nix-pills/pills/05-functions-and-imports.xml

<chapter xmlns="http://docbook.org/ns/docbook"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xi="http://www.w3.org/2001/XInclude"
      version="5.0"
      xml:id="functions-and-imports">

      <title>functions and imports</title>

<section>

      <para>
            Welcome to the fifth Nix pill. In the previous <link
    linkend="basics-of-the-language">fourth pill</link> we touched the Nix language for a moment. We introduced basic types and values of the Nix language, and basic expressions such as <literal>if</literal>, <literal>with</literal> and <literal>let</literal>. I invite you to re-read about these expressions and play with them in the repl.
      </para>

      <para>
            Functions help to build reusable components in a big repository like <link xlink:href="https://github.com/NixOS/nixpkgs/">nixpkgs</link>. The Nix manual has a <link xlink:href="https://nixos.org/nix/manual/#ss-functions">great explanation of functions</link>. Let's go: pill on one hand, Nix manual on the other hand.
      </para>

      <para>
            I remind you how to enter the Nix environment: <literal>source ~/.nix-profile/etc/profile.d/nix.sh</literal>
      </para>

</section>

<section>
      <title>Nameless and single parameter</title>

      <para>
            Functions are anonymous (lambdas), and only have a single parameter. The syntax is extremely simple. Type the parameter name, then "<literal>:</literal>", then the body of the function.
      </para>

      <screen><xi:include href="./05/anon-function.txt" parse="text" /></screen>

      <para>
            So here we defined a function that takes a parameter <literal>x</literal>, and returns <literal>x*2</literal>. The problem is that we cannot use it in any way, because it's unnamed... joke!
      </para>

      <para>
            We can store functions in variables.
      </para>

      <screen><xi:include href="./05/named-function.txt" parse="text" /></screen>

      <para>
            As usual, please ignore the special syntax for assignments inside nix-repl. So, we defined a function <literal>x: x*2</literal> that takes one parameter <literal>x</literal>, and returns <literal>x*2</literal>. This function is then assigned to the variable <literal>double</literal>. Finally we did our first function call: <literal>double 3</literal>.
      </para>

      <para>
            <emphasis role="underline">Big note:</emphasis> it's not like many other programming languages where you write <literal>double(3)</literal>. It really is <literal>double 3</literal>.
      </para>

      <para>
            In summary: to call a function, name the variable, then space, then the argument. Nothing else to say, it's as easy as that.
      </para>
</section>

<section>
      <title>More than one parameter</title>

      <para>
            How do we create a function that accepts more than one parameter? For people not used to functional programming, this may take a while to grasp. Let's do it step by step.
      </para>

      <screen><xi:include href="./05/multi-argument-function.txt" parse="text" /></screen>

      <para>
            We defined a function that takes the parameter <literal>a</literal>, the body returns another function. This other function takes a parameter <literal>b<literal> and returns <literal>a*b</literal>. Therefore, calling <literal>mul 3</literal> returns this kind of function: <literal>b: 3*b</literal>. In turn, we call the returned function with <literal>4</literal>, and get the expected result.
      </para>

      <para>
            You don't have to use parenthesis at all, Nix has sane priorities when parsing the code:
      </para>

      <screen><xi:include href="./05/no-parenthesis.txt" parse="text" /></screen>

      <para>
            Much more readable, you don't even notice that functions only receive one argument. Since the argument is separated by a space, to pass more complex expressions you need parenthesis. In other common languages you would write <literal>mul(6+7, 8+9)</literal>.
      </para>

      <para>
            Given that functions have only one parameter, it is straightforward to use <emphasis role="strong">partial application</emphasis>:
      </para>

      <screen><xi:include href="./05/partial-application.txt" parse="text" /></screen>

      <para>
            We stored the function returned by <literal>mul 3</literal> into a variable foo, then reused it.
      </para>
</section>

<section>
      <title>Arguments set</title>

      <para>
            Now this is a very cool feature of Nix. It is possible to pattern match over a set in the parameter. We write an alternative version of <literal>mul = a: b: a*b</literal> first by using a set as argument, then using pattern matching.
      </para>

      <screen><xi:include href="./05/set-argument.txt" parse="text" /></screen>

      <para>
            In the first case we defined a function that accepts a single parameter. We then access attributes <literal>a</literal> and <literal>b</literal> from the given set. Note how the parenthesis-less syntax for function calls is very elegant in this case, instead of doing <literal>mul({ a=3; b=4; })</literal> in other languages.
      </para>

