3 ms·

I don't understand. Isn't classes what the author asks for?

• masklinn 4y ago
  Lots of languages reify types as classes, but that's not necessary. You don't need classes to have types. Using classes also leaves entire segments of type-expression on the table or makes them unwieldy e.g. https://en.wikipedia.org/wiki/Tagged_union https://en.wikipedia.org/wiki/Tagged_union So it's orthogonal, classes are just one possible implementation of types.
• eyelidlessness 4y ago
  Depending on the language yes, or probably, or definitely not. A class is one of several ways to represent a type. Some languages have structural type representations and a class/instance is equivalent to a “plain old ___ object”; some languages have types but no notion of classes at all.
  • bjourne 4y ago
    Sure but the requirements given by the author are exactly represented by, for example, Java's class system. So why is the author dreaming about something that has existed for 20+ years?
• tsimionescu 4y ago
  Class as a concept is somewhat orthogonal to type, at least to people into programming language research, who consider "type" to implicitly mean compile-time type. Classes are taken to refer to support for virtual method dispatch, while types refer to compile-time expression type-checking. In languages like Java, C++ or C#, the type of an expression corresponds to the class of the value it will have at runtime. However, in Python for example, compile-time expressions always have the type "any", but can have various classes at runtime. This difference between class and type can even be seen in some of those languages. For example, the type *X has no corresponding class in C++ for any X. In Java, the type int doesn't have a class, and the types ArrayList<Integer> and ArrayList<Object> have the same class.
  • bjourne 4y ago
    According to my books classes are subtypes.