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docs/design/lexical_conventions/comments.md

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Table of contents

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Overview

A comment is a lexical element beginning with the characters // and running to the end of the line. We have no mechanism for physical line continuation, so a trailing \ does not extend a comment to subsequent lines.

A comment may be the entire content of a line, or it may follow other content on the line as a trailing comment. A full-line comment typically introduces the code below it, while a trailing comment annotates the content on its own line.

Details

After the // a whitespace character is required to make the comment valid. Newline is a whitespace character, so a line containing only // is a valid comment. The end of the file also constitutes whitespace.

All comments are removed prior to formation of tokens; a comment produces no token.

Example:

// This is a comment and is ignored. \
This is not a comment.

var Int: x; // This is a trailing comment annotating `x`.

Currently no support for block comments is provided. Commenting out larger regions of human-readable text or code is accomplished by commenting out every line in the region.

Style

Full-line and trailing comments serve different purposes:

  • Prefer a full-line comment for documentation. It has less line-length pressure and more easily stays attached to the code it describes as that code changes.
  • Use a trailing comment only to annotate or mark a specific line, in a context where a comment on the preceding line would be awkward, verbose, or imprecise.

Alternatives considered

References