factorial.js

// Run on node repl
// In node, type in
// .load factorial.js

// editor
// This side will be pure lambda calculus

const identity = (x) => x;

const constantlyFirst = (x) => (y) => x;

const constantlySecond = constantlyFirst(identity);

const True = constantlyFirst;
const False = constantlySecond;

// lines will only work on the node repl
// or require('util')
// True[util.inspect.custom] = () => 'True'; 
// False[util.inspect.custom] = () => 'False';

const not = (x) => x(False)(True);

const and = (x) => (y) => x(y)(x);
const or = (x) => (y) => x(x)(y);

// Numbers
const n0 = (f) => (x) => x;
const n1 = (f) => (x) => f(x);
const n2 = (f) => (x) => f(f(x));

const succ = (n) => (f) => (x) => f(n(f)(x));

const inc = (x) => x + 1;
const toInt = (n) => n(inc)(0);

const add = (m) => (n) => m(succ)(n);

const mul = (m) => (n) => (x) => m(n(x));

// Data structures :)
const pair = (x) => (y) => (f) => f(x)(y);

const first = (p) => p(constantlyFirst);
const second = (p) => p(constantlySecond);

const pairMul = (p) => pair(mul(first(p))(second(p)))(succ(second(p)));

const fact = (n) => first(n(pairMul)(pair(n1)(n1)));

const n5 = succ(add(n2)(n2));

console.log("Fact 5: ", toInt(fact(n5)));

intro_fp.js

// Our session will require you to read a little JS.
// You should know most of it, but if you haven't
// worked on ES6, this will help you get a gist of it


// Js now uses const instead of var
const maxValue = 100;

// setting maxValue = 300 will cause a compiler error.

// There are arrow functions
const double = (x) => x + x;

// same as
const double_ = function(x) {
    return x + x;
};

// They mostly behave like regular functions
// But they evaluate 'this' differently.
// We won't need that for now.

// arrow functions can also return arrow functions

const adder = (x) => (y) => x + y;

// same as

const adder_ = function(x) {
    return function(y) {
        return x + y;
    };
};

const addTwo = adder(2);
// what will addTwo(3) be?

adder(3)(4);

// This is the level you need to be able to read code at