<E, E2>(f: (e: E) => E2): <A, R>(
self: Effect<A, E, R>
) => Effect<A, E2, R>
<A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>Applies mapError eagerly when an effect is already resolved.
When to use
Use when an already-resolved failed effect should apply an error transformation immediately while pending effects still use regular error mapping.
Details
Success effects pass through unchanged because there is no error to
transform. Failure effects apply the mapping function immediately, and
pending effects fall back to regular mapError behavior.
Example (Mapping errors eagerly when possible)
import { Effect } from "effect"
// For resolved failure effects, the error mapping is applied immediately
const failed = Effect.fail("original error")
const mapped = Effect.mapErrorEager(failed, (err: string) => `mapped: ${err}`) // Applied eagerly
// For pending effects, behaves like regular mapError
const pending = Effect.delay(Effect.fail("error"), "100 millis")
const mappedPending = Effect.mapErrorEager(
pending,
(err: string) => `mapped: ${err}`
) // Uses regular mapErrorexport const const mapErrorEager: {
<E, E2>(f: (e: E) => E2): <A, R>(
self: Effect<A, E, R>
) => Effect<A, E2, R>
<A, E, R, E2>(
self: Effect<A, E, R>,
f: (e: E) => E2
): Effect<A, E2, R>
}
Applies mapError eagerly when an effect is already resolved.
When to use
Use when an already-resolved failed effect should apply an error
transformation immediately while pending effects still use regular error
mapping.
Details
Success effects pass through unchanged because there is no error to
transform. Failure effects apply the mapping function immediately, and
pending effects fall back to regular mapError behavior.
Example (Mapping errors eagerly when possible)
import { Effect } from "effect"
// For resolved failure effects, the error mapping is applied immediately
const failed = Effect.fail("original error")
const mapped = Effect.mapErrorEager(failed, (err: string) => `mapped: ${err}`) // Applied eagerly
// For pending effects, behaves like regular mapError
const pending = Effect.delay(Effect.fail("error"), "100 millis")
const mappedPending = Effect.mapErrorEager(
pending,
(err: string) => `mapped: ${err}`
) // Uses regular mapError
mapErrorEager: {
<function (type parameter) E in <E, E2>(f: (e: E) => E2): <A, R>(self: Effect<A, E, R>) => Effect<A, E2, R>E, function (type parameter) E2 in <E, E2>(f: (e: E) => E2): <A, R>(self: Effect<A, E, R>) => Effect<A, E2, R>E2>(f: (e: E) => E2f: (e: Ee: function (type parameter) E in <E, E2>(f: (e: E) => E2): <A, R>(self: Effect<A, E, R>) => Effect<A, E2, R>E) => function (type parameter) E2 in <E, E2>(f: (e: E) => E2): <A, R>(self: Effect<A, E, R>) => Effect<A, E2, R>E2): <function (type parameter) A in <A, R>(self: Effect<A, E, R>): Effect<A, E2, R>A, function (type parameter) R in <A, R>(self: Effect<A, E, R>): Effect<A, E2, R>R>(self: Effect<A, E, R>(parameter) self: {
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
toString: () => string;
toJSON: () => unknown;
}
interface Effect<out A, out E = never, out R = never>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.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
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<function (type parameter) A in <A, R>(self: Effect<A, E, R>): Effect<A, E2, R>A, function (type parameter) E in <E, E2>(f: (e: E) => E2): <A, R>(self: Effect<A, E, R>) => Effect<A, E2, R>E, function (type parameter) R in <A, R>(self: Effect<A, E, R>): Effect<A, E2, R>R>) => interface Effect<out A, out E = never, out R = never>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.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
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<function (type parameter) A in <A, R>(self: Effect<A, E, R>): Effect<A, E2, R>A, function (type parameter) E2 in <E, E2>(f: (e: E) => E2): <A, R>(self: Effect<A, E, R>) => Effect<A, E2, R>E2, function (type parameter) R in <A, R>(self: Effect<A, E, R>): Effect<A, E2, R>R>
<function (type parameter) A in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>A, function (type parameter) E in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>E, function (type parameter) R in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>R, function (type parameter) E2 in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>E2>(self: Effect<A, E, R>(parameter) self: {
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
toString: () => string;
toJSON: () => unknown;
}
interface Effect<out A, out E = never, out R = never>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.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
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<function (type parameter) A in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>A, function (type parameter) E in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>E, function (type parameter) R in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>R>, f: (e: E) => E2f: (e: Ee: function (type parameter) E in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>E) => function (type parameter) E2 in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>E2): interface Effect<out A, out E = never, out R = never>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.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
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<function (type parameter) A in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>A, function (type parameter) E2 in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>E2, function (type parameter) R in <A, E, R, E2>(self: Effect<A, E, R>, f: (e: E) => E2): Effect<A, E2, R>R>
} = import internalinternal.const mapErrorEager: {
<E, E2>(f: (e: E) => E2): <A, R>(
self: Effect.Effect<A, E, R>
) => Effect.Effect<A, E2, R>
<A, E, R, E2>(
self: Effect.Effect<A, E, R>,
f: (e: E) => E2
): Effect.Effect<A, E2, R>
}