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diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml deleted file mode 100644 index 4ad481ad3..000000000 --- a/doc/manual/expressions/builtins.xml +++ /dev/null @@ -1,1663 +0,0 @@ -<section 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='ssec-builtins'> - -<title>Built-in Functions</title> - -<para>This section lists the functions and constants built into the -Nix expression evaluator. (The built-in function -<function>derivation</function> is discussed above.) Some built-ins, -such as <function>derivation</function>, are always in scope of every -Nix expression; you can just access them right away. But to prevent -polluting the namespace too much, most built-ins are not in scope. -Instead, you can access them through the <varname>builtins</varname> -built-in value, which is a set that contains all built-in functions -and values. For instance, <function>derivation</function> is also -available as <function>builtins.derivation</function>.</para> - - -<variablelist> - - - <varlistentry xml:id='builtin-abort'> - <term><function>abort</function> <emphasis>s</emphasis></term> - <term><function>builtins.abort</function> <emphasis>s</emphasis></term> - - <listitem><para>Abort Nix expression evaluation, print error - message <emphasis>s</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-add'> - <term><function>builtins.add</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis> - </term> - - <listitem><para>Return the sum of the numbers - <emphasis>e1</emphasis> and - <emphasis>e2</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-all'> - <term><function>builtins.all</function> - <emphasis>pred</emphasis> <emphasis>list</emphasis></term> - - <listitem><para>Return <literal>true</literal> if the function - <emphasis>pred</emphasis> returns <literal>true</literal> - for all elements of <emphasis>list</emphasis>, - and <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-any'> - <term><function>builtins.any</function> - <emphasis>pred</emphasis> <emphasis>list</emphasis></term> - - <listitem><para>Return <literal>true</literal> if the function - <emphasis>pred</emphasis> returns <literal>true</literal> - for at least one element of <emphasis>list</emphasis>, - and <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-attrNames'> - <term><function>builtins.attrNames</function> - <emphasis>set</emphasis></term> - - <listitem><para>Return the names of the attributes in the set - <emphasis>set</emphasis> in an alphabetically sorted list. For instance, - <literal>builtins.attrNames { y = 1; x = "foo"; }</literal> - evaluates to <literal>[ "x" "y" ]</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-attrValues'> - <term><function>builtins.attrValues</function> - <emphasis>set</emphasis></term> - - <listitem><para>Return the values of the attributes in the set - <emphasis>set</emphasis> in the order corresponding to the - sorted attribute names.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-baseNameOf'> - <term><function>baseNameOf</function> <emphasis>s</emphasis></term> - - <listitem><para>Return the <emphasis>base name</emphasis> of the - string <emphasis>s</emphasis>, that is, everything following - the final slash in the string. This is similar to the GNU - <command>basename</command> command.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-bitAnd'> - <term><function>builtins.bitAnd</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return the bitwise AND of the integers - <emphasis>e1</emphasis> and - <emphasis>e2</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-bitOr'> - <term><function>builtins.bitOr</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return the bitwise OR of the integers - <emphasis>e1</emphasis> and - <emphasis>e2</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-bitXor'> - <term><function>builtins.bitXor</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return the bitwise XOR of the integers - <emphasis>e1</emphasis> and - <emphasis>e2</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-builtins'> - <term><varname>builtins</varname></term> - - <listitem><para>The set <varname>builtins</varname> contains all - the built-in functions and values. You can use - <varname>builtins</varname> to test for the availability of - features in the Nix installation, e.g., - -<programlisting> -if builtins ? getEnv then builtins.getEnv "PATH" else ""</programlisting> - - This allows a Nix expression to fall back gracefully on older Nix - installations that don’t have the desired built-in - function.