My maybe monad implementation in C++0x.

C++0x_Maybe_Monad.cpp

// This code is in the public domain, feel free to use it in anyway you'd 
// like to.
#include <iostream>
#include <functional>
using namespace std;

template<class A>
class Maybe {
  protected:
    A a;
    bool nothing;
    // a -> M a
    Maybe(A a): a(a), nothing(false) {}
    Maybe(): nothing(true) {}
  public:
    // (M a) -> (a -> M b) -> (M b)
    template<class B>
    Maybe<B> bind(function<Maybe<B> (A)> f); 
    bool isNothing(); 
    A getOrElse(A defaultValue);
};

template<class A>
class Nothing : public Maybe<A> {
  public:
    Nothing(): Maybe<A>() {}
};

template<class A>
class Just : public Maybe<A> {
  public:
    Just(A a): Maybe<A>(a) {}
};

template<class A> template<class B>
Maybe<B> Maybe<A>::bind(function<Maybe<B> (A)> f) { 
  if(nothing) 
    return Nothing<B>(); 
  else
    return f(a); 
}

template<class A>
bool Maybe<A>::isNothing() {
  return nothing;
}

template<class A>
A Maybe<A>::getOrElse(A defaultValue) {
  if(nothing)
    return defaultValue;
  else 
    return a;  
}

int main() {
  function<Maybe<int> (int)> maybePrintInt = [](int i) -> Maybe<int> {
    cout << "i = " << i << endl;
    return Just<int>(i);
  };
  function<Maybe<int> (int)> maybeAddFive = [](int i) -> Maybe<int> {
   return Just<int>(i+5);
  };

  int input;
  Maybe<int>* inputNumber;
  cout << "Please give me an int: ";
  cin >> input;
  if(cin.fail()) {
    inputNumber = new Nothing<int>();  
  } else {
    inputNumber = new Just<int>(input);
  }

  if(inputNumber->bind(maybeAddFive).bind(maybePrintInt).isNothing())
    cout << "Invalid number given!" << endl;

  delete inputNumber;
  return 0;
}