.envrc

# Usage: use_nix [...]
#
# Load environment variables from `nix-shell`.
# If you have a `default.nix` or `shell.nix` one of these will be used and
# the derived environment will be stored at ./.direnv/env-<hash>
# and symlink to it will be created at ./.direnv/default.
# Dependencies are added to the GC roots, such that the environment remains persistent.
#
# Packages can also be specified directly via e.g `use nix -p ocaml`,
# however those will not be added to the GC roots.
#
# The resulting environment is cached for better performance.
#
# To trigger switch to a different environment:
# `rm -f .direnv/default`
#
# To derive a new environment:
# `rm -rf .direnv/env-$(md5sum {shell,default}.nix 2> /dev/null | cut -c -32)`
#
# To remove cache:
# `rm -f .direnv/dump-*`
#
# To remove all environments:
# `rm -rf .direnv/env-*`
#
# To remove only old environments:
# `find .direnv -name 'env-*' -and -not -name `readlink .direnv/default` -exec rm -rf {} +`
#
use_nix() {
	set -e

	local shell="shell.nix"
	if [[ ! -f "${shell}" ]]; then
		shell="default.nix"
	fi

	local nixpkgs_version=".nixpkgs-version.json"
	if [[ ! -f "${nixpkgs_version}" ]]; then
		fail "use nix: .nixpkgs-version.json not found in the folder"
	fi

	if [[ ! -f "${shell}" ]]; then
		fail "use nix: shell.nix or default.nix not found in the folder"
	fi

	local dir="$(direnv_layout_dir)"
	local default="${dir}/default"
	if [[ ! -L "${default}" ]] || [[ ! -d $(readlink "${default}") ]]; then
		local wd="${dir}/env-$(hashFile "${shell}")-$(hashFile "${nixpkgs_version}")"
		mkdir -p "${wd}"

		local drv="${wd}/env.drv"
		if [[ ! -f "${drv}" ]]; then
			log_status "use nix: deriving new environment"
			IN_NIX_SHELL=1 nix-instantiate --add-root "${drv}" --indirect "${shell}" > /dev/null
			nix-store -r $(nix-store --query --references "${drv}") --add-root "${wd}/dep" --indirect > /dev/null
		fi

		rm -f "${default}"
		ln -s $(basename "${wd}") "${default}"
	fi

	local drv=$(readlink "${default}/env.drv")
	local dump="${dir}/dump-$(hashFile ".envrc")-$(hashFile ${drv})"

	if [[ ! -f "${dump}" ]] || [[ "${XDG_CONFIG_DIR}/direnv/direnvrc" -nt "${dump}" ]]; then
		log_status "use nix: updating cache"

		old=$(find ${dir} -name 'dump-*')
		nix-shell "${drv}" --show-trace "$@" --run "$(join_args "$direnv" dump)" > "${dump}"
		rm -f ${old}
	fi

	direnv_load cat "${dump}"

	watch_file "${default}"
	watch_file "${nixpkgs_version}"
	watch_file "${shell}"
}

hashFile() {
	if has md5sum; then
		md5sum "${@}" | cut -c -32
	elif has md5; then
		md5 -q "${@}"
	fi
}

fail() {
	log_error "${@}"
	exit 1
}

use_nix

.nixpkgs-version.json

{
  "url": "https://github.com/NixOS/nixpkgs-channels.git",
  "rev": "45a419ab5a23c93421c18f3d9cde015ded22e712",
  "date": "2018-10-16T10:33:58+01:00",
  "sha256": "00mpq5p351xsk0p682xjggw17qgd079i45yj0aa6awawpckfx37s",
  "fetchSubmodules": false
}

direnv_master.nix

{ pkgs ? import <nixpkgs> {} }:

pkgs.mkShell {
  buildInputs = [
    (pkgs.direnv.overrideAttrs (oa: rec {
      name = "direnv-unstable-${rev}";
      rev = "82c1b485117a61a015b88a8ffa04d8ff902443c0";
      src = pkgs.fetchFromGitHub {
        inherit rev;

        owner = "direnv";
        repo = "direnv";
        sha256 = "04mpq34pmbgjlax06ird3v8vphw231fbs59mwryhjciirac4hjwd";
      };
    }))
  ];
}

shell.nix

let
  # Look here for information about how to generate `nixpkgs-version.json`.
  #  → https://nixos.wiki/wiki/FAQ/Pinning_Nixpkgs
  pinnedVersion = builtins.fromJSON (builtins.readFile ./.nixpkgs-version.json);
  pinnedPkgs = import (builtins.fetchGit {
    inherit (pinnedVersion) url rev;

    ref = "nixos-unstable";
  }) {};
in

# This allows overriding pkgs by passing `--arg pkgs ...`
{ pkgs ? pinnedPkgs }:

with pkgs;

mkShell {
  buildInputs = [
    go
  ];
}