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-compareVersions'> - <term><function>builtins.compareVersions</function> - <emphasis>s1</emphasis> <emphasis>s2</emphasis></term> - - <listitem><para>Compare two strings representing versions and - return <literal>-1</literal> if version - <emphasis>s1</emphasis> is older than version - <emphasis>s2</emphasis>, <literal>0</literal> if they are - the same, and <literal>1</literal> if - <emphasis>s1</emphasis> is newer than - <emphasis>s2</emphasis>. The version comparison algorithm - is the same as the one used by <link - linkend="ssec-version-comparisons"><command>nix-env - -u</command></link>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-concatLists'> - <term><function>builtins.concatLists</function> - <emphasis>lists</emphasis></term> - - <listitem><para>Concatenate a list of lists into a single - list.</para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-concatStringsSep'> - <term><function>builtins.concatStringsSep</function> - <emphasis>separator</emphasis> <emphasis>list</emphasis></term> - - <listitem><para>Concatenate a list of strings with a separator - between each element, e.g. <literal>concatStringsSep "/" - ["usr" "local" "bin"] == "usr/local/bin"</literal></para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-currentSystem'> - <term><varname>builtins.currentSystem</varname></term> - - <listitem><para>The built-in value <varname>currentSystem</varname> - evaluates to the Nix platform identifier for the Nix installation - on which the expression is being evaluated, such as - <literal>"i686-linux"</literal> or - <literal>"x86_64-darwin"</literal>.</para></listitem> - - </varlistentry> - - - <!-- - <varlistentry><term><function>currentTime</function></term> - - <listitem><para>The built-in value <varname>currentTime</varname> - returns the current system time in seconds since 00:00:00 1/1/1970 - UTC. Due to the evaluation model of Nix expressions - (<emphasis>maximal laziness</emphasis>), it always yields the same - value within an execution of Nix.</para></listitem> - - </varlistentry> - --> - - - <!-- - <varlistentry><term><function>dependencyClosure</function></term> - - <listitem><para>TODO</para></listitem> - - </varlistentry> - --> - - - <varlistentry xml:id='builtin-deepSeq'> - <term><function>builtins.deepSeq</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>This is like <literal>seq - <emphasis>e1</emphasis> - <emphasis>e2</emphasis></literal>, except that - <emphasis>e1</emphasis> is evaluated - <emphasis>deeply</emphasis>: if it’s a list or set, its elements - or attributes are also evaluated recursively.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-derivation'> - <term><function>derivation</function> - <emphasis>attrs</emphasis></term> - <term><function>builtins.derivation</function> - <emphasis>attrs</emphasis></term> - - <listitem><para><function>derivation</function> is described in - <xref linkend='ssec-derivation' />.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-dirOf'> - <term><function>dirOf</function> <emphasis>s</emphasis></term> - <term><function>builtins.dirOf</function> <emphasis>s</emphasis></term> - - <listitem><para>Return the directory part of the string - <emphasis>s</emphasis>, that is, everything before the final - slash in the string. This is similar to the GNU - <command>dirname</command> command.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-div'> - <term><function>builtins.div</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return the quotient of the numbers - <emphasis>e1</emphasis> and - <emphasis>e2</emphasis>.</para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-elem'> - <term><function>builtins.elem</function> - <emphasis>x</emphasis> <emphasis>xs</emphasis></term> - - <listitem><para>Return <literal>true</literal> if a value equal to - <emphasis>x</emphasis> occurs in the list - <emphasis>xs</emphasis>, and <literal>false</literal> - otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-elemAt'> - <term><function>builtins.elemAt</function> - <emphasis>xs</emphasis> <emphasis>n</emphasis></term> - - <listitem><para>Return element <emphasis>n</emphasis> from - the list <emphasis>xs</emphasis>. Elements are counted - starting from 0. A fatal error occurs if the index is out of - bounds.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-fetchurl'> - <term><function>builtins.fetchurl</function> - <emphasis>url</emphasis></term> - - <listitem><para>Download the specified URL and return the path of - the downloaded file. This function is not available if <link - linkend="conf-restrict-eval">restricted evaluation mode</link> is - enabled.