carbon-lang/docs/design/structs.md

Structs

Table of contents

• TODO
• Overview
• Open questions
  • self type
  • Default access control level

TODO

This is a skeletal design, added to support the overview. It should not be treated as accepted by the core team; rather, it is a placeholder until we have more time to examine this detail. Please feel welcome to rewrite and update as appropriate.

Overview

Beyond simple tuples, Carbon of course allows defining named product types. This is the primary mechanism for users to extend the Carbon type system and fundamentally is deeply rooted in C++ and its history (C and Simula). We simply call them structs rather than other terms as it is both familiar to existing programmers and accurately captures their essence: they are a mechanism for structuring data:

struct Widget {
  var Int x;
  var Int y;
  var Int z;

  var String payload;
}

Most of the core features of structures from C++ remain present in Carbon, but often using different syntax:

struct AdvancedWidget {
  // Do a thing!
  fn DoSomething(AdvancedWidget self, Int x, Int y);

  // A nested type.
  struct NestedType {
    // ...
  }

  private var Int x;
  private var Int y;
}

fn Foo(AdvancedWidget thing) {
  thing.DoSomething(1, 2);
}

Here we provide a public object method and two private data members. The method explicitly indicates how the object parameter is passed to it, and there is no automatic scoping - you have to use self here. The self name is also a keyword, though, that explains how to invoke this method on an object. This member function accepts the object by value, which is easily expressed here along with other constraints on the object parameter. Private members work the same as in C++, providing a layer of easy validation of the most basic interface constraints.

The type itself is a compile-time constant value. All name access is done with the . notation. Constant members (including member types and member functions which do not need an implicit object parameter) can be accessed by way of that constant: AdvancedWidget.NestedType. Other members and member functions needing an object parameter (or "methods") must be accessed from an object of the type.

Some things in C++ are notably absent or orthogonally handled:

• No need for static functions, they simply don't take an initial self parameter.
• No static variables because there are no global variables. Instead, can have scoped constants.

Open questions

self type

Requiring the type of self makes method declarations quite verbose. Unclear what is the best way to mitigate this, there are many options. One is to have a special Self type.

It may be interesting to consider separating the self syntax from the rest of the parameter pattern as it doesn't seem necessary to inject all of the special rules (covariance versus contravariance, special pointer handling) for self into the general pattern matching system.

Default access control level

The default access control level, and the options for access control, are pretty large open questions. Swift and C++ (especially w/ modules) provide a lot of options and a pretty wide space to explore here. If the default isn't right most of the time, access control runs the risk of becoming a significant ceremony burden that we may want to alleviate with grouped access regions instead of per-entity specifiers. Grouped access regions have some other advantages in terms of pulling the public interface into a specific area of the type.