<A, B extends A, E2>(
refinement: Refinement<NoInfer<A>, B>,
orFailWith: (value: NoInfer<A>) => E2
): <E>(self: Result<A, E>) => Result<B, E2 | E>
<A, E2>(
predicate: Predicate<NoInfer<A>>,
orFailWith: (value: NoInfer<A>) => E2
): <E>(self: Result<A, E>) => Result<A, E2 | E>
<A, E, B extends A, E2>(
self: Result<A, E>,
refinement: Refinement<A, B>,
orFailWith: (value: A) => E2
): Result<B, E | E2>
<A, E, E2>(
self: Result<A, E>,
predicate: Predicate<A>,
orFailWith: (value: A) => E2
): Result<A, E | E2>Validates the success value of a Result using a predicate, failing with a
custom error if the predicate returns false.
When to use
Use to validate an already-successful Result value with a predicate or
refinement.
Details
- If the result is already a
Failure, it is returned as-is - If the predicate passes, the
Successis returned unchanged - If the predicate fails,
orFailWithproduces the error for a newFailure - Also accepts a
Refinementto narrow the success type - The error type of the output is the union of both error types
Example (Filtering a success value)
import { pipe, Result } from "effect"
const result = pipe(
Result.succeed(0),
Result.filterOrFail(
(n) => n > 0,
(n) => `${n} is not positive`
)
)
console.log(result)
// Output: { _tag: "Failure", failure: "0 is not positive", ... }export const const filterOrFail: {
<A, B extends A, E2>(
refinement: Refinement<NoInfer<A>, B>,
orFailWith: (value: NoInfer<A>) => E2
): <E>(self: Result<A, E>) => Result<B, E2 | E>
<A, E2>(
predicate: Predicate<NoInfer<A>>,
orFailWith: (value: NoInfer<A>) => E2
): <E>(self: Result<A, E>) => Result<A, E2 | E>
<A, E, B extends A, E2>(
self: Result<A, E>,
refinement: Refinement<A, B>,
orFailWith: (value: A) => E2
): Result<B, E | E2>
<A, E, E2>(
self: Result<A, E>,
predicate: Predicate<A>,
orFailWith: (value: A) => E2
): Result<A, E | E2>
}
Validates the success value of a Result using a predicate, failing with a
custom error if the predicate returns false.
When to use
Use to validate an already-successful Result value with a predicate or
refinement.
Details
- If the result is already a
Failure, it is returned as-is
- If the predicate passes, the
Success is returned unchanged
- If the predicate fails,
orFailWith produces the error for a new Failure
- Also accepts a
Refinement to narrow the success type
- The error type of the output is the union of both error types
Example (Filtering a success value)
import { pipe, Result } from "effect"
const result = pipe(
Result.succeed(0),
Result.filterOrFail(
(n) => n > 0,
(n) => `${n} is not positive`
)
)
console.log(result)
// Output: { _tag: "Failure", failure: "0 is not positive", ... }
filterOrFail: {
<function (type parameter) A in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>A, function (type parameter) B in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>B extends function (type parameter) A in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>A, function (type parameter) E2 in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>E2>(
refinement: Refinement<NoInfer<A>, B>refinement: interface Refinement<in A, out B extends A>A predicate that also narrows the input type when it returns true.
When to use
Use when you want a runtime check that refines A to B for TypeScript,
especially when composing type guards with
compose
or safely
checking unknown values.
Details
A refinement returns a type predicate (a is B). Use it with if or
filter to narrow types.
Example (Narrowing unknown values)
import { Predicate } from "effect"
const isString: Predicate.Refinement<unknown, string> = (u): u is string => typeof u === "string"
const data: unknown = "hello"
if (isString(data)) {
console.log(data.toUpperCase())
}
Type-level utilities for working with
Refinement
types.
When to use
Use when you need to extract input and output types from refinement
signatures while writing generic helpers over refinements.
Details
These utilities are type-only, create no runtime values, and the namespace is
erased at runtime.
Example (Extracting refinement types)
import { Predicate } from "effect"
type IsString = Predicate.Refinement<unknown, string>
type Input = Predicate.Refinement.In<IsString>
type Output = Predicate.Refinement.Out<IsString>
Refinement<type NoInfer<A> = [A][A extends any ? 0 : never]Prevents TypeScript from inferring a type parameter from a specific
position.