      <para>
            In the second case we defined an arguments set. It's like defining a set, except without values. We require that the passed set contains the keys <literal>a</literal> and <literal>b</literal>. Then we can use those <literal>a</literal> and <literal>b</literal> in the function body directly.
      </para>

      <screen><xi:include href="./05/argument-set-error.txt" parse="text" /></screen>

      <para>
            Only a set with exactly the attributes required by the function is accepted, nothing more, nothing less.
      </para>
</section>

<section>
      <title>Default and variadic attributes</title>

      <para>
            It is possible to specify <emphasis role="strong">default values</emphasis> of attributes in the arguments set:
      </para>

      <screen><xi:include href="./05/default-values.txt" parse="text" /></screen>

      <para>
            Also you can allow passing more attributes (<emphasis role="strong">variadic</emphasis>) than the expected ones:
      </para>

      <screen><xi:include href="./05/veradic-arguments.txt" parse="text" /></screen>

      <para>
            However, in the function body you cannot access the "c" attribute. The solution is to give a name to the given set with the <emphasis role="strong">@-pattern</emphasis>:
      </para>

      <screen><xi:include href="./05/named-set-argument.txt" parse="text" /></screen>

      <para>
            That's it, you give a name to the whole parameter with name@ before the set pattern.
      </para>

      <para>
            Advantages of using argument sets:
      </para>

      <itemizedlist mark='bullet'>
            <listitem>
                  <para>
                        Named unordered arguments: you don't have to remember the order of the arguments.
                  </para>
            </listitem>
            <listitem>
                  <para>
                        You can pass sets, that adds a whole new layer of flexibility and convenience. 
                  </para>
            </listitem>
      </itemizedlist>

      <para>
            Disadvantages:
      </para>

      <itemizedlist mark='bullet'>
            <listitem>
                  <para>
                        Partial application does not work with argument sets. You have to specify the whole attribute set, not part of it.
                  </para>
            </listitem>
      </itemizedlist>

      <para>
            You may find similarities with <link xlink:href="https://docs.python.org/2/faq/programming.html#how-can-i-pass-optional-or-keyword-parameters-from-one-function-to-another">Python **kwargs</link>.
      </para>
</section>

<section>
      <title>Imports</title>

      <para>
            The <literal>import</literal> function is built-in and provides a way to parse a <filename>.nix</filename> file. The natural approach is to define each component in a <filename>.nix</filename> file, then compose by importing these files.
      </para>

      <para>
            Let's start with the bare metal.
      </para>

      <para>
            <filename>a.nix</filename>:
      </para>

      <screen><xi:include href="./05/a-nix.txt" parse="text" /></screen>

      <para>
            <filename>b.nix</filename>:
      </para>

      <screen><xi:include href="./05/b-nix.txt" parse="text" /></screen>

      <para>
            <filename>mul.nix</filename>:
      </para>

      <screen><xi:include href="./05/mul-nix.txt" parse="text" /></screen>

      <screen><xi:include href="./05/import.txt" parse="text" /></screen>

      <para>
            Yes it's really that straight. You import a file, and it gets parsed as expression. Note that the scope of the imported file does not inherit the scope of the importer.
      </para>

      <para>
            <filename>test.nix</filename>:
      </para>

      <screen><xi:include href="./05/test-nix.txt" parse="text" /></screen>

      <screen><xi:include href="./05/test-import.txt" parse="text" /></screen>

      <para>
            So how do we pass information to the module? Use functions, like we did with <filename>mul.nix</filename>. A more complex example:
      </para>

      <para>
            <filename>test.nix</filename>:
      </para>

      <screen><xi:include href="./05/test-nix-2.txt" parse="text" /></screen>


      <para>
            Explaining:
      </para>

      <itemizedlist mark='bullet'>
            <listitem>
                  <para>
                        In <filename>test.nix</filename> we return a function. It accepts a set, with default attributes <literal>b</literal>, <literal>trueMsg</literal> and <literal>falseMsg</literal>.
                  </para>
            </listitem>
            <listitem>
                  <para>
                        <literal>builtins.trace</literal> is a <link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">built-in function</link> that takes two arguments. The first is the message to display, the second is the value to return. It's usually used for debugging purposes.
                  </para>
            </listitem>
            <listitem>
                  <para>
                        Then we import <filename>test.nix</filename>, and call the function with that set.
                  </para>
            </listitem>
      </itemizedlist>

      <para>
            So when is the message shown? Only when it's in need to be evaluated.
      </para>
</section>

<section>
      <title>Next pill</title>

      <para>
            ...we will finally write our first derivation.
      </para>
</section>

</chapter>