export type Loadable<T> = Init | Loading | Loaded<T> | Failure

type Init = {
    key: 'INIT'
}

export const init: Init = { key: 'INIT' }

export function isInit(l: Loadable<any>): l is Init {
    return l.key == 'INIT'
}

type Loading = {
    key: 'LOADING'
}

export const loading: Loading = { key: 'LOADING' }

export function isLoading(l: Loadable<any>): l is Loading {
    return l.key == 'LOADING'
}

type Loaded<T> = {
    key: 'LOADED',
    value: T
}

export function loaded<T>(value: T): Loaded<T> {
    return { key: 'LOADED', value }
}

export function isLoaded<T>(l: Loadable<T>): l is Loaded<T> {
    return l.key == 'LOADED'
}

type Failure = {
    key: 'FAILURE',
    error: string
}

export function failure(error: string): Failure {
    return {
        key: 'FAILURE',
        error
    }
}

export function isFailure(l: Loadable<any>): l is Failure {
    return l.key == 'FAILURE'
}

export function map<A, B>(loadable: Loadable<A>, f: (value: A) => B): Loadable<B> {
    if (loadable.key == 'LOADED') {
        return loaded(f(loadable.value))
    } else {
        return loadable
    }
}

export function map2<A, B, C>(l: [Loadable<A>, Loadable<B>], f: (a: A, b: B) => C): Loadable<C> {
    const [l1, l2] = l
    if (l1.key == 'FAILURE') {
        return l1
    } else if (l2.key == 'FAILURE') {
        return l2
    } else if (l1.key == 'LOADING' || l2.key == 'LOADING') {
        return loading
    } else if (l1.key == 'INIT' || l2.key == 'INIT') {
        return init
    } else {
        return loaded(f(l1.value, l2.value))
    }
}