Formatting Mux source

The formatter is included in Mux 0.12.0 and later releases.

Run mux format from a project directory to format its .mux files recursively. You can also select files or directories:

mux format
mux format src tests
mux format ~/test.mux

By default, the formatter uses four-space indentation and an 80-column target. It preserves literal spellings, comment contents, and declaration order. Long literals, comments, and expressions without a safe break point can exceed the target.

To change the layout, add a format object to mux-project.json:

{
  "format": {
    "indent_type": "space",
    "indent_count": 2,
    "line_width": 100,
    "brace_style": "same_line",
    "where_position": "own_line",
    "blank_lines_between_declarations": 1,
    "blank_lines_between_members": 0,
    "blank_lines_before_functions": 1,
    "trailing_comma": "multiline"
  }
}

The CLI looks for mux-project.json in the working directory and its parent directories. It stops at the nearest Git worktree root and does not search the filesystem root. One config applies to every path in a command. Without a config, the built-in defaults apply: braces stay on the declaration line, where clauses start on their own line, there is one blank line between declarations, ordinary class and type members have no extra blank lines, and multiline lists use trailing commas. Function members have one blank line above them. Invalid JSON uses all defaults; invalid settings use their individual defaults, and warnings go to stderr.

indent_type accepts space or tab. The default indent count is four spaces or one tab per level. brace_style accepts same_line or next_line, and where_position accepts own_line or same_line. trailing_comma accepts multiline, never, or always. Blank-line counts can be any nonnegative integer.

Use --check or -c to report files that would change without writing them:

mux format --check
mux format -c src

The command exits with status 0 when the files are formatted, 1 when a check finds changes, and 2 for syntax, input, or file-operation errors.

Formatting requires valid syntax, but missing imports and type errors do not prevent it. Directory traversal skips .git, target, and node_modules and does not follow discovered symlinks. Explicit symlink paths resolve to their targets. Repeated paths are processed once; empty directories succeed without changes. A missing path or an explicitly selected non-Mux file is an error.

The formatter prepares every selected file before writing, so a syntax error leaves the selected files untouched. Each changed file is replaced atomically with its permissions preserved. An I/O error during replacement can leave earlier files formatted. Changes made concurrently during preparation or staging are reported instead of overwritten, so avoid editing selected files while formatting.