<
Layers extends [
Layer<never, any, any>,
...Array<Layer<never, any, any>>
]
>(
...layers: Layers
): Layer<
Success<Layers[number]>,
Error<Layers[number]>,
Services<Layers[number]>
>Combines all the provided layers concurrently, creating a new layer with merged input, error, and output types.
When to use
Use when you need to combine multiple independent layers.
Details
All layers are built concurrently, and their outputs are merged into a single layer.
If multiple merged layers depend on the same layer value, that dependency is shared by default. Reuse a named layer value when you want services to share the same resource, such as one database pool.
Example (Merging independent layers)
import { Context, Effect, Layer } from "effect"
class Database extends Context.Service<Database, {
readonly query: (sql: string) => Effect.Effect<string>
}>()("Database") {}
class Logger extends Context.Service<Logger, {
readonly log: (msg: string) => Effect.Effect<void>
}>()("Logger") {}
const dbLayer = Layer.succeed(Database, {
query: Effect.fn("Database.query")((sql: string) => Effect.succeed("result"))
})
const loggerLayer = Layer.succeed(Logger, {
log: Effect.fn("Logger.log")((msg: string) => Effect.sync(() => console.log(msg)))
})
const mergedLayer = Layer.mergeAll(dbLayer, loggerLayer)export const const mergeAll: <
Layers extends [
Layer<never, any, any>,
...Array<Layer<never, any, any>>
]
>(
...layers: Layers
) => Layer<
Success<Layers[number]>,
Error<Layers[number]>,
Services<Layers[number]>
>
Combines all the provided layers concurrently, creating a new layer with
merged input, error, and output types.
When to use
Use when you need to combine multiple independent layers.
Details
All layers are built concurrently, and their outputs are merged into a single layer.
If multiple merged layers depend on the same layer value, that dependency is
shared by default. Reuse a named layer value when you want services to share
the same resource, such as one database pool.
Example (Merging independent layers)
import { Context, Effect, Layer } from "effect"
class Database extends Context.Service<Database, {
readonly query: (sql: string) => Effect.Effect<string>
}>()("Database") {}
class Logger extends Context.Service<Logger, {
readonly log: (msg: string) => Effect.Effect<void>
}>()("Logger") {}
const dbLayer = Layer.succeed(Database, {
query: Effect.fn("Database.query")((sql: string) => Effect.succeed("result"))
})
const loggerLayer = Layer.succeed(Logger, {
log: Effect.fn("Logger.log")((msg: string) => Effect.sync(() => console.log(msg)))
})
const mergedLayer = Layer.mergeAll(dbLayer, loggerLayer)
mergeAll = <function (type parameter) Layers in <Layers extends [Layer<never, any, any>, ...Array<Layer<never, any, any>>]>(...layers: Layers): Layer<Success<Layers[number]>, Error<Layers[number]>, Services<Layers[number]>>Layers extends [interface Layer<in ROut, out E = never, out RIn = never>A Layer describes how to build one or more services for dependency injection.
When to use
Use to model construction of application services for dependency injection,
especially when services have dependencies, can fail during construction, or
need scoped setup and release.
Details
A Layer<ROut, E, RIn> represents ROut as the services this layer
provides, E as the possible errors during layer construction, and RIn as
the services this layer requires as dependencies.
Layer<never, any, any>, ...interface Array<T>Array<interface Layer<in ROut, out E = never, out RIn = never>A Layer describes how to build one or more services for dependency injection.
When to use
Use to model construction of application services for dependency injection,
especially when services have dependencies, can fail during construction, or
need scoped setup and release.
Details
A Layer<ROut, E, RIn> represents ROut as the services this layer
provides, E as the possible errors during layer construction, and RIn as
the services this layer requires as dependencies.
Layer<never, any, any>>]>(
...layers: Layers extends [Layer<never, any, any>, ...Array<Layer<never, any, any>>]layers: function (type parameter) Layers in <Layers extends [Layer<never, any, any>, ...Array<Layer<never, any, any>>]>(...layers: Layers): Layer<Success<Layers[number]>, Error<Layers[number]>, Services<Layers[number]>>Layers
): interface Layer<in ROut, out E = never, out RIn = never>A Layer describes how to build one or more services for dependency injection.
When to use
Use to model construction of application services for dependency injection,
especially when services have dependencies, can fail during construction, or
need scoped setup and release.
Details
A Layer<ROut, E, RIn> represents ROut as the services this layer
provides, E as the possible errors during layer construction, and RIn as
the services this layer requires as dependencies.
