Even Sillier C++

The C++ try...catch construct provides a facility for discrimination of exceptions based on their types. This is a primitive "match" construct. It turns out, this is enough to encode sum types.

The program to follow uses the above idea to implement an interpreter for the language of additive expressions using exception handling for case discrimination.

#include <iostream>
#include <cassert>
#include <exception>
#include <memory>

struct expr {
  virtual ~expr() {}

  virtual void throw_ () const = 0;
};

using expr_ptr = std::shared_ptr<expr const>;

class expr is an abstract base class, class int_ and class add derived classes corresponding to the two cases of expressions. Sub-expressions are represented as std::shared_ptr<expr> instances.

struct int_ : expr { 
  int val; 
  int_ (int val) : val{val}
  {}

  void throw_ () const { throw *this; } 
};

struct add : expr { 
  expr_ptr left; 
  expr_ptr right; 

  template <class U, class V>
  add (U const& left, V const& right) 
    : left {expr_ptr{new U{left}}}
    , right {expr_ptr{new V{right}}}
  {}

  void throw_ () const { throw *this; } 
};

With the above machinery in place, here then is the "interpreter". It is implemented as a pair of mutually recursive functions.

int eval_rec ();

int eval (expr const& xpr) {
  try {
    xpr.throw_ ();
  }
  catch (...) {
    return eval_rec ();
  }
}

int eval_rec () {
  assert (std::current_exception());

  try {
    throw;
  }
  catch (int_ const& i) {
    return i.val;
  }
  catch (add const& op) {
    return eval (*op.left) +  eval (*op.right);
  }
}

This little program exercises the interpreter on the expression $(1 + 2) + 3$.

int main () {

  try{
    // (1 + 2) + 3
    std::cout << eval (add{add{int_{1}, int_{2}}, int_{3}}) << std::endl;
  }
  catch (...){
    std::cerr << "Unhandled exception\n";
  }
    
  return 0;
}

Credit to Mathias Gaunard who pointed out using a virtual function for the throwing of an expression, removed the need for explicit dynamic_cast operations in an earlier version of this program.