lambda calculus interpreter in javascript

interp.js

/* Copyright Freddy A Cubas "Superstar64"
Boost Software License - Version 1.0 - August 17th, 2003

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const isAlpha = c => c.toUpperCase() != c.toLowerCase(); // ugly hack

const leftAssociative = f => (...xs) => xs.reduce(f);

const empty = () => code => null;
// infinite backtracing
const or = leftAssociative((left, right) => () => code => left()(code) || right()(code));

const pure = item => () => code => ({ just: [item, code] });
const map = (f, ...x) => apply(pure(f), ...x);
const apply = leftAssociative((f, x) => () => code => {
    const monadF = f()(code);
    if (monadF) {
        const monadX = x()(monadF.just[1]);
        if (monadX) {
            return { just: [monadF.just[0](monadX.just[0]), monadX.just[1]] };
        }
    }
    return null;
});

const cons = x => xs => [x].concat(xs);
const many = x => () => or(map(cons, x, many(x)), pure([]))();
const some = x => map(cons, x, many(x));

const satify = check => () => code => {
    if (code.length > 0 && check(code[0])) {
        return { just: [code[0], code.slice(1)] };
    } else {
        return null;
    }
};
const letter = satify(isAlpha);
const literal = x => satify(a => a == x);
const space = many(literal(' '));
const identifier = map(x => x.reduce((a, b) => a + b, ''), some(letter));

const term = () => map(x => x.reduce((f, x) => env => f(env)(x(env))), some(termCore))();
const lambda = map(_ => _ => name => _ => _ => _ => e => env => x => e(augment(env, name, x)),
    literal('|'),
    space,
    identifier,
    space,
    literal('|'),
    space,
    term
);
const constiable = map(x => _ => env => env[x], identifier, space);
const parens = map(_ => _ => e => _ => _ => _ => e,
    literal('('),
    space,
    term,
    space,
    literal(')'),
    space
);
const termCore = or(lambda, constiable, parens);

const augment = (env, name, x) => {
    const env2 = Object.create(env);
    env2[name] = x;
    return env2;
};


const prelude = {
    two: 2,
    four: 4,
    neg: x => -x,
    add: x => y => x + y,
    sub: x => y => x - y,
    mul: x => y => x * y,
    div: x => y => x / y,
    sqrt: Math.sqrt
};

const run = string => term()(string).just[0](prelude);
// quadratic formula
const quad = run("|a| |b| |c| div (add (neg b) (sqrt ( sub (mul b b) (mul four (mul a c))))) (mul two a)");

print(quad(2)(4)(-20));