Idea for CRUD types - feedback appreciated

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3

You can use this hack I came up with to have the type checker verify that a class is a supertype of multiple types.

type Flying = {
  fly : () -> ();
};

type Swimming = {
  swim : () -> ();
};

type Walking = {
  walk : () -> ();
};

class Bird() {
  public func fly() {};
  public func walk() {};
};

// Flying <: Bird
let _ : () -> Flying = Bird;

// Walking <: Bird
let _ : () -> Walking = Bird;

class Dog() {
  public func swim() {};
  public func walk() {};
};

// Swimming <: Dog
let _ : () -> Swimming = Dog;

// Walking <: Dog
let _ : () -> Walking = Dog;

For example, if you were to say that Walking <: Dog (using the let-bound technique I showed here) and then accidentally omitted the walk method from the definition of Dog you would get this error:

main.mo:70.25-70.28: type error, expression of type
  () -> Dog
cannot produce expected type
  () -> Walking

If your class constructor takes arguments then you need to pass those along too:

class Dog(name_ : Text) {
  var name = name_;
  public func swim() {};
  public func walk() {};
};

// Swimming <: Dog
let _ : Text -> Swimming = func(name : Text) { Dog(name) };

// Walking <: Dog
let _ : Text -> Walking = func(name : Text) { Dog(name) };
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