Right fold with the accumulated value wrapped in a thunk, allowing short-circuiting

foldRThunk.js

var R = require('ramda');

// :: ((a, () -> b) -> b) -> b -> List[a] -> b
function foldRThunk(fn, acc, list) {
  return R.isEmpty(list) ? acc : fn(
    list[0],
    function () { return foldRThunk(fn, acc, R.tail(list)) }
  );
}

// :: (a -> Boolean) -> List[a] -> Boolean
function exists(p, list) {
  return foldRThunk(function(a, b) {
    return p(a) || b()
  }, false, list);
}

console.log(exists(R.eq(4), [2,4,6,8])); //=> true
console.log(exists(R.eq(4), [1,3,5,7])); //=> false