<E, A2, E2, R2>(
predicate: Predicate<Cause.Cause<E>>,
f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>
): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>
<A, E, R, A2, E2, R2>(
self: Stream<A, E, R>,
predicate: Predicate<Cause.Cause<E>>,
f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>
): Stream<A | A2, E | E2, R | R2>Recovers from stream failures by filtering the Cause and switching to a recovery stream.
Non-matching causes are re-emitted as failures.
Example (Catching matching causes)
import { Cause, Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
const failingStream = Stream.fail("NetworkError")
const recovered = Stream.catchCauseIf(
failingStream,
(cause) => Cause.hasFails(cause),
(cause) => Stream.make(`Recovered: ${Cause.squash(cause)}`)
)
const output = yield* Stream.runCollect(recovered)
yield* Console.log(output)
})
Effect.runPromise(program)
// Output: [ "Recovered: NetworkError" ]export const const catchCauseIf: {
<E, A2, E2, R2>(
predicate: Predicate<Cause.Cause<E>>,
f: (
cause: Cause.Cause<E>
) => Stream<A2, E2, R2>
): <A, R>(
self: Stream<A, E, R>
) => Stream<A | A2, E | E2, R2 | R>
<A, E, R, A2, E2, R2>(
self: Stream<A, E, R>,
predicate: Predicate<Cause.Cause<E>>,
f: (
cause: Cause.Cause<E>
) => Stream<A2, E2, R2>
): Stream<A | A2, E | E2, R | R2>
}
Recovers from stream failures by filtering the Cause and switching to a recovery stream.
Non-matching causes are re-emitted as failures.
Example (Catching matching causes)
import { Cause, Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
const failingStream = Stream.fail("NetworkError")
const recovered = Stream.catchCauseIf(
failingStream,
(cause) => Cause.hasFails(cause),
(cause) => Stream.make(`Recovered: ${Cause.squash(cause)}`)
)
const output = yield* Stream.runCollect(recovered)
yield* Console.log(output)
})
Effect.runPromise(program)
// Output: [ "Recovered: NetworkError" ]
catchCauseIf: {
<function (type parameter) E in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E, function (type parameter) A2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>A2, function (type parameter) E2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E2, function (type parameter) R2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>R2>(
predicate: Predicate<Cause.Cause<E>>predicate: interface Predicate<in A>A function that decides whether a value of type A satisfies a condition.
When to use
Use when you want a reusable boolean check for A, especially when you plan
to combine checks with
and
/
or
or pass a predicate to arrays
and iterables.
Details
A predicate returns true or false and never throws by itself. It does not
narrow types unless you use Refinement.
Example (Defining a predicate)
import { Predicate } from "effect"
const isPositive: Predicate.Predicate<number> = (n) => n > 0
console.log(isPositive(1))
Type-level utilities for working with
Predicate
types.
When to use
Use when you need to extract input types from predicate signatures while
writing generic helpers over predicate types.
Details
These utilities are type-only, create no runtime values, and the namespace is
erased at runtime.
Example (Extracting predicate input)
import { Predicate } from "effect"
type IsString = Predicate.Predicate<string>
type Input = Predicate.Predicate.In<IsString>
Predicate<import CauseCause.interface Cause<out E>A structured representation of how an Effect failed.
When to use
Use to preserve the full structured failure information for an effect instead
of collapsing it to a single error value.
Details
Access the individual failure entries through the reasons array, then
narrow each entry with
isFailReason
,
isDieReason
, or
- Use
hasFails
/
hasDies
/
hasInterrupts
to test
for the presence of specific reason kinds without iterating.
- Use
findError
/
findDefect
to extract the first value
of a given kind.
- Use
combine
to merge two causes.
Cause implements Equal — two causes with the same reasons (by value)
compare as equal.
Example (Creating and inspecting a cause)
import { Cause } from "effect"
const cause = Cause.fail("Something went wrong")
console.log(cause.reasons.length) // 1
console.log(Cause.isFailReason(cause.reasons[0])) // true
Companion namespace for the Cause interface.
Cause<function (type parameter) E in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E>>,
f: (
cause: Cause.Cause<E>
) => Stream<A2, E2, R2>
cause: Cause.Cause<E>(parameter) cause: {
reasons: ReadonlyArray<Reason<E>>;
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;
}
import CauseCause.interface Cause<out E>A structured representation of how an Effect failed.
