feat: add Result type

src/result.ts

@@ -0,0 +1,95 @@
+/**
+ * A result is either a value or an error.
+ */
+export type Result<T, E> = Ok<T> | Error<E>
+
+/**
+ * A result that contains a value.
+ */
+export interface Ok<T> {
+  readonly tag: "Ok"
+  readonly value: T
+}
+
+/**
+ * A result that contains an error.
+ */
+export interface Error<E> {
+  readonly tag: "Error"
+  readonly error: E
+}
+
+/**
+ * Creates a result that contains a value.
+ */
+export const ok = <T>(value: T): Result<T, never> => ({ tag: "Ok", value })
+
+/**
+ * Checks if a result contains a value.
+ */
+export const isOk = <T, E>(result: Result<T, E>): result is Ok<T> =>
+  result.tag === "Ok"
+
+/**
+ * Creates a result that contains an error.
+ */
+export const error = <E>(error: E): Result<never, E> => ({
+  tag: "Error",
+  error,
+})
+
+/**
+ * Checks if a result contains an error.
+ */
+export const isError = <T, E>(result: Result<T, E>): result is Error<E> =>
+  result.tag === "Error"
+
+/**
+ * Reduces a result to a value of a common type.
+ */
+export const reduce = <T, E, R>(
+  result: Result<T, E>,
+  fok: (value: T) => R,
+  ferror: (error: E) => R
+): R => (isOk(result) ? fok(result.value) : ferror(result.error))
+
+/**
+ * Maps the value of a result to a new value.
+ */
+export const map = <T, U, E>(
+  result: Result<T, E>,
+  f: (value: T) => U
+): Result<U, E> => (isOk(result) ? ok(f(result.value)) : result)
+
+/**
+ * Maps an error to a new error.
+ */
+export const mapError = <T, E, F>(
+  result: Result<T, E>,
+  f: (value: E) => F
+): Result<T, F> => (isError(result) ? error(f(result.error)) : result)
+
+export const combine = <T1, T2, TR, E1, E2, ER>(
+  result1: Result<T1, E1>,
+  result2: Result<T2, E2>,
+  fok: (value1: T1, value2: T2) => TR,
+  ferror: (error1: E1, error2: E2) => ER
+): Result<TR, E1 | E2 | ER> =>
+  isOk(result1)
+    ? isOk(result2)
+      ? ok(fok(result1.value, result2.value))
+      : result2
+    : isOk(result2)
+    ? result1
+    : error(ferror(result1.error, result2.error))
+
+export const Result = Object.freeze({
+  ok,
+  isOk,
+  error,
+  isError,
+  reduce,
+  map,
+  mapError,
+  combine,
+})