An educational pure functional programming library in TypeScript
Add prelude/result module with Result type and operations
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src/prelude/result.ts
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src/prelude/result.ts
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// Result type for error handling as values
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// Types
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export type Ok<T> = { readonly _tag: "Ok"; readonly value: T }
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export type Err<E> = { readonly _tag: "Err"; readonly error: E }
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export type Result<T, E> = Ok<T> | Err<E>
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// Constructors
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export const ok = <T>(value: T): Ok<T> => ({ _tag: "Ok", value })
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export const err = <E>(error: E): Err<E> => ({ _tag: "Err", error })
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// Type guards
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export const isOk = <T, E>(r: Result<T, E>): r is Ok<T> => r._tag === "Ok"
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export const isErr = <T, E>(r: Result<T, E>): r is Err<E> => r._tag === "Err"
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// Operations - expression only, no early returns
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export const mapResult = <T, U, E>(f: (t: T) => U) =>
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(result: Result<T, E>): Result<U, E> =>
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result._tag === "Ok" ? ok(f(result.value)) : result
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export const mapErr = <T, E, F>(f: (e: E) => F) =>
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(result: Result<T, E>): Result<T, F> =>
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result._tag === "Err" ? err(f(result.error)) : result
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export const chainResult = <T, U, E>(f: (t: T) => Result<U, E>) =>
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(result: Result<T, E>): Result<U, E> =>
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result._tag === "Ok" ? f(result.value) : result
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export const unwrapOr = <T, E>(defaultValue: T) =>
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(result: Result<T, E>): T =>
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result._tag === "Ok" ? result.value : defaultValue
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export const matchResult = <T, E, R>(
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onOk: (value: T) => R,
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onErr: (error: E) => R
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) => (result: Result<T, E>): R =>
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result._tag === "Ok" ? onOk(result.value) : onErr(result.error)
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/**
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* ESCAPE HATCH: Catches JavaScript exceptions at the boundary.
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* JS uses exceptions for errors, but FP uses Result. This bridges that gap.
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* Inside purus, we use Result - this is only for external JS interop.
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*/
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export const tryCatch = <T>(f: () => T): Result<T, unknown> => {
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try {
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return ok(f())
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} catch (e) {
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return err(e)
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}
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}