yants

This is a tiny type-checker for data in Nix, written in Nix.

Features

Checking of primitive types (int, string etc.)
Checking polymorphic types (option, list, either)
Defining & checking struct/record types
Defining & matching enum types
Defining & matching sum types
Defining function signatures (including curried functions)
Types are composable! option string! list (either int (option float))!
Type errors also compose!

Currently lacking:

Any kind of inference
Convenient syntax for attribute-set function signatures

Primitives & simple polymorphism

simple

Structs

structs

Nested structs!

nested structs

Enums!

enums

Functions!

functions

Usage

Yants can be imported from its default.nix. A single attribute (lib) can be passed, which will otherwise be imported from <nixpkgs>.

TIP: You do not need to clone the entire TVL repository to use Yants! You can clone just this project through josh: git clone https://code.tvl.fyi/depot.git:/nix/yants.git

Examples for the most common import methods would be:

Import into scope with with:

with (import ./default.nix {});
# ... Nix code that uses yants ...

Import as a named variable:

let yants = import ./default.nix {};
in yants.string "foo" # or other uses ...

Overlay into pkgs.lib:

# wherever you import your package set (e.g. from <nixpkgs>):
import <nixpkgs> {
  overlays = [
    (self: super: {
      lib = super.lib // { yants = import ./default.nix { inherit (super) lib; }; };
    })
  ];
}

# yants now lives at lib.yants, besides the other library functions!

Please see my Nix one-pager for more generic information about the Nix language and what the above constructs mean.

Stability

The current API of Yants is not yet considered stable, but it works fine and should continue to do so even if used at an older version.

Yants' tests use Nix versions above 2.2 - compatibility with older versions is not guaranteed.