Generic prototypes with Concepts

A generic prototype in Cyan takes a type (or a generic symbol) as a parameter just like generic classes in Java or template classes in C++:

object Box<T>
    func init: T value { self.value = value }
    
    func mult: T other -> T = self.value * other;
    
      // create get and set methods    
    @property
    var T value
end

For each new parameter, a new Box prototype is created (like in C++ and unlike in Java). Therefore, Box<Int> and Box<Float> and two unrelated prototypes.
In method mult: of Box, there is the sending of message * with argument other to field value of type T. That means that T should have a method

func * (T other) -> T

in order to compile correctly.

If the supplied type T in the instantiation, like Box, does not have this method, there is a compilation error:

// String does not define a * method
var Box<String> bs;

The error issued is:

The type of the real argument, 'cyan.lang.String', of expression 'other' is not sub-prototype of the type of the formal parameter,
'cyan.lang.Int' in message send with keyword '*'
Stack of generic prototype instantiations:
    main.Box<String> line 8 column 45
    main.Program line 11 column 17

    func mult: (String other)  -> String  = self.value * other;

The compiler shows code Box<String> in the error message, which is confusing. A better option is to use metaobject concept:

@concept{*
    T has [ func * T -> T ]
*}
object Box<T>
    func init: T value { self.value = value }
    
    func mult: T other -> T = self.value * other;
    
      // create get and set methods    
    @property
    var T value
end

Now, the error message is clearer:

A concept associated to generic prototype 'main.Box<String>' expected that method '* String -> String' were in prototype 'cyan.lang.String'
Stack of generic prototype instantiations:
    main.Program line 11 column 17

        var b = Box<String>("");