// optional monad in C++23: requires compiler that implements enough of
// the 2023 standard.  Compiled on M2 Mac with g++-13 -std=c++23.

// The optional type was first introduced in 2017 but the important monadic
// operations such as and_then and transform were not added until 2023.

#include<iostream>
#include<cstdlib>
#include<vector>
#include<functional>
#include<optional>
using namespace std;

optional<int> largest(vector<int>& V) {
  if (V.size()==0) return nullopt;
  int max = V[0];
  for(int i=1;i<V.size();i++) {
    if (V[i]>max) max = V[i];
  }
  return max; // no "unit" constructor needed in C++ implementation.
}//largest value in a vector, if it exists

optional<int> safediv(int a, int b) {
  if (b==0) return nullopt;
  else return a/b;
}//safe div

optional<int> get_int_arg(int i, int argc, char **argv) {
  if ((i<0) || (i>=argc) || !argv) return nullopt;
  else return atoi(argv[i]);
}
int main(int argc, char** argv) {
  vector<int> V{};
  auto push = [&V](int x) mutable -> int {V.push_back(x); return 0;};
  get_int_arg(1,argc,argv)
    .transform([&](auto x) mutable -> int {   //transform = map
       V.push_back(x);
       return x;
    });
  get_int_arg(2,argc,argv)
    .transform([&](auto x) mutable {
	     V.push_back(x);
	     return 0;
    });
  largest(V)
    .and_then([](auto x){return safediv(100,x);})  // bind
    .transform([](auto x) {
      cout << "value is now " << x << endl;
      return 0; // dummy return
    });
  return 0;
}//main

// There's another monad in C++23 called `expected`, which corresponds to
// the `Result` monad found in other languages and which I implemented in
// Java and C#.