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Diffstat (limited to 'doc/manual/writing-nix-expressions.xml')
| -rw-r--r-- | doc/manual/writing-nix-expressions.xml | 205 |
1 files changed, 202 insertions, 3 deletions
diff --git a/doc/manual/writing-nix-expressions.xml b/doc/manual/writing-nix-expressions.xml index ac7b24c2483a..546c0efb3d3d 100644 --- a/doc/manual/writing-nix-expressions.xml +++ b/doc/manual/writing-nix-expressions.xml @@ -822,9 +822,58 @@ set.</para> <simplesect><title>Functions</title> -<para>TODO</para> +<para>Functions have the following form: + +<programlisting> +{<replaceable>params</replaceable>}: <replaceable>body</replaceable></programlisting> + +This defines a function that must be called with an attribute set +containing the attributes listed in <replaceable>params</replaceable>, +which is a comma-separated list of attribute names. Optionally, for +each parameter a <emphasis>default value</emphasis> may be specified +by writing <literal><replaceable>param</replaceable> ? +<replaceable>e</replaceable></literal>, where +<replaceable>e</replaceable> is an arbitrary expression. If a +parameter has a default, the corresponding attribute may be omitted in +function calls.</para> + +<para>Note that functions do not have names. If you want to give them +a name, you can bind them to an attribute, e.g., + +<programlisting> +let { + concat = {x, y}: x + y; + body = concat {x = "foo"; y = "bar";}; +}</programlisting> + +</para> + +<para>It is also possible to define a function that takes a single +argument and that does need to be called with an attribute set as +argument. The syntax is + +<programlisting> +<replaceable>var</replaceable>: <replaceable>body</replaceable></programlisting> + +where <replaceable>var</replaceable> is the name of the argument. It +is not possible to define a default. Example: + +<programlisting> +let { + negate = x: !x; + concat = x: y: x + y; + body = if negate true then concat "foo" "bar" else ""; +}</programlisting> + +Note that <function>concat</function> is a function that takes one +arguments and returns a function that takes another argument. This +allows partial parameterisation (i.e., only filling some of the +arguments of a function); e.g., + +<programlisting> + map (concat "foo") ["bar", "bla", "abc"]</programlisting> -<para>Higher-order functions; map</para> +evaluates to <literal>["foobar" "foobla" "fooabc"]</literal>.</para> </simplesect> @@ -1059,7 +1108,157 @@ weakest binding).</para> <simplesect><title>Derivations</title> -<para>TODO</para> +<para>The most important built-in function is +<function>derivation</function>, which is used to describe a +single derivation (a build action). It takes as input an attribute +set, the attributes of which specify the inputs of the build.</para> + +<itemizedlist> + + <listitem><para>There must be an attribute named + <varname>system</varname> whose value must be a string specifying a + Nix platform identifier, such as <literal>"i686-linux"</literal> or + <literal>"powerpc-darwin"</literal><footnote><para>To figure out + your platform identifier, look at the line <quote>Checking for the + canonical Nix system name</quote> in the output of Nix's + <filename>configure</filename> script.</para></footnote> The build + can only be performed on a machine and operating system matching the + platform identifier. (Nix can automatically forward builds for + other platforms by forwarding them to other machines; see <xref + linkend='sec-distributed-builds' />.)</para></listitem> + + <listitem><para>There must be an attribute named + <varname>name</varname> whose value must be a string. This is used + as a symbolic name for the component by <command>nix-env</command>, + and it is appended to the hash in the output path of the + derivation.</para></listitem> + + <listitem><para>There must be an attribute named + <varname>builder</varname> that identifies the program that is + executed to perform the build. It can be either a derivation or a + source (a local file reference, e.g., + <filename>./builder.sh</filename>).</para></listitem> + + <listitem><para>Every attribute is passed as an environment variable + to the builder. Attribute values are translated to environment + variables as follows: + + <itemizedlist> + + <listitem><para>Strings, URIs, and integers are just passed + verbatim.