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-fetchTarball'> - <term><function>fetchTarball</function> - <emphasis>url</emphasis></term> - <term><function>builtins.fetchTarball</function> - <emphasis>url</emphasis></term> - - <listitem><para>Download the specified URL, unpack it and return - the path of the unpacked tree. The file must be a tape archive - (<filename>.tar</filename>) compressed with - <literal>gzip</literal>, <literal>bzip2</literal> or - <literal>xz</literal>. The top-level path component of the files - in the tarball is removed, so it is best if the tarball contains a - single directory at top level. The typical use of the function is - to obtain external Nix expression dependencies, such as a - particular version of Nixpkgs, e.g. - -<programlisting> -with import (fetchTarball https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz) {}; - -stdenv.mkDerivation { … } -</programlisting> - </para> - - <para>The fetched tarball is cached for a certain amount of time - (1 hour by default) in <filename>~/.cache/nix/tarballs/</filename>. - You can change the cache timeout either on the command line with - <option>--option tarball-ttl <emphasis>number of seconds</emphasis></option> or - in the Nix configuration file with this option: - <literal><xref linkend="conf-tarball-ttl" /> <emphasis>number of seconds to cache</emphasis></literal>. - </para> - - <para>Note that when obtaining the hash with <varname>nix-prefetch-url - </varname> the option <varname>--unpack</varname> is required. - </para> - - <para>This function can also verify the contents against a hash. - In that case, the function takes a set instead of a URL. The set - requires the attribute <varname>url</varname> and the attribute - <varname>sha256</varname>, e.g. - -<programlisting> -with import (fetchTarball { - url = "https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz"; - sha256 = "1jppksrfvbk5ypiqdz4cddxdl8z6zyzdb2srq8fcffr327ld5jj2"; -}) {}; - -stdenv.mkDerivation { … } -</programlisting> - - </para> - - <para>This function is not available if <link - linkend="conf-restrict-eval">restricted evaluation mode</link> is - enabled.</para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-fetchGit'> - <term> - <function>builtins.fetchGit</function> - <emphasis>args</emphasis> - </term> - - <listitem> - <para> - Fetch a path from git. <emphasis>args</emphasis> can be - a URL, in which case the HEAD of the repo at that URL is - fetched. Otherwise, it can be an attribute with the following - attributes (all except <varname>url</varname> optional): - </para> - - <variablelist> - <varlistentry> - <term>url</term> - <listitem> - <para> - The URL of the repo. - </para> - </listitem> - </varlistentry> - <varlistentry> - <term>name</term> - <listitem> - <para> - The name of the directory the repo should be exported to - in the store. Defaults to the basename of the URL. - </para> - </listitem> - </varlistentry> - <varlistentry> - <term>rev</term> - <listitem> - <para> - The git revision to fetch. Defaults to the tip of - <varname>ref</varname>. - </para> - </listitem> - </varlistentry> - <varlistentry> - <term>ref</term> - <listitem> - <para> - The git ref to look for the requested revision under. - This is often a branch or tag name. Defaults to - <literal>HEAD</literal>. - </para> - - <para> - By default, the <varname>ref</varname> value is prefixed - with <literal>refs/heads/</literal>. As of Nix 2.3.0 - Nix will not prefix <literal>refs/heads/</literal> if - <varname>ref</varname> starts with <literal>refs/</literal>. - </para> - </listitem> - </varlistentry> - <varlistentry> - <term>submodules</term> - <listitem> - <para> - A Boolean parameter that specifies whether submodules - should be checked out. Defaults to - <literal>false</literal>. - </para> - </listitem> - </varlistentry> - </variablelist> - - <para>Here are some examples of how to use - <literal>fetchGit</literal>.</para> - - <itemizedlist> - - <listitem> - <para>To fetch a private repository over SSH:</para> - - <programlisting>builtins.fetchGit { - url = "git@github.com:my-secret/repository.git"; - ref = "master"; - rev = "adab8b916a45068c044658c4158d81878f9ed1c3"; -}</programlisting> - - </listitem> - - <listitem> - <para>To fetch an arbitrary reference:</para> - - <programlisting>builtins.fetchGit { - url = "https://github.com/NixOS/nix.git"; - ref = "refs/heads/0.5-release"; -}</programlisting> - </listitem> - - <listitem> - <para> - If the revision you're looking for is in the default branch - of the git repository you don't strictly need to specify - the branch name in the <varname>ref</varname> attribute. - </para> - <para> - However, if the revision you're looking for is in a future - branch for the non-default branch you will need to specify - the the <varname>ref</varname> attribute as well. - </para> - - <programlisting>builtins.fetchGit { - url = "https://github.com/nixos/nix.git"; - rev = "841fcbd04755c7a2865c51c1e2d3b045976b7452"; - ref = "1.11-maintenance"; -}</programlisting> - - <note> - <para> - It is nice to always specify the branch which a revision - belongs to. Without the branch being specified, the - fetcher