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* Everything you always wanted to know about functions and derivations
but were afraid to ask.
This commit is contained in:
Eelco Dolstra
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ea6581b691
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@ -56,7 +56,7 @@ url='https://svn.cs.uu.nl:12443/repos/trace/release/trunk' />.</para>
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</section>
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<section><title>Setting up distributed builds</title>
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<section id='sec-distributed-builds'><title>Setting up distributed builds</title>
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<para>You can enable distributed builds by setting the environment
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variable <envar>NIX_BUILD_HOOK</envar> to point to a program that Nix
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@ -822,9 +822,58 @@ set.</para>
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<simplesect><title>Functions</title>
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<para>TODO</para>
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<para>Functions have the following form:
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<para>Higher-order functions; map</para>
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<programlisting>
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{<replaceable>params</replaceable>}: <replaceable>body</replaceable></programlisting>
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This defines a function that must be called with an attribute set
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containing the attributes listed in <replaceable>params</replaceable>,
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which is a comma-separated list of attribute names. Optionally, for
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each parameter a <emphasis>default value</emphasis> may be specified
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by writing <literal><replaceable>param</replaceable> ?
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<replaceable>e</replaceable></literal>, where
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<replaceable>e</replaceable> is an arbitrary expression. If a
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parameter has a default, the corresponding attribute may be omitted in
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function calls.</para>
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<para>Note that functions do not have names. If you want to give them
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a name, you can bind them to an attribute, e.g.,
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<programlisting>
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let {
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concat = {x, y}: x + y;
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body = concat {x = "foo"; y = "bar";};
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}</programlisting>
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</para>
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<para>It is also possible to define a function that takes a single
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argument and that does need to be called with an attribute set as
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argument. The syntax is
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<programlisting>
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<replaceable>var</replaceable>: <replaceable>body</replaceable></programlisting>
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where <replaceable>var</replaceable> is the name of the argument. It
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is not possible to define a default. Example:
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<programlisting>
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let {
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negate = x: !x;
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concat = x: y: x + y;
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body = if negate true then concat "foo" "bar" else "";
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}</programlisting>
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Note that <function>concat</function> is a function that takes one
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arguments and returns a function that takes another argument. This
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allows partial parameterisation (i.e., only filling some of the
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arguments of a function); e.g.,
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<programlisting>
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map (concat "foo") ["bar", "bla", "abc"]</programlisting>
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evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para>
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</simplesect>
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@ -1059,7 +1108,157 @@ weakest binding).</para>
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<simplesect><title>Derivations</title>
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<para>TODO</para>
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<para>The most important built-in function is
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<function>derivation</function>, which is used to describe a
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single derivation (a build action). It takes as input an attribute
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set, the attributes of which specify the inputs of the build.</para>
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<itemizedlist>
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<listitem><para>There must be an attribute named
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<varname>system</varname> whose value must be a string specifying a
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Nix platform identifier, such as <literal>"i686-linux"</literal> or
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<literal>"powerpc-darwin"</literal><footnote><para>To figure out
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your platform identifier, look at the line <quote>Checking for the
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canonical Nix system name</quote> in the output of Nix's
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<filename>configure</filename> script.</para></footnote> The build
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can only be performed on a machine and operating system matching the
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platform identifier. (Nix can automatically forward builds for
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other platforms by forwarding them to other machines; see <xref
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linkend='sec-distributed-builds' />.)</para></listitem>
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<listitem><para>There must be an attribute named
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<varname>name</varname> whose value must be a string. This is used
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as a symbolic name for the component by <command>nix-env</command>,
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and it is appended to the hash in the output path of the
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derivation.</para></listitem>
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<listitem><para>There must be an attribute named
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<varname>builder</varname> that identifies the program that is
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executed to perform the build. It can be either a derivation or a
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source (a local file reference, e.g.,
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<filename>./builder.sh</filename>).</para></listitem>
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<listitem><para>Every attribute is passed as an environment variable
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to the builder. Attribute values are translated to environment
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variables as follows:
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<itemizedlist>
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<listitem><para>Strings, URIs, and integers are just passed
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verbatim.</para></listitem>
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<listitem><para>A <emphasis>path</emphasis> (e.g.,
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<filename>../foo/sources.tar</filename>) causes the referenced
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file to be copied to the store; its location in the store is put
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in the environment variable. The idea is that all sources
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should reside in the Nix store, since all inputs to a derivation
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should reside in the Nix store.</para></listitem>
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<listitem><para>A <emphasis>derivation</emphasis> causes that
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derivation to be built prior to the present derivation; the
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output path is put in the environment
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variable.</para></listitem>
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<listitem><para>Lists of the previous types are also allowed.
