Add Option fold and methods/functions for Option/Either conversion

src/either.ts

@@ -1,3 +1,5 @@
+import { Option } from './option'
+
 /**
  * Represents the failure case of an Either
  * @typeParam E - The type of the error
@@ -77,6 +79,50 @@ export const fold =
   (fa: Either<A, E>): B =>
     fa._tag === 'Left' ? onLeft(fa.left) : onRight(fa.right)
 
+/**
+ * Converts an Option to an Either, providing a default error for None.
+ * @typeParam A - Value type
+ * @typeParam E - Error type
+ * @param onNone - Error value for None case
+ * @returns Function that converts Option<A> to Either<A, E>
+ */
+export const fromOption =
+  <E>(onNone: E) => <A>(option: Option<A>): Either<A, E> =>
+    option._tag === 'Some' ? right(option.value) : left(onNone)
+
+/**
+ * Lazily converts an Option to an Either (error evaluated only when needed).
+ * @typeParam A - Value type
+ * @typeParam E - Error type
+ * @param onNone - Function returning error for None case
+ * @returns Function that converts Option<A> to Either<A, E>
+ */
+export const fromOptionL =
+  <E>(onNone: () => E) => <A>(option: Option<A>): Either<A, E> =>
+    option._tag === 'Some' ? right(option.value) : left(onNone())
+
+/**
+ * Converts a Promise<Option> to Promise<Either> with a fixed error.
+ * @typeParam A - Value type
+ * @typeParam E - Error type
+ * @param onNone - Error value for None
+ * @returns Function that converts Promise<Option<A>> to Promise<Either<A, E>>
+ */
+export const fromPromiseOption =
+  <E>(onNone: E) => <A>(p: Promise<Option<A>>): Promise<Either<A, E>> =>
+    p.then(option => fromOption(onNone)(option))
+
+/**
+ * Lazily converts a Promise<Option> to Promise<Either>.
+ * @typeParam A - Value type
+ * @typeParam E - Error type
+ * @param onNone - Function returning error for None
+ * @returns Function that converts Promise<Option<A>> to Promise<Either<A, E>>
+ */
+export const fromPromiseOptionL =
+  <E>(onNone: () => E) => <A>(p: Promise<Option<A>>): Promise<Either<A, E>> =>
+    p.then(option => fromOptionL(onNone)(option))
+
 /**
  * A class for working with Either in a do-notation style
  * @typeParam T - The type of the accumulated scope
@@ -277,8 +323,7 @@ export class AsyncDoEither<T extends Record<string, unknown>, E> {
    * @param f - Function returning Either or Promise of Either
    * @returns A new AsyncDoEither instance with the same scope
    */
-  doL(f:
+  doL(f: (scope: T) =>
-    (scope: T) =>
     | Either<unknown, E>
     | Promise<Either<unknown, E>>): AsyncDoEither<T, E>
   {

src/option.ts

@@ -1,3 +1,5 @@
+import { Either, left, right } from './either'
+
 /**
  * Represents the absence of a value
  */
@@ -52,6 +54,54 @@ export const chain =
   <A, B>(f: (a: A) => Option<B>) => (fa: Option<A>): Option<B> =>
     fa._tag === 'Some' ? f(fa.value) : none
 
+/**
+ * Folds an Option into a single value by providing handlers for both cases (Some/None)
+ * @typeParam A - The value type contained in the Option
+ * @typeParam B - The output type of the fold operation
+ * @param onNone - Function to handle the None case
+ * @param onSome - Function to handle the Some case
+ * @returns A function that takes an Option and returns the folded value
+ * @example
+ * const result = fold(
+ *   () => 'No value',
+ *   (value: number) => `Value: ${value}`
+ * )(some(42)) // Returns "Value: 42"
+ *
+ * @example
+ * const result = fold(
+ *   () => 'No value',
+ *   (value: number) => `Value: ${value}`
+ * )(none) // Returns "No value"
+ */
+export const fold =
+  <A, B>(onNone: () => B, onSome: (value: A) => B) => (option: Option<A>): B =>
+    option._tag === 'Some' ? onSome(option.value) : onNone()
+
+/**
+ * A curried version of fold that takes the Option first
+ * @typeParam A - The value type contained in the Option
+ * @typeParam B - The output type of the fold operation
+ * @param option - The Option to fold
+ * @returns An object with methods to handle both cases
+ * @example
+ * some(42).fold({
+ *   onNone: () => 'No value',
+ *   onSome: (value) => `Value: ${value}`
+ * }) // Returns "Value: 42"
+ */
+export const foldC = <A>(option: Option<A>) => ({
+  /**
+   * @param handlers - Object containing both case handlers
+   * @param handlers.onNone - Function to handle None case
+   * @param handlers.onSome - Function to handle Some case
+   * @returns The folded value
+   */
+  fold: <B>(handlers: {onNone: () => B, onSome: (value: A) => B}): B =>
+    option._tag === 'Some'
+      ? handlers.onSome(option.value)
+      : handlers.onNone(),
+})
+
 /**
  * Extracts the value from an Option or returns a default
  * @typeParam A - The value type
@@ -155,6 +205,42 @@ export class DoOption<T extends Record<string, unknown>> {
     return this.option
   }
 
