Record.ReadonlyRecord — API

Record.ReadonlyRecordtypeeffect/Record.ts:45

ReadonlyRecord<K, A>

Represents a readonly record with keys of type K and values of type A. This is the foundational type for immutable key-value mappings in Effect.

Example (Defining a readonly record type)

import type { Record } from "effect"

// Creating a readonly record type
type UserRecord = Record.ReadonlyRecord<"name" | "age", string | number>

const user: UserRecord = {
  name: "John",
  age: 30
}

models

Source effect/Record.ts:4573 lines

export type ReadonlyRecord<in out K extends string | symbol, out A> = {
  readonly [P in K]: A
}

/**
 * Namespace containing utility types for working with readonly records.
 * These types help with type-level operations on record keys and values.
 *
 * **Example** (Using readonly record helper types)
 *
 * ```ts
 * import type { Record } from "effect"
 *
 * // Using NonLiteralKey to convert literal keys to generic types
 * type GenericKey = Record.ReadonlyRecord.NonLiteralKey<"foo" | "bar"> // string
 *
 * // Using IntersectKeys to find common keys between record types
 * type CommonKeys = Record.ReadonlyRecord.IntersectKeys<"a" | "b", "b" | "c"> // "b"
 * ```
 *
 * @since 2.0.0
 */
export declare namespace ReadonlyRecord {
  type IsFiniteString<T extends string> = T extends "" ? true :
    [T] extends [`${infer Head}${infer Rest}`]
      ? string extends Head ? false : `${number}` extends Head ? false : Rest extends "" ? true : IsFiniteString<Rest>
    : false

  /**
   * Represents a type that converts literal string keys to generic string type and symbol keys to generic symbol type.
   * This is useful for maintaining type safety while allowing flexible key types in record operations.
   *
   * **Example** (Converting literal keys to non-literal keys)
   *
   * ```ts
   * import type { Record } from "effect"
   *
   * // For literal string keys, this becomes 'string'
   * type Example1 = Record.ReadonlyRecord.NonLiteralKey<"foo" | "bar"> // string
   *
   * // For symbol keys, this becomes 'symbol'
   * type Example2 = Record.ReadonlyRecord.NonLiteralKey<symbol> // symbol
   * ```
   *
   * @category models
   * @since 2.0.0
   */
  export type NonLiteralKey<K extends string | symbol> = K extends string ? IsFiniteString<K> extends true ? string : K
    : symbol

  /**
   * Represents the intersection of two key types, handling both literal and non-literal string keys.
   * This type is used in record operations that need to compute overlapping keys.
   *
   * **Example** (Intersecting record keys)
   *
   * ```ts
   * import type { Record } from "effect"
   *
   * // Intersection of literal keys
   * type Example1 = Record.ReadonlyRecord.IntersectKeys<"a" | "b", "b" | "c"> // "b"
   *
   * // Intersection with generic string
   * type Example2 = Record.ReadonlyRecord.IntersectKeys<string, "a" | "b"> // string
   * ```
   *
   * @category models
   * @since 2.0.0
   */
  export type IntersectKeys<K1 extends string, K2 extends string> = [string] extends [K1 | K2] ?
    NonLiteralKey<K1> & NonLiteralKey<K2>
    : K1 & K2
}