carbon-lang/docs/design/variables.md

Variables

Table of contents

• TODO
• Overview
  • Declaring constants
• Alternatives
  • Declaring constants
  • Global variables

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

Blocks introduce nested scopes and can contain local variable declarations that work similarly to function parameters.

For example:

fn Foo() {
  var Int: x = 42;
}

This introduces a local variable named x into the block's scope. It has the type Int and is initialized with the value 42. These variable declarations (and function declarations) have a lot more power than what we're covering just yet, but this gives you the basic idea.

While there can be global constants, there are no global variables.

Declaring constants

Constants will use template-like syntax for declarations. For example, a simple integer constant looks like:

var Int:$$ MyVal = 42;

Alternatives

Declaring constants

There is other syntax that could be used for declaring constants. There are serious problems with the use of const in C++ as part of the type system. Another alternative is let from Swift, although there are some questions around how intuitive it is for this to introduce a constant. Another candidate is val from Kotlin. Another thing we need to contend with is the surprise of const and reference (semantic) types. At present we are leaning towards the tempalte-like syntax for consistency within Carbon.

Global variables

We are exploring several different ideas for how to design less bug-prone patterns to replace the important use cases programmers still have for global variables. We may be unable to fully address them, at least for migrated code, and be forced to add some limited form of global variables back. We may also discover that their convenience outweighs any improvements afforded.