+  /**
+   * Converts the DoOption to an Either, providing a default error value for the None case
+   * @typeParam E - The type of the error to use if the Option is None
+   * @param onNone - The error value to use if the Option is None
+   * @returns An Either - Right with the accumulated scope if the Option is Some,
+   *          or Left with the provided error if the Option is None
+   * @example
+   * DoOption.start()
+   *   .bind('id', some(1))
+   *   .toEither('Missing value')
+   *   // Returns Right({ id: 1 }) or Left('Missing value')
+   */
+  toEither<E>(onNone: E): Either<T, E> {
+    return this.option._tag === 'Some'
+      ? right(this.option.value)
+      : left(onNone)
+  }
+
+  /**
+   * Converts the DoOption to an Either, lazily providing an error value for the None case
+   * @typeParam E - The type of the error to use if the Option is None
+   * @param onNone - A function that returns the error value when Option is None
+   * @returns An Either - Right with the accumulated scope if the Option is Some,
+   *          or Left with the computed error if the Option is None
+   * @example
+   * DoOption.start()
+   *   .bind('config', loadConfig())
+   *   .toEitherL(() => new Error('Config missing'))
+   *   // Returns Right(config) or Left(Error)
+   */
+  toEitherL<E>(onNone: () => E): Either<T, E> {
+    return this.option._tag === 'Some'
+      ? right(this.option.value)
+      : left(onNone())
+  }
+
   /**
    * Executes an effect without modifying the scope
    * @param fa - The Option effect to execute (does not depend on scope)
@@ -303,6 +389,46 @@ export class AsyncDoOption<T extends Record<string, unknown>> {
     )
   }
 
+  /**
+   * Converts the AsyncDoOption to a Promise of Either, providing a default error value for the None case
+   * @typeParam E - The type of the error to use if the Option is None
+   * @param onNone - The error value to use if the Option is None
+   * @returns A Promise that resolves to an Either - Right with the accumulated scope if the Option is Some,
+   *          or Left with the provided error if the Option is None
+   * @example
+   * AsyncDoOption.start()
+   *   .bind('user', getUserAsync())
+   *   .toEither('User not found')
+   *   .then(either => { ... }) // either is Either<{ user: User }, string>
+   */
+  toEither<E>(onNone: E): Promise<Either<T, E>> {
+    return this.promise.then(option =>
+      option._tag === 'Some'
+        ? right(option.value)
+        : left(onNone)
+    )
+  }
+
+  /**
+   * Converts the AsyncDoOption to a Promise of Either, lazily providing an error value for the None case
+   * @typeParam E - The type of the error to use if the Option is None
+   * @param onNone - A function that returns the error value to use if the Option is None
+   * @returns A Promise that resolves to an Either - Right with the accumulated scope if the Option is Some,
+   *          or Left with the error from onNone if the Option is None
+   * @example
+   * AsyncDoOption.start()
+   *   .bind('data', fetchData())
+   *   .toEitherL(() => new Error('Data fetch failed'))
+   *   .then(either => { ... }) // either is Either<{ data: Data }, Error>
+   */
+  toEitherL<E>(onNone: () => E): Promise<Either<T, E>> {
+    return this.promise.then(option =>
+      option._tag === 'Some'
+        ? right(option.value)
+        : left(onNone())
+    )
+  }
+
   /**
    * Returns the current scope
    * @returns A promise of the current Option scope