might fail if the default branch changes. - Additionally, it can be confusing to try a commit from a - non-default branch and see the fetch fail. If the branch - is specified the fault is much more obvious. - </para> - </note> - </listitem> - - <listitem> - <para> - If the revision you're looking for is in the default branch - of the git repository you may omit the - <varname>ref</varname> attribute. - </para> - <programlisting>builtins.fetchGit { - url = "https://github.com/nixos/nix.git"; - rev = "841fcbd04755c7a2865c51c1e2d3b045976b7452"; -}</programlisting> - </listitem> - - <listitem> - <para>To fetch a specific tag:</para> - <programlisting>builtins.fetchGit { - url = "https://github.com/nixos/nix.git"; - ref = "refs/tags/1.9"; -}</programlisting> - </listitem> - - <listitem> - <para>To fetch the latest version of a remote branch:</para> - <programlisting>builtins.fetchGit { - url = "ssh://git@github.com/nixos/nix.git"; - ref = "master"; -}</programlisting> - <note><para>Nix will refetch the branch in accordance to - <xref linkend="conf-tarball-ttl" />.</para></note> - <note><para>This behavior is disabled in <emphasis>Pure - evaluation mode</emphasis>.</para></note> - </listitem> - </itemizedlist> - </listitem> - </varlistentry> - - - <varlistentry><term><function>builtins.filter</function> - <emphasis>f</emphasis> <emphasis>xs</emphasis></term> - - <listitem><para>Return a list consisting of the elements of - <emphasis>xs</emphasis> for which the function - <emphasis>f</emphasis> returns - <literal>true</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-filterSource'> - <term><function>builtins.filterSource</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem> - - <para>This function allows you to copy sources into the Nix - store while filtering certain files. For instance, suppose that - you want to use the directory <filename>source-dir</filename> as - an input to a Nix expression, e.g. - -<programlisting> -stdenv.mkDerivation { - ... - src = ./source-dir; -} -</programlisting> - - However, if <filename>source-dir</filename> is a Subversion - working copy, then all those annoying <filename>.svn</filename> - subdirectories will also be copied to the store. Worse, the - contents of those directories may change a lot, causing lots of - spurious rebuilds. With <function>filterSource</function> you - can filter out the <filename>.svn</filename> directories: - -<programlisting> - src = builtins.filterSource - (path: type: type != "directory" || baseNameOf path != ".svn") - ./source-dir; -</programlisting> - - </para> - - <para>Thus, the first argument <emphasis>e1</emphasis> - must be a predicate function that is called for each regular - file, directory or symlink in the source tree - <emphasis>e2</emphasis>. If the function returns - <literal>true</literal>, the file is copied to the Nix store, - otherwise it is omitted. The function is called with two - arguments. The first is the full path of the file. The second - is a string that identifies the type of the file, which is - either <literal>"regular"</literal>, - <literal>"directory"</literal>, <literal>"symlink"</literal> or - <literal>"unknown"</literal> (for other kinds of files such as - device nodes or fifos — but note that those cannot be copied to - the Nix store, so if the predicate returns - <literal>true</literal> for them, the copy will fail). If you - exclude a directory, the entire corresponding subtree of - <emphasis>e2</emphasis> will be excluded.</para> - - </listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-foldl-prime'> - <term><function>builtins.foldl’</function> - <emphasis>op</emphasis> <emphasis>nul</emphasis> <emphasis>list</emphasis></term> - - <listitem><para>Reduce a list by applying a binary operator, from - left to right, e.g. <literal>foldl’ op nul [x0 x1 x2 ...] = op (op - (op nul x0) x1) x2) ...</literal>. The operator is applied - strictly, i.e., its arguments are evaluated first. For example, - <literal>foldl’ (x: y: x + y) 0 [1 2 3]</literal> evaluates to - 6.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-functionArgs'> - <term><function>builtins.functionArgs</function> - <emphasis>f</emphasis></term> - - <listitem><para> - Return a set containing the names of the formal arguments expected - by the function <emphasis>f</emphasis>. - The value of each attribute is a Boolean denoting whether the corresponding - argument has a default value. For instance, - <literal>functionArgs ({ x, y ? 