C++ templates and concepts can mix type construction, overload selection, and behavior requirements. Musi splits that apart: data builds values, classes name required behavior, instances supply it.
template <class T>
struct Show {
virtual std::string show(const T& value) const = 0;
};let Show [T] := class {
let show (value : T) : String;
};
export instance Show[Int] {
let show (value : Int) : String := "number";
};Reading Templates, Concepts, Classes, and Laws from C++17
On the Musi side, Musi classes name behavior a type can provide, instances provide it, and laws document promises callers rely on. Read the shared example through C++17 eyes: keep the useful instinct, then let Musi name shape, behavior, absence, and outside work in separate places.
False friend
Do not confuse a Musi class with stored object state, inheritance, or a metatable. Shape and behavior are separate. For a C++ reader, the trap is turning every Musi value into a class-shaped design; Musi class is closer to a concept or trait than a C++ class; records/data store shape, instances satisfy behavior.
When this pays off
Use classes when many types share an operation such as compare, show, step, encode, or measure. The C++17 instinct still helps here: Keep the C++ habit of asking which operation is generic and which value owns shape.