Effect type applicative computation - deferred runtime error

effect.js

const Data = Tcons => Dcons => {
  const Data = x => {
    const t = new Tcons();
    t[`run${Tcons.name}`] = x;
    t.tag = Tcons.name
    return t;
  };

  return Dcons(Data);
};


const Eff = Data(function Eff() {}) (Eff => thunk => Eff(thunk));

const runEff = f => tf =>
  f(tf.runEff());
  

const map = f => tx =>
  Eff(() => f(tx.runEff()));
  
const of = x =>
  Eff(() => x);
  
const ap = tf => tx =>
  Eff(() => tf.runEff() (tx.runEff()));
  
const chain = mx => fm =>
  Eff(() => fm(mx.runEff()).runEff());


const append = s => t => s.concat(t);
const id = x => x;
const readLn = Eff(() => window.prompt());
const log = x => Eff(() => console.log(x));


const tx = ap(map(append) (log("foo")))
  (readLn); // malicious code

// execution is deferred just like the runtime error...

runEff(id) (tx);