//: So far the recipes we define can't run each other. Let's change that.
:(scenario "calling_recipe")
recipe main [
  f
]
recipe f [
  3:integer <- add 2:literal, 2:literal
]
+mem: storing 4 in location 3

:(before "struct routine {")
// Everytime a recipe runs another, we interrupt it and start running the new
// recipe. When that finishes, we continue this one where we left off.
// This requires maintaining a 'stack' of interrupted recipes or 'calls'.
struct call {
  recipe_number running_recipe;
  size_t pc;
  // End Call Fields
  call(recipe_number r) :running_recipe(r), pc(0) {}
};
typedef stack<call> call_stack;

:(replace{} "struct routine")
struct routine {
  call_stack calls;
  // End Routine Fields
  routine(recipe_number r);
};
:(code)
  routine::routine(recipe_number r) {
    calls.push(call(r));
  }
//: now update routine's helpers
:(replace{} "inline size_t& running_at(routine& rr)")
inline size_t& running_at(routine& rr) {
  return rr.calls.top().pc;
}
:(replace{} "inline string recipe_name(routine& rr)")
inline string recipe_name(routine& rr) {
  return Recipe[rr.calls.top().running_recipe].name;
}
:(replace{} "inline vector<instruction>& steps(routine& rr)")
inline vector<instruction>& steps(routine& rr) {
  return Recipe[rr.calls.top().running_recipe].steps;
}

:(replace{} "default:" following "End Primitive Recipe Implementations.")
default: {
  // not a primitive; try to look for a matching recipe
  if (Recipe.find(instructions[pc].operation) == Recipe.end()) {
    raise << "undefined operation " << instructions[pc].operation << ": " << instructions[pc].name << '\n';
    break;
  }
  rr.calls.push(call(instructions[pc].operation));
  continue;  // not done with caller; don't increment pc
}

//: finally, we need to fix the termination conditions for the run loop

:(replace{} "inline bool done(routine& rr)")
inline bool done(routine& rr) {
  return rr.calls.empty();
}

:(before "Running one instruction.")
// when we reach the end of one call, we may reach the end of the one below
// it, and the one below that, and so on
while (running_at(rr) >= steps(rr).size()) {
  rr.calls.pop();
  if (rr.calls.empty()) return;
  // todo: no results returned warning
  ++running_at(rr);
}

:(before "End Includes")
#include <stack>
using std::stack;