bugs.js

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

function encode_numeral(n) {
  if (n === 0) return z;
  return succ(encode_numeral(n-1));
}

function pp_numeral(n) {
  const pp_rest = r => r === 'x' ? 'f x' : `f (${r})`;
  return "λf.λx. " + n(r => pp_rest(r))("x");
}

const nil = c => n => n
const cons = e => l => c => n => c(e)(l(c)(n))

function encode_list(l) {
  if (l.length === 0) return nil;

  const [x, ...xs] = l;
  return cons(x)(encode_list(xs))
}

function encode_string(s) {
  const char_list = Array.from(s).map(c => c.charCodeAt(0));
  return encode_list(char_list.map(encode_numeral))
}

function pp_string(l) {
  const pp_concat = e => l => l === 'n' ? `c (${pp_numeral(e)}) n` : `c (${pp_numeral(e)}) (${l})`;
  return `λc.λn. ${l(pp_concat)('n')}`;
}

console.log(pp_string(encode_string('bugs')));