Tiny and simple lisp implementation in node

lisp.js

class Env {
    constructor(p, e = {}) {
        this.p = p;
        this.e = e;
    }

    get(name) {
        let val = this.e[name];

        if (typeof val !== 'undefined')
            return val;
        
        if (this.p)
            return this.p.get(name);

        throw `Identifier ${name} is not defined`;
    }

    set(name, val) {
        let exists = !!this.e[name];

        if (!exists)
            this.e[name] = val;
        else
            throw `Identifier ${name} is already defined`;
    }
}

function Func(params, body, _env) {
    return (x) => {
        let env = new Env(_env, params.reduce((acc, p, i) => { 
            acc[p.v] = x[i]; 
            return acc;
        }, {}));
        return eval(body, env);
    };
}

let proc = process,
    con = console,
    cmpop = (x, f) => x.slice(1).every(f),
    globals = {
    '+':    (x)    => x.reduce((acc, val) => acc + val),
    '-':    (x)    => x.reduce((acc, val) => acc - val),
    '/':    (x)    => x.reduce((acc, val) => { if (val === 0) throw "Cannot divide by 0"; return acc / val; }),
    '*':    (x)    => x.reduce((acc, val) => acc * val),
    '**':   (x)    => x.reduce((acc, val) => Math.pow(acc, val)),
    '>':    (x)    => cmpop(x, (v) => x[0] > v),
    '>=':   (x)    => cmpop(x, (v) => x[0] >= v),
    '<':    (x)    => cmpop(x, (v) => x[0] < v),
    '<=':   (x)    => cmpop(x, (v) => x[0] <= v),
    '==':   (x)    => cmpop(x, (v) => x[0] === v),
    'car':  (x)    => x[0],
    'cdr':  (x)    => x.slice(1), 
    'cons': (x)    => [x[0]].concat(x.slice(1))
  },
  genv = new Env(null, globals),
  rl = require('readline').createInterface({
    input: proc.stdin,
    output: proc.stdout,
    prompt: '\u039b> '
  });


function lex(source) {
    return source.replace(/\(|\)/g, ' $& ').match(/".*?"|\S+/g);
}

function parse(tokens) {
    if (tokens.length === 0) throw 'Unexpected EOF';

    let token = tokens.shift();
    switch (token) {
        case '(':
            let list = [];
            while (tokens[0] !== ')') list.push(parse(tokens));

            tokens.shift();
            return { t: 'expression', args: list };
        case ')':
            throw 'Unmatched delimiter )';
        default:
            if (token[0] === '"') return { t: 'string', v: token.substr(1, token.length-2) };

            let numberVal = Number(token);
            return isNaN(numberVal) ? { t: 'identifier', v: token } : { t: 'number', v: numberVal };
    }
}

function eval(exp, env) {
    switch (exp.t) {
        case 'expression': 
            let args = exp.args,
                v = args[0].v;
            
            if (v === 'if') {
                return eval(args[1], env) ? eval(args[2], env) : eval(args[3], env);
            } else if (v === 'define') {
                env.set(args[1].v, eval(args[2], env));
            } else if (v === 'lambda') {
                return Func(args[1].args, args[2], env);
            } else if (v === 'quote') {
                return args[1];
            }
            else {
                let f = eval(args[0], env),
                    vals = args.slice(1).map((e) => eval(e, env));
                return f(vals);
            }
        case 'string':
        case 'number':
            return exp.v;
        case 'identifier':
            return env.get(exp.v);
        default:
            return exp.v;
    }
    
}

rl.prompt();
rl.on('line', (l) => {
    if (l) {
        try {
            let r = eval(parse(lex(l.trim())), genv);
            if (typeof r !== 'undefined') con.log(r);
        } catch (e) {
            con.error(`Error: ${e}`);
        }
    }
    rl.prompt();
}).on('close', () => proc.exit(0));