123}: ...) = { x = false; y = true; }</literal>. - </para> - - <para>"Formal argument" here refers to the attributes pattern-matched by - the function. Plain lambdas are not included, e.g. - <literal>functionArgs (x: ...) = { }</literal>. - </para></listitem> - </varlistentry> - - - <varlistentry xml:id='builtin-fromJSON'> - <term><function>builtins.fromJSON</function> <emphasis>e</emphasis></term> - - <listitem><para>Convert a JSON string to a Nix - value. For example, - -<programlisting> -builtins.fromJSON ''{"x": [1, 2, 3], "y": null}'' -</programlisting> - - returns the value <literal>{ x = [ 1 2 3 ]; y = null; - }</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-genList'> - <term><function>builtins.genList</function> - <emphasis>generator</emphasis> <emphasis>length</emphasis></term> - - <listitem><para>Generate list of size - <emphasis>length</emphasis>, with each element - <emphasis>i</emphasis> equal to the value returned by - <emphasis>generator</emphasis> <literal>i</literal>. For - example, - -<programlisting> -builtins.genList (x: x * x) 5 -</programlisting> - - returns the list <literal>[ 0 1 4 9 16 ]</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-getAttr'> - <term><function>builtins.getAttr</function> - <emphasis>s</emphasis> <emphasis>set</emphasis></term> - - <listitem><para><function>getAttr</function> returns the attribute - named <emphasis>s</emphasis> from - <emphasis>set</emphasis>. Evaluation aborts if the - attribute doesn’t exist. This is a dynamic version of the - <literal>.</literal> operator, since <emphasis>s</emphasis> - is an expression rather than an identifier.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-getEnv'> - <term><function>builtins.getEnv</function> - <emphasis>s</emphasis></term> - - <listitem><para><function>getEnv</function> returns the value of - the environment variable <emphasis>s</emphasis>, or an empty - string if the variable doesn’t exist. This function should be - used with care, as it can introduce all sorts of nasty environment - dependencies in your Nix expression.</para> - - <para><function>getEnv</function> is used in Nix Packages to - locate the file <filename>~/.nixpkgs/config.nix</filename>, which - contains user-local settings for Nix Packages. (That is, it does - a <literal>getEnv "HOME"</literal> to locate the user’s home - directory.)</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-hasAttr'> - <term><function>builtins.hasAttr</function> - <emphasis>s</emphasis> <emphasis>set</emphasis></term> - - <listitem><para><function>hasAttr</function> returns - <literal>true</literal> if <emphasis>set</emphasis> has an - attribute named <emphasis>s</emphasis>, and - <literal>false</literal> otherwise. This is a dynamic version of - the <literal>?</literal> operator, since - <emphasis>s</emphasis> is an expression rather than an - identifier.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-hashString'> - <term><function>builtins.hashString</function> - <emphasis>type</emphasis> <emphasis>s</emphasis></term> - - <listitem><para>Return a base-16 representation of the - cryptographic hash of string <emphasis>s</emphasis>. The - hash algorithm specified by <emphasis>type</emphasis> must - be one of <literal>"md5"</literal>, <literal>"sha1"</literal>, - <literal>"sha256"</literal> or <literal>"sha512"</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-hashFile'> - <term><function>builtins.hashFile</function> - <emphasis>type</emphasis> <emphasis>p</emphasis></term> - - <listitem><para>Return a base-16 representation of the - cryptographic hash of the file at path <emphasis>p</emphasis>. The - hash algorithm specified by <emphasis>type</emphasis> must - be one of <literal>"md5"</literal>, <literal>"sha1"</literal>, - <literal>"sha256"</literal> or <literal>"sha512"</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-head'> - <term><function>builtins.head</function> - <emphasis>list</emphasis></term> - - <listitem><para>Return the first element of a list; abort - evaluation if the argument isn’t a list or is an empty list. You - can test whether a list is empty by comparing it with - <literal>[]</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-import'> - <term><function>import</function> - <emphasis>path</emphasis></term> - <term><function>builtins.import</function> - <emphasis>path</emphasis></term> - - <listitem><para>Load, parse and return the Nix expression in the - file <emphasis>path</emphasis>. If <emphasis>path - </emphasis> is a directory, the file <filename>default.nix - </filename> in that directory is loaded. Evaluation aborts if the - file doesn’t exist or contains an incorrect Nix expression. - <function>import</function> implements Nix’s module system: you - can put any Nix expression (such as a set or a function) in a - separate file, and use it from Nix expressions in other - files.</para> - - <note><para>Unlike some languages, <function>import</function> is a regular - function in Nix. Paths using the angle bracket syntax (e.g., <function> - import</function> <emphasis><foo></emphasis>) are normal path - values (see <xref linkend='ssec-values' />).