Tangential:

Pure impl v2:

  class foo {
  public:
    //enum is a keyword, so let's call it _enum
    const _enum current_operator;
    foo(_enum co) : current_operator(co) { }
  // ... evalOp() as in your stateful impl example ...
  };

  //Example use:
  Reduce(foo{ADD}, 1, 2, 3, 4, 5, 6);

Point being, functional purity doesn't necessarily mean only functions taking all their inputs as arguments. It means no mutable state. In the above example, the class foo essentially represents a partially applied evalOp(). If you have multiple, related functions working on similar sets of parameters, you could put them in such a class with const members to create what is essentially a package of partially applied functions.