</para></listitem> + + <listitem><para>A <emphasis>path</emphasis> (e.g., + <filename>../foo/sources.tar</filename>) causes the referenced + file to be copied to the store; its location in the store is put + in the environment variable. The idea is that all sources + should reside in the Nix store, since all inputs to a derivation + should reside in the Nix store.</para></listitem> + + <listitem><para>A <emphasis>derivation</emphasis> causes that + derivation to be built prior to the present derivation; the + output path is put in the environment + variable.</para></listitem> + + <listitem><para>Lists of the previous types are also allowed. + They are simply concatenated, separated by + spaces.</para></listitem> + + </itemizedlist> + + </para></listitem> + + <listitem><para>The optional argument <varname>args</varname> + specifies command-line arguments to be passed to the builder. It + should be a list.</para></listitem> + +</itemizedlist> + +<para>(Note that <function>mkDerivation</function> in the standard +environment is a wrapper around <function>derivation</function> that +adds a default value for <varname>system</varname> and always uses +Bash as the builder, to which the supplied builder is passed as a +command-line argument. See <xref linkend='sec-standard-environment' +/>.)</para> + +<para>The builder is executed as follows: + +<itemizedlist> + + <listitem><para>A temporary directory is created under the directory + specified by <envar>TMPDIR</envar> (default + <filename>/tmp</filename>) where the build will take place. The + current directory is changed to this directory.</para></listitem> + + <listitem><para>The environment is cleared and set to the derivation + attributes, as specified above.</para></listitem> + + <listitem><para>In addition, the following variables are set: + + <itemizedlist> + + <listitem><para><envar>NIX_BUILD_TOP</envar> contains the path of + the temporary directory for this build.</para></listitem> + + <listitem><para>Also, <envar>TMPDIR</envar>, + <envar>TEMPDIR</envar>, <envar>TMP</envar>, <envar>TEMP</envar> + are set to point to the temporary directory. This is to prevent + the builder from accidentally writing temporary files anywhere + else. Doing so might cause interference by other + processes.</para></listitem> + + <listitem><para><envar>PATH</envar> is set to + <filename>/path-not-set</filename> to prevent shells from + initialising it to their built-in default value.</para></listitem> + + <listitem><para><envar>HOME</envar> is set to + <filename>/homeless-shelter</filename> to prevent programs from + using <filename>/etc/passwd</filename> or the like to find the + user's home directory, which could cause impurity. Usually, when + <envar>HOME</envar> is set, it is used as the location of the home + directory, even if it points to a non-existent + path.</para></listitem> + + <listitem><para><envar>NIX_STORE</envar> is set to the path of the + top-level Nix store directory (typically, + <filename>/nix/store</filename>).</para></listitem> + + <listitem><para><envar>out</envar> is set to point to the output + path of the derivation, which is a subdirectory of the Nix store. + The output path is a concatenation of the cryptographic hash of + all build inputs, and the <varname>name</varname> + attribute.</para></listitem> + + </itemizedlist> + + </para></listitem> + + <listitem><para>If the output path already exists, it is removed. + Also, locks are acquired to prevent multiple Nix instances from + performing the same build at the same time.</para></listitem> + + <listitem><para>A log of the combined standard output and error is + written to <filename>/nix/var/log/nix</filename>.</para></listitem> + + <listitem><para>The builder is executed with the arguments specified + by the attribute <varname>args</varname>. If it exit with exit code + 0, it is considered to have succeeded.</para></listitem> + + <listitem><para>The temporary directory is removed (unless the + <option>-K</option> option was specified).</para></listitem> + + <listitem><para>If the build was succesful, Nix scans the output for + references to the paths of the inputs. These so-called + <emphasis>retained dependencies</emphasis> could be used when the + output of the derivation is used (e.g., when it's executed or used + as input to another derivation), so if we deploy the derivation, we + should copy the retained dependencies as well. The scan is + performed by looking for the hash parts of file names of the + inputs.</para></listitem> + +</itemizedlist> + +</para> </simplesect> |