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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.