7. Coding conventions

7.1. Syntax

  • Use 2 spaces of indentation per indentation level in Nix expressions, 4 spaces in shell scripts.

  • Do not use tab characters, i.e. configure your editor to use soft tabs. For instance, use ``(setq-default

    indent-tabs-mode nil)`` in Emacs. Everybody has different tab

    settings so it’s asking for trouble.

  • Use lowerCamelCase for variable names, not UpperCamelCase. TODO: naming of attributes in all-packages.nix?

  • Function calls with attribute set arguments are written as

    foo {
      arg = ...;


      arg = ...;

    Also fine is

    foo { arg = ...; }

    if it’s a short call.

  • In attribute sets or lists that span multiple lines, the attribute names or list elements should be aligned:

    # A long list.
    list =
      [ elem1
    # A long attribute set.
    attrs =
      { attr1 = short_expr;
        attr2 =
          if true then big_expr else big_expr;
    # Alternatively:
    attrs = {
      attr1 = short_expr;
      attr2 =
        if true then big_expr else big_expr;
  • Short lists or attribute sets can be written on one line:

    # A short list.
    list = [ elem1 elem2 elem3 ];
    # A short set.
    attrs = { x = 1280; y = 1024; };
  • Breaking in the middle of a function argument can give hard-to-read code, like

    someFunction { x = 1280;
      y = 1024; } otherArg

    (especially if the argument is very large, spanning multiple lines).


      { x = 1280; y = 1024; }


    let res = { x = 1280; y = 1024; };
    in someFunction res otherArg yetAnotherArg
  • The bodies of functions, asserts, and withs are not indented to prevent a lot of superfluous indentation levels, i.e.

    { arg1, arg2 }:
    assert system == "i686-linux";
    stdenv.mkDerivation { ...


    { arg1, arg2 }:
      assert system == "i686-linux";
        stdenv.mkDerivation { ...
  • Function formal arguments are written as:

    { arg1, arg2, arg3 }:

    but if they don’t fit on one line they’re written as:

    { arg1, arg2, arg3
    , arg4, ...
    , # Some comment...
  • Functions should list their expected arguments as precisely as possible. That is, write

    { stdenv, fetchurl, perl }:

    instead of

    args: with args;


    { stdenv, fetchurl, perl, ... }:

    For functions that are truly generic in the number of arguments (such as wrappers around mkDerivation) that have some required arguments, you should write them using an @-pattern:

    { stdenv, doCoverageAnalysis ? false, ... } @ args:
    stdenv.mkDerivation (args // {
       if doCoverageAnalysis then "bla" else ""

    instead of

    args.stdenv.mkDerivation (args // {
       if args ? doCoverageAnalysis && args.doCoverageAnalysis then "bla" else ""

7.2. Package naming

In Nixpkgs, there are generally three different names associated with a package:

  • The name attribute of the derivation (excluding the version part). This is what most users see, in particular when using nix-env.

  • The variable name used for the instantiated package in all-packages.nix, and when passing it as a dependency to other functions. This is what Nix expression authors see. It can also be used when installing using ``nix-env


  • The filename for (the directory containing) the Nix expression.

Most of the time, these are the same. For instance, the package e2fsprogs has a name attribute "e2fsprogs-", is bound to the variable name e2fsprogs in all-packages.nix, and the Nix expression is in pkgs/os-specific/linux/e2fsprogs/default.nix. However, identifiers in the Nix language don’t allow certain characters (e.g. dashes), so sometimes a different variable name should be used. For instance, the module-init-tools package is bound to the module_init_tools variable in all-packages.nix.

There are a few naming guidelines:

  • Generally, try to stick to the upstream package name.
  • Don’t use uppercase letters in the name attribute — e.g., "mplayer-1.0rc2" instead of "MPlayer-1.0rc2".
  • The version part of the name attribute must start with a digit (following a dash) — e.g., "hello-0.3-pre-r3910" instead of "hello-svn-r3910", as the latter would be seen as a package named hello-svn by nix-env.
  • Dashes in the package name should be changed to underscores in variable names, rather than to camel case — e.g., module_init_tools instead of moduleInitTools.
  • If there are multiple versions of a package, this should be reflected in the variable names in all-packages.nix, e.g. hello_0_3 and hello_0_4. If there is an obvious “default” version, make an attribute like hello = hello_0_4;.

