Expression Syntax

Here is a Nix expression for GNU Hello:

{ stdenv, fetchurl, perl }: ①

stdenv.mkDerivation { ②
  name = "hello-2.1.1"; ③
  builder = ./; ④
  src = fetchurl { ⑤
    url = "";
    sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
  inherit perl; ⑥

This file is actually already in the Nix Packages collection in pkgs/applications/misc/hello/ex-1/default.nix. It is customary to place each package in a separate directory and call the single Nix expression in that directory default.nix. The file has the following elements (referenced from the figure by number):

  1. This states that the expression is a function that expects to be called with three arguments: stdenv, fetchurl, and perl. They are needed to build Hello, but we don't know how to build them here; that's why they are function arguments. stdenv is a package that is used by almost all Nix Packages; it provides a “standard” environment consisting of the things you would expect in a basic Unix environment: a C/C++ compiler (GCC, to be precise), the Bash shell, fundamental Unix tools such as cp, grep, tar, etc. fetchurl is a function that downloads files. perl is the Perl interpreter.

    Nix functions generally have the form { x, y, ..., z }: e where x, y, etc. are the names of the expected arguments, and where e is the body of the function. So here, the entire remainder of the file is the body of the function; when given the required arguments, the body should describe how to build an instance of the Hello package.

  2. So we have to build a package. Building something from other stuff is called a derivation in Nix (as opposed to sources, which are built by humans instead of computers). We perform a derivation by calling stdenv.mkDerivation. mkDerivation is a function provided by stdenv that builds a package from a set of attributes. A set is just a list of key/value pairs where each key is a string and each value is an arbitrary Nix expression. They take the general form { name1 = expr1; ... nameN = exprN; }.

  3. The attribute name specifies the symbolic name and version of the package. Nix doesn't really care about these things, but they are used by for instance nix-env -q to show a “human-readable” name for packages. This attribute is required by mkDerivation.

  4. The attribute builder specifies the builder. This attribute can sometimes be omitted, in which case mkDerivation will fill in a default builder (which does a configure; make; make install, in essence). Hello is sufficiently simple that the default builder would suffice, but in this case, we will show an actual builder for educational purposes. The value ./ refers to the shell script shown in the next section, discussed below.

  5. The builder has to know what the sources of the package are. Here, the attribute src is bound to the result of a call to the fetchurl function. Given a URL and a SHA-256 hash of the expected contents of the file at that URL, this function builds a derivation that downloads the file and checks its hash. So the sources are a dependency that like all other dependencies is built before Hello itself is built.

    Instead of src any other name could have been used, and in fact there can be any number of sources (bound to different attributes). However, src is customary, and it's also expected by the default builder (which we don't use in this example).

  6. Since the derivation requires Perl, we have to pass the value of the perl function argument to the builder. All attributes in the set are actually passed as environment variables to the builder, so declaring an attribute

    perl = perl;

    will do the trick: it binds an attribute perl to the function argument which also happens to be called perl. However, it looks a bit silly, so there is a shorter syntax. The inherit keyword causes the specified attributes to be bound to whatever variables with the same name happen to be in scope.