Tix

A type checker for Nix, built on MLsub/SimpleSub — Hindley-Milner extended with subtyping, union types, and intersection types.

The goal is to have inference do as much of the work as possible but when it gets really nasty allow manually specified types through doc comments.

TLDR: try to be typescript for nix

Its currently still in progress so you might see weird type errors but has been decently usable on real projects

Quick Start

Installation

nix run github:JRMurr/tix -- inspect my-file.nix

{
  inputs.tix.url = "github:JRMurr/tix";
}

# configuration.nix
{ inputs, pkgs, ... }:
{
  environment.systemPackages = [
    inputs.tix.packages.${pkgs.system}.default
  ];
}

nix profile install github:JRMurr/tix

# configuration.nix
let
  tix = import (builtins.fetchTarball "https://github.com/JRMurr/tix/archive/main.tar.gz") {};
in
{
  environment.systemPackages = [
    tix.packages.${builtins.currentSystem}.default
  ];
}

let
  tix = import (builtins.fetchTarball {
    url = "https://github.com/JRMurr/tix/archive/<rev>.tar.gz";
    sha256 = "<hash>";  # nix-prefetch-url --unpack <url>
  }) {};
in
  tix.packages.${builtins.currentSystem}.default

nix-env -f https://github.com/JRMurr/tix/archive/main.tar.gz -iA packages.x86_64-linux.default

Type-check a file

tix inspect my-file.nix

Use the LSP

tix lsp

Works with any editor that supports LSP. Provides hover types, completions, go-to-definition, rename, inlay hints, and more.

CLI flags

tix inspect <file.nix> [--stubs path/to/stubs/] [--no-default-stubs] [--config tix.toml]
tix stubs generate nixos [--flake .] [--hostname myhost] [-o nixos.tix]
tix stubs generate home-manager [--flake .] [--username jr] [-o hm.tix]

What does it do?

Given a Nix file like:

let
  add = a: b: a + b;
  result = add 1 2;
  greeting = if result > 0 then "positive" else null;
in
{ inherit add result greeting; }

Tix infers:

add     :: int -> int -> int
result  :: int
greeting :: string | null

Union types fall out naturally — if-then-else with different branch types, heterogeneous lists, etc. all just work.

Documentation

Full docs (type system, stubs, configuration, internals): jrmurr.github.io/tix

To build locally:

cd docs && mdbook serve

Links

• nil — Nix LSP that does some type inference. Read over its source a lot and took some code from there to get started.
• nix-types RFC — good spec for parsing doc comments
• The Simple Essence of Algebraic Subtyping — the paper this is based on