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# Expression Syntax
Here is a Nix expression for GNU Hello:
```nix
{ stdenv, fetchurl, perl }: ①
stdenv.mkDerivation { ②
name = "hello-2.1.1"; ③
builder = ./builder.sh; ④
src = fetchurl { ⑤
url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
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 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 `./builder.sh` refers to the
shell script shown in the [next section](build-script.md),
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
```nix
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.
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