Comonads in C++.

Comonads.cpp

//
// Comonads.cpp: Comonads in C++
//
// Author: Nathan Bedell
//

//
// "Your scientists were so preoccupied with whether or not they could, they didn't stop 
// to think if they should."
//

#include <iostream>
#include <string> 

template <template <typename> class w>
struct Comonad {
    template <typename a>
    static a& extract(const w<a>);

    template <typename a>
    static w<w<a>>& duplicate(w<a>);
};

template <typename a, typename b>
struct Store {
    a internalState;
    b (*view)(a);
};

template <typename e, typename a>
struct Moore {
    a internalState;
    Moore<e,a> (*onEvent)(e);
};

template <template <typename> class f, typename a>
struct Cofree {
    a state;
    f<Cofree<f, a>> next;
};

// Note: I'm not sure if this is right.
template <typename A, typename R>
struct CalculatorFVisitor {
    R onEnterSymbol(char* symbol, A (*rest)(A));
    R onEnterNumber(int number, A (*rest)(A));
    R onQueryResult(int number, int (*rest)(A));
};

template <typename A>
struct CalculatorF {
    template <typename R>
    R accept(CalculatorFVisitor<A, R> visitor);
};

template <typename A>
using Calculator = Cofree<CalculatorF, A>;

/*
Calculator<int> calculator(int state) {
  return {
    .state = state,
    .next = {
      // Generic lambda not accepted here?
      .accept = []<typename R>(CalculatorFVisitor<int, R> visitor) {
        return visitor.onEnterSymbol()
      }
    } 
  }
}
*/

int main() {

  Store<int,int> store = {
      .internalState = 40,
      .view =  [](auto x) { return x + 2; }
  };

  std::cout << std::to_string(store.view(store.internalState));
}