When to use
Use to preserve the full structured failure information for an effect instead
of collapsing it to a single error value.
Details
Access the individual failure entries through the reasons array, then
narrow each entry with
isFailReason
,
isDieReason
, or
- Use
hasFails
/
hasDies
/
hasInterrupts
to test
for the presence of specific reason kinds without iterating.
- Use
findError
/
findDefect
to extract the first value
of a given kind.
- Use
combine
to merge two causes.
Cause implements Equal — two causes with the same reasons (by value)
compare as equal.
Example (Creating and inspecting a cause)
import { Cause } from "effect"
const cause = Cause.fail("Something went wrong")
console.log(cause.reasons.length) // 1
console.log(Cause.isFailReason(cause.reasons[0])) // true
Companion namespace for the Cause interface.
Cause<function (type parameter) E in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E>) => interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>A2, function (type parameter) E2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E2, function (type parameter) R2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>R2>
): <function (type parameter) A in <A, R>(self: Stream<A, E, R>): Stream<A | A2, E | E2, R2 | R>A, function (type parameter) R in <A, R>(self: Stream<A, E, R>): Stream<A | A2, E | E2, R2 | R>R>(
self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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; <…;
}
interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, R>(self: Stream<A, E, R>): Stream<A | A2, E | E2, R2 | R>A, function (type parameter) E in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E, function (type parameter) R in <A, R>(self: Stream<A, E, R>): Stream<A | A2, E | E2, R2 | R>R>
) => interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, R>(self: Stream<A, E, R>): Stream<A | A2, E | E2, R2 | R>A | function (type parameter) A2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>A2, function (type parameter) E in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E | function (type parameter) E2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>E2, function (type parameter) R2 in <E, A2, E2, R2>(predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): <A, R>(self: Stream<A, E, R>) => Stream<A | A2, E | E2, R2 | R>R2 | function (type parameter) R in <A, R>(self: Stream<A, E, R>): Stream<A | A2, E | E2, R2 | R>R>
<function (type parameter) A in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A, function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E, function (type parameter) R in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R, function (type parameter) A2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A2, function (type parameter) E2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E2, function (type parameter) R2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R2>(
self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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; <…;
}
interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A, function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E, function (type parameter) R in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R>,
predicate: Predicate<Cause.Cause<E>>predicate: interface Predicate<in A>A function that decides whether a value of type A satisfies a condition.
When to use
Use when you want a reusable boolean check for A, especially when you plan
to combine checks with
and
/
or
or pass a predicate to arrays
and iterables.
Details
A predicate returns true or false and never throws by itself. It does not
narrow types unless you use Refinement.
Example (Defining a predicate)
import { Predicate } from "effect"
const isPositive: Predicate.Predicate<number> = (n) => n > 0
console.log(isPositive(1))
Type-level utilities for working with
Predicate
types.
When to use
Use when you need to extract input types from predicate signatures while
writing generic helpers over predicate types.
Details
These utilities are type-only, create no runtime values, and the namespace is
erased at runtime.
Example (Extracting predicate input)
import { Predicate } from "effect"
type IsString = Predicate.Predicate<string>
type Input = Predicate.Predicate.In<IsString>
Predicate<import CauseCause.interface Cause<out E>A structured representation of how an Effect failed.
When to use
Use to preserve the full structured failure information for an effect instead
of collapsing it to a single error value.
Details
Access the individual failure entries through the reasons array, then
narrow each entry with
isFailReason
,
isDieReason
, or
- Use
hasFails
/
hasDies
/
hasInterrupts
to test
for the presence of specific reason kinds without iterating.
- Use
findError
/
findDefect
to extract the first value
of a given kind.
- Use
combine
to merge two causes.
Cause implements Equal — two causes with the same reasons (by value)
compare as equal.
Example (Creating and inspecting a cause)
import { Cause } from "effect"
const cause = Cause.fail("Something went wrong")
console.log(cause.reasons.length) // 1
console.log(Cause.isFailReason(cause.reasons[0])) // true
Companion namespace for the Cause interface.
Cause<function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E>>,
f: (
cause: Cause.Cause<E>
) => Stream<A2, E2, R2>
cause: Cause.Cause<E>(parameter) cause: {
reasons: ReadonlyArray<Reason<E>>;
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;
}
import CauseCause.interface Cause<out E>A structured representation of how an Effect failed.
When to use
Use to preserve the full structured failure information for an effect instead
of collapsing it to a single error value.
