{ nixpkgs }: <co xml:id='ex-hello-co-nixpkgs' />

  pkgs = import nixpkgs {}; <co xml:id='ex-hello-co-import-nixpkgs' />

  pkgs = import &lt;nixpkgs&gt; {}; <co xml:id='ex-hello-co-import-nixpkgs' />

      { helloSrc }: <co xml:id='ex-hello-co-tarball-args' />

        src = helloSrc;

        src = &lt;hello&gt;; <co xml:id='ex-hello-co-tarball-args' />

      { tarball ? jobs.tarball {}        <co xml:id='ex-hello-co-build-args' />
      , system ? builtins.currentSystem
      }:

      { system ? builtins.currentSystem }:  <co xml:id='ex-hello-co-build-args' />

      let pkgs = import nixpkgs { inherit system; }; in

      let pkgs = import <nixpkgs> { inherit system; }; in

        src = tarball;

        src = jobs.tarball;

        <callout arearefs='ex-hello-co-nixpkgs'>

        <callout arearefs='ex-hello-co-import-nixpkgs'>

            This specifies a function of one named arguments,
            <varname>nixpkgs</varname>.  This function and those
            defined below is called by Hydra.  Here the
            <varname>nixpkgs</varname> argument is meant to be a
            checkout of the <link
            xlink:href="http://nixos.org/nixpkgs/">Nixpkgs</link>
            software distribution.

            This defines a variable <varname>pkgs</varname> holding
            the set of packages provided by <link
            xlink:href="http://nixos.org/nixpkgs/">Nixpkgs</link>.

            Hydra inspects the formal argument list of the function
            (here, the <varname>nixpkgs</varname> argument) and passes
            it the corresponding parameter specified as a build input
            on Hydra's web interface.  In this case, the web interface
            should show a <varname>nixpkgs</varname> build input,
            which is a checkout of the Nixpkgs source code repository.
          </para>
        </callout>
        <callout arearefs='ex-hello-co-import-nixpkgs'>
          <para>
            This defines a variable <varname>pkgs</varname> holding
            the set of packages provided by Nixpkgs.

            Since <varname>nixpkgs</varname> appears in angle brackets,
            there must be a build input of that name in the Nix search
            path.  In this case, the web interface should show a
            <varname>nixpkgs</varname> build input, which is a checkout
            of the Nixpkgs source code repository; Hydra then adds this
            and other build inputs to the Nix search path when
            evaluating <filename>release.nix</filename>.

            The <varname>tarball</varname> takes an additional
            argument called <varname>helloSrc</varname>.  Again, this
            argument is passed by Hydra and is meant to be a checkout
            of GNU Hello's source code repository.

            The <varname>tarball</varname> jobs expects a
            <varname>hello</varname> build input to be available in the
            Nix search path.  Again, this input is passed by Hydra and
            is meant to be a checkout of GNU Hello's source code
            repository.

            The <varname>build</varname> function takes two additional
            parameter: <varname>tarball</varname>, which is meant to
            be the result of the <varname>tarball</varname> job, and
            <varname>system</varname>, which should be a string

            The <varname>build</varname> function takes one
            parameter, <varname>system</varname>, which should be a string

            <literal>"x86_64-linux"</literal>.

            <literal>"x86_64-linux"</literal>.  Additionally, it refers
            to <varname>jobs.tarball</varname>, seen above.

            Again, these parameters are passed by Hydra when it calls
            <varname>build</varname>.  Thus, they must be defined as
            build inputs in Hydra: <varname>tarball</varname> should
            have type <literal>Build Output</literal>, its value being
            the latest output of the <varname>tarball</varname> job,
            and <varname>system</varname> should be a string.

            Hydra inspects the formal argument list of the function
            (here, the <varname>system</varname> argument) and passes it
            the corresponding parameter specified as a build input on
            Hydra's web interface.  Here, <varname>system</varname> is
            passed by Hydra when it calls <varname>build</varname>.
            Thus, it must be defined as a build input of type string in
            Hydra, which could take one of several values.

            The question mark after <literal>tarball</literal> and
            <literal>system</literal> defines default values for these
            arguments, and is only useful for debugging.

            The question mark after <literal>system</literal> defines
            the default value for this argument, and is only useful when
            debugging locally.

error: cannot auto-call a function that has an argument without a default value (`nixpkgs')

error: user-thrown exception: file `hello' was not found in the Nix search path (add it using $NIX_PATH or -I)

      This is because no value was specified for the
      <varname>nixpkgs</varname> argument of the Nix expression.
    </para>
    <para>
      This is fixed by providing a default value for that argument in
      the Nix expression, which will allow <command>nix-build</command>
      to auto-call the function: instfead of writing <literal>{ nixpkgs
      }:</literal>, we now write <literal>{ nixpkgs ? &lt;nixpkgs&gt;
      }:</literal>.  What it means is that, by default, the
      <varname>nixpkgs</varname> variable will be bound to the absolute
      path of any <filename>nixpkgs</filename> file found in the Nix
      search path.  Similarly, a default value for
      <varname>helloSrc</varname> needs to be provided.
    </para>

    <para>
      Thus, assuming a checkout of Nixpkgs is available under
      <filename>$HOME/src/nixpkgs</filename>, the
      <varname>tarball</varname> jobset can now be evaluated by running:

       The error is self-explanatory.  Assuming
       <filename>$HOME/src/hello</filename> points to a checkout of
       Hello, this can be fixed this way:

  { tarball ? jobs.tarball {}
  , system ? builtins.currentSystem
  }:

  { system ? builtins.currentSystem }:

    src = tarball;

    src = jobs.tarball;

  { tarball ? jobs.tarball {}
  , system ? builtins.currentSystem
  }:

  { system ? builtins.currentSystem }:

    build = jobs.build { inherit tarball system; };

    build = jobs.build { inherit system; };