monad in c++ 03

monad03.cpp

//
//http://c2.com/cgi/wiki?CeePlusPlusMonadsExample
//
//see also: http://c2.com/cgi/wiki?OnMonads
//

#include <iostream>

template<typename A_> 
struct monad
{
  template<typename B_> 
  monad<B_>& operator >>=(monad<B_>& (*f)(A_))
  {
    return *new monad_bind<A_,B_>(*this,f);
  }

  static monad<A_>& return_value(A_ a)
  {
    return *new monad_return<A_>(a);
  }

  virtual A_ execute() = 0;    // this should be protected

private:        
  template<typename A_,typename B_> 
  struct monad_bind : public monad<B_>
  {
    monad_bind(monad<A_>& ma,monad<B_>& (*f)(A_)) 
      : _ma(ma)
      , _f(f)
    {
    }

    monad<A_>& _ma;
    monad<B_>& (*_f)(A_);

    virtual B_ execute()
    {
      A_ a=_ma.execute();
      monad<B_>& mb=_f(a);
      return mb.execute();
    }
  };            

  template<typename A_> 
  struct monad_return : public monad<A_>
  {
    monad_return(A_ a) : _a(a) {}
  
    A_ _a;

    virtual A_ execute()
    {
      return _a;
    }
  };
};        

struct unit
{
};

monad<unit>& write_character(char c)
{
  std::cout << c;
  return monad<unit>::return_value(unit());
}

monad<char>& read_character(unit)
{
  char c;
  std::cin >> c;
  return monad<char>::return_value(c);
}

monad<char>& is_it_capitalized(char c)
{
  return
    isupper(c)?
    monad<char>::return_value('y')
    :
  monad<char>::return_value('n');
}

monad<unit>& program(monad<unit>& start)
{
  return (((start >>= read_character) >>= is_it_capitalized) >>= write_character);
}

void main(void)
{
  monad<unit>& result=program(monad<unit>::return_value(unit()));
  unit end=result.execute();
}

//program = read_character >>= is_it_capitalized >>= write_character
//where is_it_capitalized c = if isupper c
//then return 'y' 
//else return 'n'