</para></note> - - <para>A Nix expression loaded by <function>import</function> must - not contain any <emphasis>free variables</emphasis> (identifiers - that are not defined in the Nix expression itself and are not - built-in). Therefore, it cannot refer to variables that are in - scope at the call site. For instance, if you have a calling - expression - -<programlisting> -rec { - x = 123; - y = import ./foo.nix; -}</programlisting> - - then the following <filename>foo.nix</filename> will give an - error: - -<programlisting> -x + 456</programlisting> - - since <varname>x</varname> is not in scope in - <filename>foo.nix</filename>. If you want <varname>x</varname> - to be available in <filename>foo.nix</filename>, you should pass - it as a function argument: - -<programlisting> -rec { - x = 123; - y = import ./foo.nix x; -}</programlisting> - - and - -<programlisting> -x: x + 456</programlisting> - - (The function argument doesn’t have to be called - <varname>x</varname> in <filename>foo.nix</filename>; any name - would work.)</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-intersectAttrs'> - <term><function>builtins.intersectAttrs</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return a set consisting of the attributes in the - set <emphasis>e2</emphasis> that also exist in the set - <emphasis>e1</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-isAttrs'> - <term><function>builtins.isAttrs</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to a set, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-isList'> - <term><function>builtins.isList</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to a list, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-isFunction'><term><function>builtins.isFunction</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to a function, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-isString'> - <term><function>builtins.isString</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to a string, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-isInt'> - <term><function>builtins.isInt</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to an int, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-isFloat'> - <term><function>builtins.isFloat</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to a float, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-isBool'> - <term><function>builtins.isBool</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to a bool, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - <varlistentry><term><function>builtins.isPath</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to a path, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-isNull'> - <term><function>isNull</function> - <emphasis>e</emphasis></term> - <term><function>builtins.isNull</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return <literal>true</literal> if - <emphasis>e</emphasis> evaluates to <literal>null</literal>, - and <literal>false</literal> otherwise.</para> - - <warning><para>This function is <emphasis>deprecated</emphasis>; - just write <literal>e == null</literal> instead.</para></warning> - - </listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-length'> - <term><function>builtins.length</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return the length of the list - <emphasis>e</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-lessThan'> - <term><function>builtins.lessThan</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return <literal>true</literal> if the number - <emphasis>e1</emphasis> is less than the number - <emphasis>e2</emphasis>, and <literal>false</literal> - otherwise. Evaluation aborts if either - <emphasis>e1</emphasis> or <emphasis>e2</emphasis> - does not evaluate to a number.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-listToAttrs'> - <term><function>builtins.listToAttrs</function> - <emphasis>e</emphasis></term> - - <listitem><para>Construct a set from a list specifying the names - and values of each attribute. Each element of the list should be - a set consisting of a string-valued attribute - <varname>name</varname> specifying the name of the attribute, and - an attribute <varname>value</varname> specifying its value. - Example: - -<programlisting> -builtins.listToAttrs - [ { name = "foo"; value = 123; } - { name = "bar"; value = 456; } - ] -</programlisting> - - evaluates to - -<programlisting> -{ foo = 123; bar = 456; } -</programlisting> - - </para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-map'> - <term><function>map</function> - <emphasis>f</emphasis> <emphasis>list</emphasis></term> - <term><function>builtins.map</function> - <emphasis>f</emphasis> <emphasis>list</emphasis></term> - - <listitem><para>Apply the function <emphasis>f</emphasis> to - each element in