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They are simply concatenated, separated by
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spaces.</para></listitem>
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</itemizedlist>
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</para></listitem>
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<listitem><para>The optional argument <varname>args</varname>
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specifies command-line arguments to be passed to the builder. It
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should be a list.</para></listitem>
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</itemizedlist>
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<para>(Note that <function>mkDerivation</function> in the standard
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environment is a wrapper around <function>derivation</function> that
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adds a default value for <varname>system</varname> and always uses
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Bash as the builder, to which the supplied builder is passed as a
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command-line argument. See <xref linkend='sec-standard-environment'
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/>.)</para>
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<para>The builder is executed as follows:
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<itemizedlist>
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<listitem><para>A temporary directory is created under the directory
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specified by <envar>TMPDIR</envar> (default
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<filename>/tmp</filename>) where the build will take place. The
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current directory is changed to this directory.</para></listitem>
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<listitem><para>The environment is cleared and set to the derivation
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attributes, as specified above.</para></listitem>
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<listitem><para>In addition, the following variables are set:
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<itemizedlist>
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<listitem><para><envar>NIX_BUILD_TOP</envar> contains the path of
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the temporary directory for this build.</para></listitem>
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<listitem><para>Also, <envar>TMPDIR</envar>,
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<envar>TEMPDIR</envar>, <envar>TMP</envar>, <envar>TEMP</envar>
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are set to point to the temporary directory. This is to prevent
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the builder from accidentally writing temporary files anywhere
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else. Doing so might cause interference by other
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processes.</para></listitem>
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<listitem><para><envar>PATH</envar> is set to
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<filename>/path-not-set</filename> to prevent shells from
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initialising it to their built-in default value.</para></listitem>
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<listitem><para><envar>HOME</envar> is set to
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<filename>/homeless-shelter</filename> to prevent programs from
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using <filename>/etc/passwd</filename> or the like to find the
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user's home directory, which could cause impurity. Usually, when
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<envar>HOME</envar> is set, it is used as the location of the home
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directory, even if it points to a non-existent
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path.</para></listitem>
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<listitem><para><envar>NIX_STORE</envar> is set to the path of the
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top-level Nix store directory (typically,
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<filename>/nix/store</filename>).</para></listitem>
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<listitem><para><envar>out</envar> is set to point to the output
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path of the derivation, which is a subdirectory of the Nix store.
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The output path is a concatenation of the cryptographic hash of
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all build inputs, and the <varname>name</varname>
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attribute.</para></listitem>
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</itemizedlist>
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</para></listitem>
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<listitem><para>If the output path already exists, it is removed.
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Also, locks are acquired to prevent multiple Nix instances from
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performing the same build at the same time.</para></listitem>
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<listitem><para>A log of the combined standard output and error is
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written to <filename>/nix/var/log/nix</filename>.</para></listitem>
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<listitem><para>The builder is executed with the arguments specified
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by the attribute <varname>args</varname>. If it exit with exit code
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0, it is considered to have succeeded.</para></listitem>
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<listitem><para>The temporary directory is removed (unless the
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<option>-K</option> option was specified).</para></listitem>
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<listitem><para>If the build was succesful, Nix scans the output for
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references to the paths of the inputs. These so-called
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<emphasis>retained dependencies</emphasis> could be used when the
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output of the derivation is used (e.g., when it's executed or used
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as input to another derivation), so if we deploy the derivation, we
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should copy the retained dependencies as well. The scan is
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performed by looking for the hash parts of file names of the
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inputs.</para></listitem>
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</itemizedlist>
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</para>
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</simplesect>
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