Layer<
type Success<T extends Any> =
T extends Layer<
infer _ROut,
infer _E,
infer _RIn
>
? _ROut
: never
Extracts the service output type (ROut) from a Layer type.
When to use
Use to derive the services provided by an existing or generic Layer without
restating its ROut type parameter.
Success<function (type parameter) Layers in <Layers extends [Layer<never, any, any>, ...Array<Layer<never, any, any>>]>(...layers: Layers): Layer<Success<Layers[number]>, Error<Layers[number]>, Services<Layers[number]>>Layers[number]>,
type Error<T extends Any> =
T extends Layer<
infer _ROut,
infer _E,
infer _RIn
>
? _E
: never
Extracts the error type (E) from a Layer type.
When to use
Use to derive a layer construction error type for helper types, wrappers, or
APIs that preserve a layer failure channel.
Error<function (type parameter) Layers in <Layers extends [Layer<never, any, any>, ...Array<Layer<never, any, any>>]>(...layers: Layers): Layer<Success<Layers[number]>, Error<Layers[number]>, Services<Layers[number]>>Layers[number]>,
type Services<T extends Any> =
T extends infer L
? L extends Layer<
infer _ROut,
infer _E,
infer _RIn
>
? _RIn
: never
: never
Extracts the service requirements (RIn) from a Layer type.
When to use
Use to derive the dependency requirements of a generic or inferred Layer
without restating its RIn type parameter.
Services<function (type parameter) Layers in <Layers extends [Layer<never, any, any>, ...Array<Layer<never, any, any>>]>(...layers: Layers): Layer<Success<Layers[number]>, Error<Layers[number]>, Services<Layers[number]>>Layers[number]>
> => const fromBuild: <ROut, E, RIn>(
build: (
memoMap: MemoMap,
scope: Scope.Scope
) => Effect<Context.Context<ROut>, E, RIn>
) => Layer<ROut, E, RIn>
Constructs a Layer from a function that uses a MemoMap and Scope to
build the layer.
Details
The function receives a MemoMap for memoization and a Scope for resource management.
A child scope is created, and if the build fails, the child scope is closed.
Example (Constructing a layer from a build function)
import { Context, Effect, Layer } from "effect"
class Database extends Context.Service<Database, {
readonly query: (sql: string) => Effect.Effect<string>
}>()("Database") {}
const databaseLayer = Layer.fromBuild(() =>
Effect.sync(() =>
Context.make(Database, {
query: (sql: string) => Effect.succeed("result")
})
)
)
fromBuild((memoMap: MemoMap(parameter) memoMap: {
get: <RIn, E, ROut>(layer: Layer<ROut, E, RIn>, scope: Scope.Scope) => Effect<Context.Context<ROut>, E, RIn> | undefined;
getOrElseMemoize: <RIn, E, ROut>(layer: Layer<ROut, E, RIn>, scope: Scope.Scope, build: (memoMap: MemoMap, scope: Scope.Scope) => Effect<Context.Context<ROut>, E, RIn>) => Effect<Context.Context<ROut>, E, RIn>;
}
scope: Scope.Scope(parameter) scope: {
strategy: "sequential" | "parallel";
state: State.Open | State.Closed | State.Empty;
}
const mergeAllEffect: <
Layers extends [
Layer<never, any, any>,
...Array<Layer<never, any, any>>
]
>(
layers: Layers,
memoMap: MemoMap,
scope: Scope.Scope
) => Effect<
Context.Context<
{
[k in keyof Layers]: Success<Layers[k]>
}[number]
>,
{
[k in keyof Layers]: Error<Layers[k]>
}[number],
{
[k in keyof Layers]: Services<Layers[k]>
}[number]
>
layers: Layers extends [Layer<never, any, any>, ...Array<Layer<never, any, any>>]layers, memoMap: MemoMap(parameter) memoMap: {
get: <RIn, E, ROut>(layer: Layer<ROut, E, RIn>, scope: Scope.Scope) => Effect<Context.Context<ROut>, E, RIn> | undefined;
getOrElseMemoize: <RIn, E, ROut>(layer: Layer<ROut, E, RIn>, scope: Scope.Scope, build: (memoMap: MemoMap, scope: Scope.Scope) => Effect<Context.Context<ROut>, E, RIn>) => Effect<Context.Context<ROut>, E, RIn>;
}
scope: Scope.Scope(parameter) scope: {
strategy: "sequential" | "parallel";
state: State.Open | State.Closed | State.Empty;
}