conformist

Status: early / pre-1.0. conformist is a clean copy of treefmt v2.5.0 that adds first-class linting on top of treefmt's formatter multiplexing, per RFC 0001. The [linter.<name>] config section, the conformist check subcommand, and repair-mode autofixes are implemented. Expect rough edges before 1.0.

conformist runs all your formatters and linters with one command. It inherits treefmt's model: conformist walks the tree, matches files to tools by glob, and runs the matched tools in parallel, only on files that changed since the last run.

The linter additions (RFC 0001):

• A [linter.<name>] config section parallel to [formatter.<name>]. Its command is a read-only check; an optional repair-command applies autofixes.
• First-class repair mode (the default — applies formatter and linter fixes) and check mode (read-only — reports without writing), exposed via a conformist check subcommand.
• A sandbox-copy-and-diff strategy so fix-only formatters can be checked without ever writing to your source tree (so checks work even on a read-only tree).

With Nix (the supported path):

nix build github:amarbel-llc/conformist
./result/bin/conformist --help

Or from source with Go ≥ 1.26:

go build -o conformist .

Generate a starter config (writes conformist.toml):

conformist --init

Format the tree (repair mode):

conformist

Check the tree read-only — runs every formatter and linter without writing. Exits 0 when clean, 1 when findings are detected, 2 on an operational error:

conformist check

Print version:

conformist version

Formatters are specified in conformist.toml (or .conformist.toml), discovered by searching upward from the working directory:

[formatter.nix]
command = "nixfmt"
includes = ["*.nix"]

[formatter.rust]
command = "rustfmt"
options = ["--edition", "2018"]
includes = ["*.rs"]

A formatter may declare a native read-only check via check-command / check-options for use by conformist check; without one, the formatter is checked through a sandbox copy. Set sandbox = true to force sandbox checking even when a check command exists.

Linters use a parallel [linter.<name>] section. Its command is the read-only check (it must exit non-zero on findings); add repair-command / repair-options to apply autofixes in repair mode. A linter with no repair-command is a no-op in repair mode:

[linter.shellcheck]
command = "shellcheck"
includes = ["*.sh"]

[linter.ruff]
command = "ruff"
options = ["check"]
repair-command = "ruff"
repair-options = ["check", "--fix"]
includes = ["*.py"]

conformist ships a Nix module (like treefmt-nix, extended to cover linters). A flake declares its formatters and linters once and gets a generated config, a nix fmt wrapper, and a read-only conformist check flake check — with tool binaries resolved from your pinned nixpkgs.

# flake.nix
inputs.conformist.url = "github:amarbel-llc/conformist";

# ... in eachDefaultSystem:
conformistEval = conformist.lib.evalModule pkgs {
  package = conformist.packages.${system}.default; # required: conformist isn't in nixpkgs
  programs.gofmt.enable = true;
  programs.nixfmt.enable = true;
  linters.shellcheck.enable = true;
};
# ...
formatter = conformistEval.config.build.wrapper;       # nix fmt
checks.formatting = conformistEval.config.build.check self; # nix flake check

A flake-parts flakeModule is also exported. conformist itself self-consumes this module for its own nix fmt / formatting check. See the Nix module guide for the full surface: declaring tools the registry doesn't ship, the formatter-vs-linter namespaces, and the build outputs.

conformist runs tools that follow the formatter specification: files are passed as arguments, the tool writes back only on change, and it exits non-zero on error.

conformist is derived from numtide/treefmt v2.5.0 and is an independent project (not a GitHub fork). treefmt's original copyright and MIT license are retained in LICENSE; see NOTICE for provenance. conformist is released under the MIT license.