import type ChainFn from "./Chain";
import type FlattenFn from "./Flatten";
import type { Fn1, PartialApply } from "../../HKT";
import type { ValueOf } from "../../Obj/ValueOf";
import type { Eq, Lazied, Lazied$Get } from "../../helpers";
import type { Applicative } from "../Applicative";
import type { HKT, Kind } from "../HKT";

/**
 * A type class for types that can be converted to a string.
 */
export type Monad<T = unknown> = Lazied$Get<MonadL<T>>;
/**
 * {@link Lazied} version of {@link Monad}.
 */
export type MonadL<T = unknown> = Lazied<Kind<MonadImpl[keyof MonadImpl][0], T>>;

/**
 * Type class constraints for type class {@link Monad}.
 */
export interface TypeClass$$Monad<F extends Applicative> {
  /**
   * Flatten a nested structure of a {@link Monad}.
   *
   * **⚠️ Warning:** Correctness of the method is not fully checked, so be careful when implementing
   * this method.
   *
   * Sig: `<F<~>, T>(f: F<F<T>>) => F<T>`
   */
  Flatten: Fn1<Kind<F, F>, F>;
}

/**
 * Implementations for type class {@link Monad}.
 */
export interface MonadImpl {}

/**
 * Helper type for implementing type class {@link Monad}.
 */
export type ImplMonadFor<F extends Applicative, TypeClass extends TypeClass$$Monad<F>> = [
  F,
  TypeClass,
];

/**
 * Get the matching entry of {@link MonadImpl} for `F`.
 * @private
 */
type _Monad$GetMatch<F extends Monad> = ValueOf<{
  [P in keyof MonadImpl as F extends MonadImpl[P][0] ? P : never]: MonadImpl[P];
}>;

/**
 * Get the constructor of `F` from {@link MonadImpl}.
 */
export type Monad$GetConstruct<F extends Monad> =
  Eq<F, Monad> extends true ? Monad
  : _Monad$GetMatch<F> extends [infer F extends HKT, unknown] ? F
  : never;
/**
 * The **unsafe** version of {@link Monad$GetConstruct} (i.e. no type checking with `F`).
 */
export type Monad$GetConstructW<F> = F extends Monad ? Monad$GetConstruct<F> : never;

/**
 * Get the type class of `F` from {@link MonadImpl}.
 */
export type Monad$GetTypeClass<F extends Monad> =
  _Monad$GetMatch<F> extends [unknown, infer TypeClass] ? TypeClass : never;
/**
 * The **unsafe** version of {@link Monad$GetTypeClass} (i.e. no type checking with `F`).
 */
export type Monad$GetTypeClassW<F> = F extends Monad ? Monad$GetTypeClass<F> : never;

/***********
 * Methods *
 ***********/
/**
 * Methods for `Monad`.
 */
export namespace Monad {
  /**
   * [Fn] Flatten a {@link Monad} of a {@link Monad} into a {@link Monad}.
   *
   * Sig: `<F<~>, T>(f: F<F<T>>) => F<T>`
   */
  export type Flatten = FlattenFn;

  /**
   * [Fn] Map a function over a {@link Monad} and flatten the result.
   *
   * Sig: `<F<~>, T, U>[f: (x: T) => F<U>](fa: F<T>) => F<U>`
   */
  export type Chain<F extends Fn1<never, Monad>> = PartialApply<ChainFn, [F]>;
}