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 : 40                      }, {                          page : ‘About‘,                          view : 10                      }, {                          page : ‘Help‘,                          view : 4                      });const Sum = x => ({    x,    concat   : ({ x: y }) => Sum(x + y),    toString : () => `Sum(${x})`});Sum.empty = () => Sum(0);const res = stats.foldMap(x => fromNullable(x.view).map(Sum), Right(Sum(0)));console.log(res.toString()); // Right(Sum(54))

let stats = List.of({                          page : ‘Home‘,                          view : 40                      }, {                          page : ‘About‘,                          view : 10                      }, {                          page : ‘Help‘,                          view : null                      });const Sum = x => ({    x,    concat   : ({ x: y }) => Sum(x + y),    toString : () => `Sum(${x})`});Sum.empty = () => Sum(0);const res = stats.foldMap(x => fromNullable(x.view).map(Sum), Right(Sum(0)));console.log(res.toString()); // Right(Sum(50))