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I was doing this for #851 initially, but I think #438 and #826 hadn't made it in (possibly intentionally due to #851? I don't recall). Co-authored-by: Chandler Carruth <chandlerc@gmail.com> Co-authored-by: Richard Smith <richard@metafoo.co.uk>
77 lines
2.5 KiB
Markdown
77 lines
2.5 KiB
Markdown
# Variables
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<!--
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Part of the Carbon Language project, under the Apache License v2.0 with LLVM
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Exceptions. See /LICENSE for license information.
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SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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-->
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<!-- toc -->
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## Table of contents
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- [Overview](#overview)
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- [Notes](#notes)
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- [Global variables](#global-variables)
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- [Alternatives considered](#alternatives-considered)
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- [References](#references)
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<!-- tocstop -->
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## Overview
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Carbon's local variable syntax is:
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- `var` _identifier_`:` _< expression |_ `auto` _> [_ `=` _value ]_`;`
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Blocks introduce nested scopes and can contain local variable declarations that
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work similarly to function parameters.
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For example:
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```
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fn Foo() {
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var x: i32 = 42;
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}
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```
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This introduces a local variable named `x` into the block's scope. It has the
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type `Int` and is initialized with the value `42`. These variable declarations
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(and function declarations) have a lot more power than what we're covering just
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yet, but this gives you the basic idea.
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If `auto` is used in place of the type, [type inference](type_inference.md) is
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used to automatically determine the variable's type.
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While there can be global constants, there are no global variables.
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## Notes
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> TODO: Constant syntax is an ongoing discussion.
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### Global variables
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We are exploring several different ideas for how to design less bug-prone
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patterns to replace the important use cases programmers still have for global
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variables. We may be unable to fully address them, at least for migrated code,
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and be forced to add some limited form of global variables back. We may also
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discover that their convenience outweighs any improvements afforded.
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## Alternatives considered
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- [No `var` introducer keyword](/proposals/p0339.md#no-var-introducer-keyword)
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- [Name of the `var` statement introducer](/proposals/p0339.md#name-of-the-var-statement-introducer)
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- [Colon between type and identifier](/proposals/p0339.md#colon-between-type-and-identifier)
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- [Type elision](/proposals/p0339.md#type-elision)
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- [Type ordering](/proposals/p0618.md#type-ordering)
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- [Elide the type instead of using `auto`](/proposals/p0851.md#elide-the-type-instead-of-using-auto)
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## References
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- Proposal
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[#339: `var` statement](https://github.com/carbon-language/carbon-lang/pull/339)
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- Proposal
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[#618: `var` ordering](https://github.com/carbon-language/carbon-lang/pull/618)
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- Proposal
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[#851: auto keyword for vars](https://github.com/carbon-language/carbon-lang/pull/851)
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