import type * as P from "@principia/prelude";
import * as HKT from "@principia/prelude/HKT";

import { identity } from "../Function";
import { right } from "./constructors";
import { Functor } from "./functor";
import { isLeft } from "./guards";
import type { Either, URI, V } from "./model";

/*
 * -------------------------------------------
 * Extend Either
 * -------------------------------------------
 */

/**
 * ```haskell
 * extend_ :: Extend w => (w a, (w a -> b)) -> w b
 * ```
 *
 * @category Extend
 * @since 1.0.0
 */
export const extend_ = <E, A, B>(wa: Either<E, A>, f: (wa: Either<E, A>) => B): Either<E, B> =>
   isLeft(wa) ? wa : right(f(wa));

/**
 * ```haskell
 * extend :: Extend w => (w a -> b) -> w a -> w b
 * ```
 *
 * @category Extend
 * @since 1.0.0
 */
export const extend = <E, A, B>(f: (wa: Either<E, A>) => B) => (wa: Either<E, A>): Either<E, B> => extend_(wa, f);

/**
 * ```haskell
 * duplicate :: Extend w => w a -> w (w a)
 * ```
 *
 * @category Extend
 * @since 1.0.0
 */
export const duplicate = <E, A>(wa: Either<E, A>): Either<E, Either<E, A>> => extend_(wa, identity);

/**
 * @category Instances
 * @since 1.0.0
 */
export const Extend: P.Extend<[URI], V> = HKT.instance({
   ...Functor,
   extend
});