An educational pure functional programming library in TypeScript
Add prelude/brand module with Branded and Refined types
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src/prelude/brand.ts
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src/prelude/brand.ts
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import type { Option } from "./option"
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import { some, none } from "./option"
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// Brand symbol for nominal typing
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declare const __brand: unique symbol
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type Brand<B> = { [__brand]: B }
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/**
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* Branded type - creates distinct types from primitives.
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* Example: type UserId = Branded<string, "UserId">
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*/
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export type Branded<T, B> = T & Brand<B>
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/**
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* Create a branded value (unchecked).
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*/
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export const brand = <T, B>(value: T): Branded<T, B> => value as Branded<T, B>
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// Refinement symbol
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declare const __refinement: unique symbol
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type Refinement<R> = { [__refinement]: R }
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/**
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* Refined type - branded with validation guarantee.
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*/
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export type Refined<T, R> = T & Refinement<R>
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/**
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* Create a refinement validator.
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* Returns Option to avoid throwing on invalid input.
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*/
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export const refine = <T, R>(predicate: (value: T) => boolean) =>
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(value: T): Option<Refined<T, R>> =>
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predicate(value) ? some(value as Refined<T, R>) : none
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// Common numeric refinements
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export type Positive = "Positive"
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export type NonNegative = "NonNegative"
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export type Normalized = "Normalized"
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export type Integer = "Integer"
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export const positive = refine<number, Positive>(x => x > 0)
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export const nonNegative = refine<number, NonNegative>(x => x >= 0)
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export const normalized = refine<number, Normalized>(x => x >= 0 && x <= 1)
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export const integer = refine<number, Integer>(x => Number.isInteger(x))