PierceCPP

code.cpp

#include <functional>
class A {};
class B {};

typedef std::function<B(A)> F;
typedef std::function<B(F)> G;
typedef std::function<B(G)> H;

F proof(H h){
  auto t = [](A a){
    return [a](F f) { return f (a); };
  };
  return [h,t](A a){ return h(t(a)); };
}

int main(){}