Companion code to my essay on lists (http://niltag.net/essays/lists.html)

list_essay_code.js

'use strict';

/**
 * Our little list implementation
 */
 
///// The foundation
let nil  = (f) => (x) => x;
let cons = (h) => (t) => (f) => (x) => f(h)(t(f)(x));
let foldr = (list) => (f) => (x) => list(f)(x);

///// List utilities
let length = (xs) => foldr(xs)((a) => (b) => b + 1)(0);
let map = (f) => (xs) => foldr(xs)( (y) => (ys) => cons(f(y))(ys) )(nil);
let filter = (cond) => (xs) => foldr
    (xs)
    ( (value) => (already_filtered) => (f) => (x) =>
        cond(value)
            ? f(value)(foldr(already_filtered)(f)(x))
            : foldr(already_filtered)(f)(x))
    (nil);
let concat = (xs) => (ys) => foldr(xs)(cons)(ys);
let flatten = (xs) => foldr(xs)(concat)(nil);
let flatMap = (list) => (f) => flatten( map (f) (list) );
let unit = (x) => cons (x) (nil); // constructs a unit list (one item)

// Allow us to print lists to the console
let show = (list) => list ( (a) => (b) => "" + a.toString() + b ) ("");

///// Demonstrations

let sum = (list) => foldr (list) ( (a) => (b) => a + b ) (0);
let list_1 = cons(1) (cons(2) (cons(3) (nil)));

let six = sum(list_1);
let three = length(list_1);
let twelve = sum ( map ( (x) => x * 2 ) (list_1) );
let one_and_three = filter ( (x) => x % 2 !== 0 ) (list_1);

let person_a_groceries = cons ('rice') (cons ('oranges') (nil));
let person_b_groceries = cons ('monster') (cons ('nos') (cons ('hot cheetos')
                              (nil)));
let shopping_list = flatten( cons (person_a_groceries)
                           ( cons (person_b_groceries)
                           ( nil )));
                           
///// More involved flatMap example
let customers = cons ('john') (cons ('jacob') (cons ('jingleheimer') (cons ('schmidt') (nil))));
let categories = cons ('A') (cons ('B') (cons ('C') (nil)));
let products = function (category) {
    if (category === 'A') {
        return cons ('a1') (cons ('a2') (cons ('a3') (nil)));
    } else if (category === 'B') {
        return cons ('b1') (cons ('b2') (nil));
    } else if (category === 'C') {
        return cons ('c1') (cons ('c2') (cons ('c3') (cons ('c4') (cons ('c5') (nil)))));
    } else {
        return nil;
    }
};

let likes = function(customer,category) {
    if (customer === 'john' && (category === 'A' || category === 'C')
    || (customer === 'jacob' && (category === 'B'))
    || (customer === 'jingleheimer' && (category === 'B' || category === 'C'))
    || (customer === 'schmidt' && (category === 'A'))) {
        return true;
    }
    return false;
};

let ads =
    flatMap (customers)  ((customer) =>
    flatMap (categories) ((category) =>
        (likes(customer,category))
            ? flatMap (products(category)) ((product) =>
                unit("Ad to: " + customer + " for " + product))
            : nil));
            
console.log(show(ads));