formatter-spec.md

outline
deep

Formatter Specification

To keep the design of treefmt simple, we only support formatters that adhere to a certain standard. This document outlines that standard.

In this design, we rely on treefmt to do the tree traversal, and only invoke the code formatter on the selected files.

!!! note

If the formatter you would like to use doesn't comply with the rules, it's often possible to create a wrapper script
that transforms the usage to match the specification.

Rules

In order for the formatter to comply with this spec, it MUST satisfy the following:

1. Files passed as arguments

The formatter's CLI must be of the form:

<command> [options] [...<files>]

Where:

• <command> is the name of the formatting tool.
• [options] is any number of flags and options that the formatter accepts.
• [...<files>] is one or more files given to the formatter for processing.

Example:

$ rustfmt --edition 2018 src/main.rs src/lib.rs

!!! note

It _MUST_ process the specified files. For example, it _MUST_ NOT ignore files because they are not tracked by a VCS.

It _SHOULD_ processes only the specified files. Files that are not passed _SHOULD_ never be formatted.

2. Write to changed files

Whenever there is a change to the code formatting, the code formatter MUST write those changes back to the original location.

If there are no changes to the original file, the formatter MUST NOT write to the original location.

3. Idempotent

The code formatter SHOULD be indempotent. Meaning that it produces stable outputs.

4. Reliable

We expect the formatter to be reliable and not break the semantics of the formatted files.