depot/doc/languages-frameworks/nim.section.md

Nim

The Nim compiler and a builder function is available. Nim programs are built using buildNimPackage and a lockfile containing Nim dependencies.

The following example shows a Nim program that depends only on Nim libraries:

{ lib, buildNimPackage, fetchFromGitHub }:

buildNimPackage (finalAttrs: {
  pname = "ttop";
  version = "1.2.7";

  src = fetchFromGitHub {
    owner = "inv2004";
    repo = "ttop";
    rev = "v${finalAttrs.version}";
    hash = "sha256-oPdaUqh6eN1X5kAYVvevOndkB/xnQng9QVLX9bu5P5E=";
  };

  lockFile = ./lock.json;

  nimFlags = [
    "-d:NimblePkgVersion=${finalAttrs.version}"
  ];
})

buildNimPackage parameters

The buildNimPackage function takes an attrset of parameters that are passed on to stdenv.mkDerivation.

The following parameters are specific to buildNimPackage:

• lockFile: JSON formatted lockfile.
• nimbleFile: Specify the Nimble file location of the package being built rather than discover the file at build-time.
• nimRelease ? true: Build the package in release mode.
• nimDefines ? []: A list of Nim defines. Key-value tuples are not supported.
• nimFlags ? []: A list of command line arguments to pass to the Nim compiler. Use this to specify defines with arguments in the form of -d:${name}=${value}.
• nimDoc ? false`: Build and install HTML documentation.

Lockfiles

Nim lockfiles are created with the nim_lk utility. Run nim_lk with the source directory as an argument and it will print a lockfile to stdout.

$ cd nixpkgs
$ nix build -f . ttop.src
$ nix run -f . nim_lk ./result | jq --sort-keys > pkgs/by-name/tt/ttop/lock.json

Overriding Nim packages

The buildNimPackage function generates flags and additional build dependencies from the lockFile parameter passed to buildNimPackage. Using overrideAttrs on the final package will apply after this has already been generated, so this can't be used to override the lockFile in a package built with buildNimPackage. To be able to override parameters before flags and build dependencies are generated from the lockFile, use overrideNimAttrs instead with the same syntax as overrideAttrs:

pkgs.nitter.overrideNimAttrs {
  # using a different source which has different dependencies from the standard package
  src = pkgs.fetchFromGithub { /* … */ };
  # new lock file generated from the source
  lockFile = ./custom-lock.json;
}

Lockfile dependency overrides

The buildNimPackage function matches the libraries specified by lockFile to attrset of override functions that are then applied to the package derivation. The default overrides are maintained as the top-level nimOverrides attrset at pkgs/top-level/nim-overrides.nix.

For example, to propagate a dependency on SDL2 for lockfiles that select the Nim sdl2 library, an overlay is added to the set in the nim-overrides.nix file:

{ lib
/* … */
, SDL2
/* … */
}:

{
  /* … */
  sdl2 =
    lockAttrs:
    { buildInputs ? [ ], ... }:
    {
      buildInputs = buildInputs ++ [ SDL2 ];
    };
  /* … */
}

The annotations in the nim-overrides.nix set are functions that take two arguments and return a new attrset to be overlaid on the package being built.

• lockAttrs: the attrset for this library from within a lockfile. This can be used to implement library version constraints, such as marking libraries as broken or insecure.
• prevAttrs: the attrset produced by initial arguments to buildNimPackage and any preceding lockfile overlays.

Overriding an Nim library override

The nimOverrides attrset makes it possible to modify overrides in a few different ways.

Override a package internal to its definition:

{ lib, buildNimPackage, nimOverrides, libressl }:

let
  buildNimPackage' = buildNimPackage.override {
    nimOverrides = nimOverrides.override { openssl = libressl; };
  };
in buildNimPackage' (finalAttrs: {
  pname = "foo";
  # …
})

Override a package externally:

{ pkgs }: {
  foo = pkgs.foo.override {
    buildNimPackage = pkgs.buildNimPackage.override {
      nimOverrides = pkgs.nimOverrides.override { openssl = libressl; };
    };
  };
}