invariants.cpp

struct invariants;

struct invariants_builder 
{
 void set(std::string const& index, std::any item);

 invariants commit();
};

/// A map of type -> any.
/// 
/// Maps are immutable, however, it is possible to create a shallow copy and
/// mutate/add elemeents in the copy. The added/mutated elements shall exist
/// only in the copy. All other elements are available in the copy, via the 
/// parent 
struct invariants
{
 invariants set(std::string const& index, std::any item) const;
 invariants_builder bulk_update() const;
 
 template<class T> 
 T const* 
 query(std::string const& index) const;

 template<class T> 
 T const& 
 require(std::string const& index) const;
};


void 
test()
{
    invariants i1;  //empty map
    
    auto i1 = i1.set("base_currency", "BTC");
    auto i2 = i1.set("term_currency", "USD");
    
    // i2 contains "base_currency" and "term_currency"
    // i1 contains only "base_currency", which is shared with i2

    
    // now perform a bulk update
    
    auto updated = i2.bulk_update();
    updated.set("markup", 0.001);
    updated.set("source_exchange", "FTX");
    updated.set("target_exchange", "Binance");
    auto i3 = updated.commit();
    
    // i3 contains:
    // "base_currency"
    // "term_currency"
    // "markup"
    // "source_exchange"
    // "target_exchange"
}