7.3. File naming and organisation

Names of files and directories should be in lowercase, with dashes between words — not in camel case. For instance, it should be all-packages.nix, not allPackages.nix or AllPackages.nix.

7.3.1. Hierachy

Each package should be stored in its own directory somewhere in the pkgs/ tree, i.e. in pkgs////. Below are some rules for picking the right category for a package. Many packages fall under several categories; what matters is the primary purpose of a package. For example, the libxml2 package builds both a library and some tools; but it’s a library foremost, so it goes under pkgs/development/libraries.

When in doubt, consider refactoring the pkgs/ tree, e.g. creating new categories or splitting up an existing category.

If it’s used to support software development:

If it’s a library used by other packages:
development/libraries (e.g. libxml2)
If it’s a compiler:
development/compilers (e.g. gcc)
If it’s an interpreter:
development/interpreters (e.g. guile)

If it’s a (set of) development tool(s):

If it’s a parser generator (including lexers):
development/tools/parsing (e.g. bison, flex)
If it’s a build manager:
development/tools/build-managers (e.g. gnumake)
development/tools/misc (e.g. binutils)
If it’s a (set of) tool(s):

(A tool is a relatively small program, especially one intented to be used non-interactively.)

If it’s for networking:
tools/networking (e.g. wget)
If it’s for text processing:
tools/text (e.g. diffutils)

If it’s a system utility, i.e., something related or essential to the operation of a system:

tools/system (e.g. cron)
If it’s an archiver (which may include a compression function):
tools/archivers (e.g. zip, tar)
If it’s a compression program:
tools/compression (e.g. gzip, bzip2)
If it’s a security-related program:
tools/security (e.g. nmap, gnupg)
If it’s a shell:
shells (e.g. bash)

If it’s a server:

If it’s a web server:
servers/http (e.g. apache-httpd)
If it’s an implementation of the X Windowing System:
servers/x11 (e.g. xorg — this includes the client libraries and programs)
If it’s a desktop environment (including window managers):
desktops (e.g. kde, gnome, enlightenment)
If it’s an application:

A (typically large) program with a distinct user interface, primarily used interactively.

If it’s a version management system:
applications/version-management (e.g. subversion)
If it’s for video playback / editing:
applications/video (e.g. vlc)
If it’s for graphics viewing / editing:
applications/graphics (e.g. gimp)

If it’s for networking:

If it’s a mailreader:
applications/networking/mailreaders (e.g. thunderbird)
If it’s a newsreader:
applications/networking/newsreaders (e.g. pan)
If it’s a web browser:
applications/networking/browsers (e.g. firefox)

If it’s data (i.e., does not have a straight-forward executable semantics):

If it’s a font:

If it’s related to SGML/XML processing:

If it’s an XML DTD:
data/sgml+xml/schemas/xml-dtd (e.g. docbook)
If it’s an XSLT stylesheet:

(Okay, these are executable...)

data/sgml+xml/stylesheets/xslt (e.g. docbook-xsl)

If it’s a game:

7.3.2. Versioning

Because every version of a package in Nixpkgs creates a potential maintenance burden, old versions of a package should not be kept unless there is a good reason to do so. For instance, Nixpkgs contains several versions of GCC because other packages don’t build with the latest version of GCC. Other examples are having both the latest stable and latest pre-release version of a package, or to keep several major releases of an application that differ significantly in functionality.

If there is only one version of a package, its Nix expression should be named e2fsprogs/default.nix. If there are multiple versions, this should be reflected in the filename, e.g. e2fsprogs/1.41.8.nix and e2fsprogs/1.41.9.nix. The version in the filename should leave out unnecessary detail. For instance, if we keep the latest Firefox 2.0.x and 3.5.x versions in Nixpkgs, they should be named firefox/2.0.nix and firefox/3.5.nix, respectively (which, at a given point, might contain versions and 3.5.4). If a version requires many auxiliary files, you can use a subdirectory for each version, e.g. firefox/2.0/default.nix and firefox/3.5/default.nix.

All versions of a package must be included in all-packages.nix to make sure that they evaluate correctly.