/**
 *  Consider to use `Nullable<T>`, `Undefinable<T>`, or `Maybe<T>` to express an absence of a value.
 *  In JavaScript, they satisfy almost use cases. Probably, you might not have to use this type.
 *
 *  --------
 *  This is [_option type_](https://en.wikipedia.org/wiki/Option_type).
 *
 *  ## CAUTION:
 *
 *  ### Be careful to use `===` or `Object.is()` to compare the equality of this type
 *
 *  You should use `equal` operator to check the equality for two objects of this type
 *  instead of `===` or `Object.is()`.
 *  Operators for this type sometimes return the inputted object directly
 *  to avoid an unnecessary objecti allocation.
 *
 *  We use this design by the assumption that we would not compare `a` and `b` usually in the following case.
 *  It usually suggest some design problems if you would like to compare these `a` and `b`.
 *
 *  ```typescript
 *  const a = createSome(val);
 *  const b = andThen(a, someOperation);
 *  ```
 *
 *  ## Remarks
 *  In almost of cases of JavaScript, it's more ergonomic to use `Nullable<T>`, `Undefinable<T>`, or `Maybe<T>`
 *  to represent that either there is some value or not.
 *  We also recommend to use them instead of this type generally.
 *
 *  However, this type is usefil in following case:
 *
 *      1. Migration solution from `PlainResult<T, void>` to `Nullable<T>`, `Undefinable<T>`, or `Maybe<T>`.
 *      2. You need to treat `null` or `undefined` as some value rather than absence of value.
 *
 *  Then this tagged type helps you powerfully.
 *
 *  @deprecated 37.1.0
 */
export type Option<T> = Some<T> | None;
/**
 *  This type contain some value of _T_.
 *
 *  You can create this type value and get an inner value in this type by hand.
 *  But we recommend to use factory and utility functions for forward compatibility.
 *  And we don't recommend to implement this type for your type too.
 *
 *  - {@link createSome()} to create a value of `Some(T)`.
 *  - {@link isSome()} to check whether the value is `Some(T)`.
 *  - {@link unwrapSome()} to get an inner value in `Some(T)`.
 *  - `unwrapOr()` to get either an inner value in `Some(T)` or a fallback default value.
 *  - ...and more.
 */
export interface Some<out T> {
    /**
     *  Don't touch this property directly from an user project
     *  except 3rd party project that does not install this package but uses a value returned from an other project.
     *  Instead, use {@link isSome()} or {@link isNone()} operator to get an inner value.
     *
     *  Historically, this type was created to target a JSVM that supports ES5.
     *  Then there was no well optimized `Symbol` to achieve a private property.
     *  We don't have a plan to change this into private property keep the backward compatibility.
     */
    readonly ok: true;
    /**
     *  Don't touch this property directly from an user project
     *  except 3rd party project that does not install this package but uses a value returned from an other project.
     *  Instead, use {@link unwrapSome()} operator to get an inner value.
     *
     *  Historically, this type was created to target a JSVM that supports ES5.
     *  Then there was no well optimized `Symbol` to achieve a private property.
     *  We don't have a plan to change this into private property keep the backward compatibility.
     */
    readonly val: T;
}
export declare function isSome<T>(input: Option<T>): input is Some<T>;
export declare function createSome<T>(val: T): Some<T>;
/**
 *  This type represents no value.
 *  This is a pair of `Some(T)`.
 *
 *  You can create this type value and get an inner value in this type by hand.
 *  But we recommend to use factory and utility functions for forward compatibility.
 *  And we don't recommend to implement this type for your type too.
 *
 *  - {@link createNone()} to create a value of `None`.
 *  - {@link isNone()} to check whether the value is `None`.
 *  - ...and more.
 */
export interface None {
    /**
     *  Don't touch this property directly from an user project
     *  except 3rd party project that does not install this package but uses a value returned from an other project.
     *  Instead, use {@link isSome()} or {@link isNone()} operator to get an inner value.
     *
     *  Historically, this type was created to target a JSVM that supports ES5.
     *  Then there was no well optimized `Symbol` to achieve a private property.
     *  We don't have a plan to change this into private property keep the backward compatibility.
     */
    readonly ok: false;
    /**
     *  Don't touch this property directly from an user project
     *  except 3rd party project that does not install this package but uses a value returned from an other project.
     *  Instead, use {@link unwrapSome()} operator to get an inner value.
     *
     *  Historically, this type was created to target a JSVM that supports ES5.
     *  Then there was no well optimized `Symbol` to achieve a private property.
     *  We don't have a plan to change this into private property keep the backward compatibility.
     */
    readonly val: null;
}
export declare function isNone<T>(input: Option<T>): input is None;
export declare function createNone(): None;
/**
 *  Return the inner `T` of a `Some(T)`.
 *
 *  @throws {TypeError}
 *      Throws if the self is a `None`.
 */
export declare function unwrapSome<T>(input: Option<T>): T;
/**
 *  Return _input_ as `T` if the passed _input_ is `Some(T)`.
 *  Otherwise, throw `TypeError` with the passed `msg`.
 *
 *  @throws {TypeError}
 *      Throws if the self is a `None`.
 */
export declare function expectSome<T>(input: Option<T>, msg: string): T;