Details
Access the individual failure entries through the reasons array, then
narrow each entry with
isFailReason
,
isDieReason
, or
- Use
hasFails
/
hasDies
/
hasInterrupts
to test
for the presence of specific reason kinds without iterating.
- Use
findError
/
findDefect
to extract the first value
of a given kind.
- Use
combine
to merge two causes.
Cause implements Equal — two causes with the same reasons (by value)
compare as equal.
Example (Creating and inspecting a cause)
import { Cause } from "effect"
const cause = Cause.fail("Something went wrong")
console.log(cause.reasons.length) // 1
console.log(Cause.isFailReason(cause.reasons[0])) // true
Companion namespace for the Cause interface.
Cause<function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E>) => interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A2, function (type parameter) E2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E2, function (type parameter) R2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R2>
): interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A | function (type parameter) A2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A2, function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E | function (type parameter) E2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E2, function (type parameter) R in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R | function (type parameter) R2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R2>
} = dual<(...args: Array<any>) => any, <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>) => Stream<A | A2, E | E2, R | R2>>(arity: 3, body: <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>) => Stream<A | A2, E | E2, R | R2>): ((...args: Array<any>) => any) & (<A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>) => Stream<A | A2, E | E2, R | R2>) (+1 overload)Creates a function that can be called in data-first style or data-last
(pipe-friendly) style.
When to use
Use to expose one implementation through both direct and pipe-friendly
call styles.
Details
Pass either the arity of the uncurried function or a predicate that decides
whether the current call is data-first. Arity is the common case. Use a
predicate when optional arguments make arity ambiguous.
Example (Selecting data-first or data-last style by arity)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(2, (self, that) => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Defining overloads with call signatures)
import { Function, pipe } from "effect"
const sum: {
(that: number): (self: number) => number
(self: number, that: number): number
} = Function.dual(2, (self: number, that: number): number => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Selecting data-first or data-last style with a predicate)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(
(args) => args.length === 2,
(self, that) => self + that
)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
dual(3, <function (type parameter) A in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A, function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E, function (type parameter) R in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R, function (type parameter) A2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A2, function (type parameter) E2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E2, function (type parameter) R2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R2>(
self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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; <…;
}
interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A, function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E, function (type parameter) R in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R>,
predicate: Predicate<Cause.Cause<E>>predicate: interface Predicate<in A>A function that decides whether a value of type A satisfies a condition.
When to use
Use when you want a reusable boolean check for A, especially when you plan
to combine checks with
and
/
or
or pass a predicate to arrays
and iterables.
Details
A predicate returns true or false and never throws by itself. It does not
narrow types unless you use Refinement.
Example (Defining a predicate)
import { Predicate } from "effect"
const isPositive: Predicate.Predicate<number> = (n) => n > 0
console.log(isPositive(1))
Type-level utilities for working with
Predicate
types.
When to use
Use when you need to extract input types from predicate signatures while
writing generic helpers over predicate types.
Details
These utilities are type-only, create no runtime values, and the namespace is
erased at runtime.
Example (Extracting predicate input)
import { Predicate } from "effect"
type IsString = Predicate.Predicate<string>
type Input = Predicate.Predicate.In<IsString>
Predicate<import CauseCause.interface Cause<out E>A structured representation of how an Effect failed.
When to use
Use to preserve the full structured failure information for an effect instead
of collapsing it to a single error value.
Details
Access the individual failure entries through the reasons array, then
narrow each entry with
isFailReason
,
isDieReason
, or
- Use
hasFails
/
hasDies
/
hasInterrupts
to test
for the presence of specific reason kinds without iterating.
- Use
findError
/
findDefect
to extract the first value
of a given kind.
- Use
combine
to merge two causes.
Cause implements Equal — two causes with the same reasons (by value)
compare as equal.
Example (Creating and inspecting a cause)
import { Cause } from "effect"
const cause = Cause.fail("Something went wrong")
console.log(cause.reasons.length) // 1
console.log(Cause.isFailReason(cause.reasons[0])) // true
Companion namespace for the Cause interface.
Cause<function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E>>,
f: (
cause: Cause.Cause<E>
) => Stream<A2, E2, R2>
cause: Cause.Cause<E>(parameter) cause: {
reasons: ReadonlyArray<Reason<E>>;
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;
}
import CauseCause.interface Cause<out E>A structured representation of how an Effect failed.
