Sketch of JSON Decoder in JavaScript

decoder.js

declare opaque type Decoder<T>;

declare var num: Decoder<number>;
declare var str: Decoder<string>;
declare var bool: Decoder<boolean>;
declare function array<T>(decoder: Decoder<T>): Decoder<T[]>;
declare function field<T>(key: string, decoder: Decoder<T>): Decoder<T>;
declare function nullable<T>(decoder: Decoder<T>): Decoder<?T>;
declare function maybe<T>(decoder: Decoder<T>): Decoder<?T>;
declare function at<T>(keys: string[], decoder: Decoder<T>): Decoder<T>;
declare function oneOf<T>(decoders: Decoder<T>[]): Decoder<T>;
declare function succeed<T>(value: T): Decoder<T>;
declare function fail<T>(error: string): Decoder<T>;
declare function nul<T>(defaultValue: T): Decoder<T>;
declare function andThen<A, B>(f: (A) => Decoder<B>, Decoder<A>): Decoder<B>;

declare function map<A, T>(
    tagger: (A) => T,
    a: Decoder<A>,
): Decoder<T>;
declare function map2<A, B, T>(
    tagger: (A, B) => T,
    a: Decoder<A>,
    b: Decoder<B>,
): Decoder<T>;
declare function map3<A, B, C, T>(
    tagger: (A, B) => T,
    a: Decoder<A>,
    b: Decoder<B>,
    c: Decoder<C>,
): Decoder<T>;

declare function decodeString<T>(decoder: Decoder<T>, json: string): Promise<T>;
declare function decodeValue<T>(decoder: Decoder<T>, value: any): Promise<T>;

example.js

type Parent = {
    name: string,
    children: Child[],
};

type Child = {
    name: string,
};

const childDecoder = map(
    (name) => ({ name }),
    filed('name', str),
);

const parentDecoder = map2(
    (name, children) => ({ name, children }),
    field('name', str),
    filed('children', array(childDecoder)),
);