An educational pure functional programming library in TypeScript
Add prelude/match module with exhaustive pattern matching
+41
+1
src/prelude/index.ts
+1
src/prelude/index.ts
+40
src/prelude/match.ts
+40
src/prelude/match.ts
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/**
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* Exhaustive pattern matching for discriminated unions.
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* Requires handlers for all variants - compiler enforces exhaustiveness.
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*/
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export const match = <T extends { _tag: string }>(value: T) =>
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<R>(cases: { [K in T["_tag"]]: (v: Extract<T, { _tag: K }>) => R }): R =>
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(cases as unknown as Record<string, (v: T) => R>)[value._tag]!(value)
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/**
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* Pattern matching with default case for partial matches.
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*/
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export const matchOr = <T extends { _tag: string }, R>(defaultValue: R) =>
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(value: T) =>
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(cases: Partial<{ [K in T["_tag"]]: (v: Extract<T, { _tag: K }>) => R }>): R => {
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const handler = (cases as unknown as Record<string, ((v: T) => R) | undefined>)[value._tag]
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return handler !== undefined ? handler(value) : defaultValue
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}
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/**
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* Guard-based pattern matching with predicates.
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* Evaluates guards in order, returns first match or default.
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*/
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export const when = <T>(value: T) =>
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<R>(...guards: ReadonlyArray<readonly [(v: T) => boolean, (v: T) => R]>) =>
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(defaultCase: (v: T) => R): R => {
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const go = (index: number): R =>
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index >= guards.length
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? defaultCase(value)
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: guards[index]
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? guards[index]
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: go(index + 1)
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return go(0)
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}
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/**
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* Literal value matching.
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*/
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export const matchLiteral = <T extends string | number | boolean>(value: T) =>
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<R>(cases: { [K in T & PropertyKey]: R }, defaultCase?: R): R =>
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(value as PropertyKey) in cases ? cases[value as T & PropertyKey] : (defaultCase as R)