jmt

Bridge between treefmt and jj fix.

Problem

Nix flakes with treefmt-nix define formatters declaratively, but jj fix requires separate configuration. Maintaining both is tedious and error-prone.

Additionally, jj fix requires tools to read from stdin and write to stdout, but many formatters (deadnix, ruff --fix) only support in-place file modification.

Solution

jmt automatically generates jj fix config from your existing treefmt setup:

• Single source of truth: Define formatters once in treefmt-nix, use everywhere
• Smart caching: Only rebuilds formatter when flake.nix/flake.lock changes
• stdin/stdout adaptation: Wraps incompatible tools with temp file workarounds

Why jj fix?

Unlike traditional formatters that modify files in your working copy:

• No merge conflicts: Formatting is applied per-commit, not to working copy
• Works with jj absorb: Format changes are automatically absorbed into the right commits
• Clean history: Each commit is formatted independently, even in stacked PRs

# Format all commits in current stack
jmt

# Then absorb any other changes - no conflicts!
jj absorb

Usage

jmt                          # Sync config + run jj fix
jmt --sync                   # Only sync config
jmt --print                  # Print generated config
jmt --list                   # List available formatters
jmt --only=nixfmt,ruff       # Run specific tools only
jmt -- -s @-                 # Pass args to jj fix (format parent)

Install

{
  inputs.jmt.url = "github:mulatta/jmt";
}

nix run github:mulatta/jmt

stdin/stdout Handling

jj fix requires tools to read stdin and write stdout. jmt handles this automatically:

Tool Type	Strategy	Example
Native stdin	Pass through	nixfmt, alejandra
Needs flag	Add stdin arg	yamlfmt -, taplo -
In-place only	Temp file wrapper	deadnix, ruff check --fix
Linters	Passthrough (stderr only)	shellcheck, statix

Wrapper script pattern for in-place tools:

t=$(mktemp --suffix=.nix); cat >"$t"; deadnix -e "$t"; cat "$t"; rm "$t"

How it works

1. Build nix build .#formatter.<system> (cached by mtime)
2. Extract treefmt.toml path from wrapper script
3. Parse formatters, generate jj fix TOML with stdin-compatible commands
4. Write to .jj/repo/config.toml
5. Run jj fix --include-unchanged-files

License

MIT