carbon-lang/docs/design/variables.md

Variables

Table of contents

• Overview
• Notes
  • Global variables
• Alternatives considered
• References

Overview

Carbon's local variable syntax is:

• var identifier: < expression | auto > [ = value ];

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

For example:

fn Foo() {
  var x: i32 = 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.

If auto is used in place of the type, type inference is used to automatically determine the variable's type.

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

Notes

TODO: Constant syntax is an ongoing discussion.

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.

Alternatives considered

• No var introducer keyword
• Name of the var statement introducer
• Colon between type and identifier
• Type elision
• Type ordering
• Elide the type instead of using auto

References

• Proposal #339: var statement
• Proposal #618: var ordering
• Proposal #851: auto keyword for vars