const { List } = require('immutable-ext');

const Right = x => ({

    chain    : f => f(x),

    ap       : other => other.map(x),

    traverse : (of, f) => f(x).map(Right),

    map      : f => Right(f(x)),

    fold     : (f, g) => g(x),

    concat   : o => o.fold(_ => Right(x), y => Right(x.concat(y))),

    toString : () => `Right(${x})`

})

const Left = x => ({

    chain    : f => Left(x),

    ap       : other => Left(x),

    traverse : (of, f) => of(Left(x)),

    map      : f => Left(x),

    fold     : (f, g) => f(x),

    concat   : o => o.fold(_ => Left(x), y => o),

    toString : () => `Left(${x})`

});

const fromNullable = x => x != null ?

                          Right(x) :

                          Left(null);

const tryCatch = f => {

    try {

        return Right(f())

    }

    catch( e ) {

        return Left(e)

    }

};

let stats = List.of({

                          page : 'Home',

                          view :

                      }, {

                          page : 'About',

                          view :

                      }, {

                          page : 'Help',

                          view :

                      });

const Sum = x => ({

    x,

    concat   : ({ x: y }) => Sum(x + y),

    toString : () => `Sum(${x})`

});

Sum.empty = () => Sum();

const res = stats.foldMap(x => fromNullable(x.view)

.map(Sum), Right(Sum()));

console.log(res.toString()); // Right(Sum(54))

let stats = List.of({

                          page : 'Home',

                          view :

                      }, {

                          page : 'About',

                          view :

                      }, {

                          page : 'Help',

                          view : null

                      });

const Sum = x => ({

    x,

    concat   : ({ x: y }) => Sum(x + y),

    toString : () => `Sum(${x})`

});

Sum.empty = () => Sum();

const res = stats.foldMap(x => fromNullable(x.view)

.map(Sum), Right(Sum()));

console.log(res.toString()); // Right(Sum(50))