# Source files

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## Table of contents

-   [Overview](#overview)
-   [Encoding](#encoding)
-   [Alternatives considered](#alternatives-considered)
-   [References](#references)

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## Overview

A Carbon _source file_ is a sequence of Unicode code points in Unicode
Normalization Form C ("NFC"), and represents a portion of the complete text of a
program.

Program text can come from a variety of sources, such as an interactive
programming environment (a so-called "Read-Evaluate-Print-Loop" or REPL), a
database, a memory buffer of an IDE, or a command-line argument.

The canonical representation for Carbon programs is in files stored as a
sequence of bytes in a file system on disk. Such files have a `.carbon`
extension.

## Encoding

The on-disk representation of a Carbon source file is encoded in UTF-8. Such
files may begin with an optional UTF-8 BOM, that is, the byte sequence
EF<sub>16</sub>,BB<sub>16</sub>,BF<sub>16</sub>. This prefix, if present, is
ignored.

No Unicode normalization is performed when reading an on-disk representation of
a Carbon source file, so the byte representation is required to be normalized in
Normalization Form C. The Carbon source formatting tool will convert source
files to NFC as necessary.

## Alternatives considered

-   [Character encoding](/proposals/p0142.md#character-encoding-1)
-   [Byte order marks](/proposals/p0142.md#byte-order-marks)
-   [Normalization forms](/proposals/p0142.md#normalization-forms)

## References

-   Proposal
    [#142: Unicode source files](https://github.com/carbon-language/carbon-lang/pull/142)

-   [Unicode](https://www.unicode.org/versions/latest/) is a universal character
    encoding, maintained by the
    [Unicode Consortium](https://home.unicode.org/basic-info/overview/). It is
    the canonical encoding used for textual information interchange across all
    modern technology.

    Carbon is based on Unicode 13.0, which is currently the latest version of
    the Unicode standard. Newer versions will be considered for adoption as they
    are released.

-   [Unicode Standard Annex #15: Unicode Normalization Forms](https://www.unicode.org/reports/tr15/tr15-50.html)

-   [Wikipedia Unicode equivalence page: Normal forms](https://en.wikipedia.org/wiki/Unicode_equivalence#Normal_forms)