When to use
Use when a function parameter must match an inferred type without becoming
an inference source.
Details
The parameter using NoInfer must still match the inferred type.
Example (Controlling inference)
import type { Types } from "effect"
declare function withDefault<T>(value: T, fallback: Types.NoInfer<T>): T
// T is inferred as "a" | "b" from the first argument only
const result = withDefault<"a" | "b">("a", "b")
NoInfer<function (type parameter) A in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>A>, function (type parameter) B in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>B>,
orFailWith: (value: NoInfer<A>) => E2orFailWith: (value: NoInfer<A>value: type NoInfer<A> = [A][A extends any ? 0 : never]Prevents TypeScript from inferring a type parameter from a specific
position.
When to use
Use when a function parameter must match an inferred type without becoming
an inference source.
Details
The parameter using NoInfer must still match the inferred type.
Example (Controlling inference)
import type { Types } from "effect"
declare function withDefault<T>(value: T, fallback: Types.NoInfer<T>): T
// T is inferred as "a" | "b" from the first argument only
const result = withDefault<"a" | "b">("a", "b")
NoInfer<function (type parameter) A in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>A>) => function (type parameter) E2 in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>E2
): <function (type parameter) E in <E>(self: Result<A, E>): Result<B, E2 | E>E>(self: Result<A, E>self: type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>A, function (type parameter) E in <E>(self: Result<A, E>): Result<B, E2 | E>E>) => type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) B in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>B, function (type parameter) E2 in <A, B extends A, E2>(refinement: Refinement<NoInfer<A>, B>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<B, E2 | E>E2 | function (type parameter) E in <E>(self: Result<A, E>): Result<B, E2 | E>E>
<function (type parameter) A in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>A, function (type parameter) E2 in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>E2>(
predicate: Predicate<NoInfer<A>>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<type NoInfer<A> = [A][A extends any ? 0 : never]Prevents TypeScript from inferring a type parameter from a specific
position.
When to use
Use when a function parameter must match an inferred type without becoming
an inference source.
Details
The parameter using NoInfer must still match the inferred type.
Example (Controlling inference)
import type { Types } from "effect"
declare function withDefault<T>(value: T, fallback: Types.NoInfer<T>): T
// T is inferred as "a" | "b" from the first argument only
const result = withDefault<"a" | "b">("a", "b")
NoInfer<function (type parameter) A in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>A>>,
orFailWith: (value: NoInfer<A>) => E2orFailWith: (value: NoInfer<A>value: type NoInfer<A> = [A][A extends any ? 0 : never]Prevents TypeScript from inferring a type parameter from a specific
position.
When to use
Use when a function parameter must match an inferred type without becoming
an inference source.
Details
The parameter using NoInfer must still match the inferred type.
Example (Controlling inference)
import type { Types } from "effect"
declare function withDefault<T>(value: T, fallback: Types.NoInfer<T>): T
// T is inferred as "a" | "b" from the first argument only
const result = withDefault<"a" | "b">("a", "b")
NoInfer<function (type parameter) A in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>A>) => function (type parameter) E2 in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>E2
): <function (type parameter) E in <E>(self: Result<A, E>): Result<A, E2 | E>E>(self: Result<A, E>self: type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>A, function (type parameter) E in <E>(self: Result<A, E>): Result<A, E2 | E>E>) => type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>A, function (type parameter) E2 in <A, E2>(predicate: Predicate<NoInfer<A>>, orFailWith: (value: NoInfer<A>) => E2): <E>(self: Result<A, E>) => Result<A, E2 | E>E2 | function (type parameter) E in <E>(self: Result<A, E>): Result<A, E2 | E>E>
<function (type parameter) A in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>A, function (type parameter) E in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>E, function (type parameter) B in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>B extends function (type parameter) A in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>A, function (type parameter) E2 in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>E2>(
self: Result<A, E>self: type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>A, function (type parameter) E in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>E>,
refinement: Refinement<A, B>refinement: interface Refinement<in A, out B extends A>A predicate that also narrows the input type when it returns true.
When to use
Use when you want a runtime check that refines A to B for TypeScript,
especially when composing type guards with
compose
or safely
checking unknown values.
Details
A refinement returns a type predicate (a is B). Use it with if or
filter to narrow types.
