature

declare const pick: <Tags extends ReadonlyArray<Tag<any, any>>>(...tags: Tags) => <Services>(self: Context<Services>) => Context<Services & Tag.Identifier<Tags[number]>>

Example

import * as assert from "node:assert"
import { pipe, Context, Option } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")
const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")

const someContext = pipe(
  Context.make(Port, { PORT: 8080 }),
  Context.add(Timeout, { TIMEOUT: 5000 })
)

const Services = pipe(someContext, Context.pick(Port))

assert.deepStrictEqual(Context.getOption(Services, Port), Option.some({ PORT: 8080 }))
assert.deepStrictEqual(Context.getOption(Services, Timeout), Option.none())

Signature

declare const ReferenceTypeId: unique symbol

ReferenceTypeId

type

Since v3.11.0

Signature

type ReferenceTypeId = typeof ReferenceTypeId

TagTypeId

Since v2.0.0

Signature

declare const TagTypeId: unique symbol

TagTypeId

type

Since v2.0.0

Signature

type TagTypeId = typeof TagTypeId

TypeId

type

Since v2.0.0

Signature

type TypeId = typeof TypeId

Unsafe

unsafeGet

Since v2.0.0

Get a service from the context that corresponds to the given tag. This function is unsafe because if the tag is not present in the context, a runtime error will be thrown.

For a safer version see getOption.

Signature

declare const unsafeGet: {
  <S, I>(tag: Tag<I, S>): <Services>(self: Context<Services>) => S;
  <Services, S, I>(self: Context<Services>, tag: Tag<I, S>): S;
}

Example

import * as assert from "node:assert"
import { Context } from "effect"

const Port = Context.GenericTag<{ PORT: number }>("Port")
const Timeout = Context.GenericTag<{ TIMEOUT: number }>("Timeout")

const Services = Context.make(Port, { PORT: 8080 })

assert.deepStrictEqual(Context.unsafeGet(Services, Port), { PORT: 8080 })
assert.throws(() => Context.unsafeGet(Services, Timeout))