When to use
Use to preserve the full structured failure information for an effect instead
of collapsing it to a single error value.
Details
Access the individual failure entries through the reasons array, then
narrow each entry with
isFailReason
,
isDieReason
, or
- Use
hasFails
/
hasDies
/
hasInterrupts
to test
for the presence of specific reason kinds without iterating.
- Use
findError
/
findDefect
to extract the first value
of a given kind.
- Use
combine
to merge two causes.
Cause implements Equal — two causes with the same reasons (by value)
compare as equal.
Example (Creating and inspecting a cause)
import { Cause } from "effect"
const cause = Cause.fail("Something went wrong")
console.log(cause.reasons.length) // 1
console.log(Cause.isFailReason(cause.reasons[0])) // true
Companion namespace for the Cause interface.
Cause<function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E>) => interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A2, function (type parameter) E2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E2, function (type parameter) R2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R2>
): interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A | function (type parameter) A2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>A2, function (type parameter) E in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E | function (type parameter) E2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>E2, function (type parameter) R in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R | function (type parameter) R2 in <A, E, R, A2, E2, R2>(self: Stream<A, E, R>, predicate: Predicate<Cause.Cause<E>>, f: (cause: Cause.Cause<E>) => Stream<A2, E2, R2>): Stream<A | A2, E | E2, R | R2>R2> =>
const fromChannel: <
Arr extends Arr.NonEmptyReadonlyArray<any>,
E,
R
>(
channel: Channel.Channel<
Arr,
E,
void,
unknown,
unknown,
unknown,
R
>
) => Stream<
Arr extends Arr.NonEmptyReadonlyArray<infer A>
? A
: never,
E,
R
>
Creates a stream from a array-emitting Channel.
Example (Creating a stream from an array-emitting channel)
import { Channel, Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
const channel = Channel.succeed([1, 2, 3] as const)
const stream = Stream.fromChannel(channel)
const result = yield* Stream.runCollect(stream)
yield* Console.log(result)
})
// Output: [ 1, 2, 3 ]
fromChannel(
import ChannelChannel.const catchCauseIf: {
<
OutErr,
OutElem1,
OutErr1,
OutDone1,
InElem1,
InErr1,
InDone1,
Env1
>(
predicate: Predicate.Predicate<
Cause.Cause<OutErr>
>,
f: (
cause: Cause.Cause<OutErr>
) => Channel<
OutElem1,
OutErr1,
OutDone1,
InElem1,
InErr1,
InDone1,
Env1
>
): <
OutElem,
OutDone,
InElem,
InErr,
InDone,
Env
>(
self: Channel<
OutElem,
OutErr,
OutDone,
InElem,
InErr,
InDone,
Env
>
) => Channel<
OutElem | OutElem1,
OutErr | OutErr1,
OutDone | OutDone1,
InElem & InElem1,
InErr & InErr1,
InDone & InDone1,
Env | Env1
>
<
OutElem,
OutErr,
OutDone,
InElem,
InErr,
InDone,
Env,
OutElem1,
OutErr1,
OutDone1,
InElem1,
InErr1,
InDone1,
Env1
>(
self: Channel<
OutElem,
OutErr,
OutDone,
InElem,
InErr,
InDone,
Env
>,
predicate: Predicate.Predicate<
Cause.Cause<OutErr>
>,
f: (
cause: Cause.Cause<OutErr>
) => Channel<
OutElem1,
OutErr1,
OutDone1,
InElem1,
InErr1,
InDone1,
Env1
>
): Channel<
OutElem | OutElem1,
OutErr | OutErr1,
OutDone | OutDone1,
InElem & InElem1,
InErr & InErr1,
InDone & InDone1,
Env | Env1
>
}
self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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; <…;
}
Stream<A, E, R>.channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>(property) Stream<A, E, R>.channel: {
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; <…;
}
predicate: Predicate<Cause.Cause<E>>predicate,
(cause: Cause.Cause<E>(parameter) cause: {
reasons: ReadonlyArray<Reason<E>>;
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;
}
f: (
cause: Cause.Cause<E>
) => Stream<A2, E2, R2>
cause: Cause.Cause<E>(parameter) cause: {
reasons: ReadonlyArray<Reason<E>>;
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;
}
Stream<A2, E2, R2>.channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>(property) Stream<A2, E2, R2>.channel: {
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; <…;
}