Example (Narrowing unknown values)
import { Predicate } from "effect"
const isString: Predicate.Refinement<unknown, string> = (u): u is string => typeof u === "string"
const data: unknown = "hello"
if (isString(data)) {
console.log(data.toUpperCase())
}
Type-level utilities for working with
Refinement
types.
When to use
Use when you need to extract input and output types from refinement
signatures while writing generic helpers over refinements.
Details
These utilities are type-only, create no runtime values, and the namespace is
erased at runtime.
Example (Extracting refinement types)
import { Predicate } from "effect"
type IsString = Predicate.Refinement<unknown, string>
type Input = Predicate.Refinement.In<IsString>
type Output = Predicate.Refinement.Out<IsString>
Refinement<function (type parameter) A in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>A, function (type parameter) B in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>B>,
orFailWith: (value: A) => E2orFailWith: (value: Avalue: function (type parameter) A in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>A) => function (type parameter) E2 in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>E2
): type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) B in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>B, function (type parameter) E in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>E | function (type parameter) E2 in <A, E, B extends A, E2>(self: Result<A, E>, refinement: Refinement<A, B>, orFailWith: (value: A) => E2): Result<B, E | E2>E2>
<function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E, function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E2>(self: Result<A, E>self: type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E>, predicate: Predicate<A>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<function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A>, orFailWith: (value: A) => E2orFailWith: (value: Avalue: function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A) => function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E2): type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E | function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E2>
} = dual<(...args: Array<any>) => any, <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2) => Result<A, E | E2>>(arity: 3, body: <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2) => Result<A, E | E2>): ((...args: Array<any>) => any) & (<A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2) => Result<A, E | E2>) (+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, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E, function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E2>(
self: Result<A, E>self: type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E>,
predicate: Predicate<A>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<function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A>,
orFailWith: (value: A) => E2orFailWith: (value: Avalue: function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A) => function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E2
): type Result<A, E = never> = Success<A, E> | Failure<A, E>A value that is either Success<A, E> or Failure<A, E>.
When to use
Use when both success and failure should remain available as data and
Option would lose failure information.
Details
- Use
succeed
/
fail
to construct
- Use
match
to fold both branches
- Use
isSuccess
/
isFailure
to narrow the type
E defaults to never, so Result<number> means a result that cannot fail.
Example (Creating and matching a Result)
import { Result } from "effect"
const success = Result.succeed(42)
const failure = Result.fail("something went wrong")
const message = Result.match(success, {
onSuccess: (value) => `Success: ${value}`,
onFailure: (error) => `Error: ${error}`
})
console.log(message)
// Output: "Success: 42"
Namespace containing type-level utilities for extracting the inner types
of a Result.
Example (Extracting inner types)
import type { Result } from "effect"
type R = Result.Result<number, string>
// number
type A = Result.Result.Success<R>
// string
type E = Result.Result.Failure<R>
Result<function (type parameter) A in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>A, function (type parameter) E in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E | function (type parameter) E2 in <A, E, E2>(self: Result<A, E>, predicate: Predicate<A>, orFailWith: (value: A) => E2): Result<A, E | E2>E2> => const flatMap: {
<A, A2, E2>(f: (a: A) => Result<A2, E2>): <E>(
self: Result<A, E>
) => Result<A2, E | E2>
<A, E, A2, E2>(
self: Result<A, E>,
f: (a: A) => Result<A2, E2>
): Result<A2, E | E2>
}
self: Result<A, E>self, (a: Aa) => predicate: Predicate<A>predicate(a: Aa) ? const succeed: <A>(right: A) => Result<A>Creates a Result holding a Success value.
Details
- Use when you have a value and want to lift it into the
Result type
- The error type
E defaults to never
Example (Wrapping a value)
import { Result } from "effect"
const result = Result.succeed(42)
console.log(Result.isSuccess(result))
// Output: true
succeed(a: Aa) : const fail: <E>(
left: E
) => Result<never, E>
Creates a Result holding a Failure value.
When to use
Use to represent a failed Result with a typed failure value.
Details
- The success type
A defaults to never
Example (Creating a failure)
import { Result } from "effect"
const result = Result.fail("Something went wrong")
console.log(Result.isFailure(result))
// Output: true
fail(orFailWith: (value: A) => E2orFailWith(a: Aa))))