the list <emphasis>list</emphasis>. For - example, - -<programlisting> -map (x: "foo" + x) [ "bar" "bla" "abc" ]</programlisting> - - evaluates to <literal>[ "foobar" "foobla" "fooabc" - ]</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-match'> - <term><function>builtins.match</function> - <emphasis>regex</emphasis> <emphasis>str</emphasis></term> - - <listitem><para>Returns a list if the <link - xlink:href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04">extended - POSIX regular expression</link> <emphasis>regex</emphasis> - matches <emphasis>str</emphasis> precisely, otherwise returns - <literal>null</literal>. Each item in the list is a regex group. - -<programlisting> -builtins.match "ab" "abc" -</programlisting> - -Evaluates to <literal>null</literal>. - -<programlisting> -builtins.match "abc" "abc" -</programlisting> - -Evaluates to <literal>[ ]</literal>. - -<programlisting> -builtins.match "a(b)(c)" "abc" -</programlisting> - -Evaluates to <literal>[ "b" "c" ]</literal>. - -<programlisting> -builtins.match "[[:space:]]+([[:upper:]]+)[[:space:]]+" " FOO " -</programlisting> - -Evaluates to <literal>[ "foo" ]</literal>. - - </para></listitem> - </varlistentry> - - <varlistentry xml:id='builtin-mul'> - <term><function>builtins.mul</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return the product of the numbers - <emphasis>e1</emphasis> and - <emphasis>e2</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-parseDrvName'> - <term><function>builtins.parseDrvName</function> - <emphasis>s</emphasis></term> - - <listitem><para>Split the string <emphasis>s</emphasis> into - a package name and version. The package name is everything up to - but not including the first dash followed by a digit, and the - version is everything following that dash. The result is returned - in a set <literal>{ name, version }</literal>. Thus, - <literal>builtins.parseDrvName "nix-0.12pre12876"</literal> - returns <literal>{ name = "nix"; version = "0.12pre12876"; - }</literal>.</para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-path'> - <term> - <function>builtins.path</function> - <emphasis>args</emphasis> - </term> - - <listitem> - <para> - An enrichment of the built-in path type, based on the attributes - present in <emphasis>args</emphasis>. All are optional - except <varname>path</varname>: - </para> - - <variablelist> - <varlistentry> - <term>path</term> - <listitem> - <para>The underlying path.</para> - </listitem> - </varlistentry> - <varlistentry> - <term>name</term> - <listitem> - <para> - The name of the path when added to the store. This can - used to reference paths that have nix-illegal characters - in their names, like <literal>@</literal>. - </para> - </listitem> - </varlistentry> - <varlistentry> - <term>filter</term> - <listitem> - <para> - A function of the type expected by - <link linkend="builtin-filterSource">builtins.filterSource</link>, - with the same semantics. - </para> - </listitem> - </varlistentry> - <varlistentry> - <term>recursive</term> - <listitem> - <para> - When <literal>false</literal>, when - <varname>path</varname> is added to the store it is with a - flat hash, rather than a hash of the NAR serialization of - the file. Thus, <varname>path</varname> must refer to a - regular file, not a directory. This allows similar - behavior to <literal>fetchurl</literal>. Defaults to - <literal>true</literal>. - </para> - </listitem> - </varlistentry> - <varlistentry> - <term>sha256</term> - <listitem> - <para> - When provided, this is the expected hash of the file at - the path. Evaluation will fail if the hash is incorrect, - and providing a hash allows - <literal>builtins.path</literal> to be used even when the - <literal>pure-eval</literal> nix config option is on. - </para> - </listitem> - </varlistentry> - </variablelist> - </listitem> - </varlistentry> - - <varlistentry xml:id='builtin-pathExists'> - <term><function>builtins.pathExists</function> - <emphasis>path</emphasis></term> - - <listitem><para>Return <literal>true</literal> if the path - <emphasis>path</emphasis> exists at evaluation time, and - <literal>false</literal> otherwise.</para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-placeholder'> - <term><function>builtins.placeholder</function> - <emphasis>output</emphasis></term> - - <listitem><para>Return a placeholder string for the specified - <emphasis>output</emphasis> that will be substituted by the - corresponding output path at build time. Typical outputs would be - <literal>"out"</literal>, <literal>"bin"</literal> or - <literal>"dev"</literal>.</para></listitem> - </varlistentry> - - <varlistentry xml:id='builtin-readDir'> - <term><function>builtins.readDir</function> - <emphasis>path</emphasis></term> - - <listitem><para>Return the contents of the directory - <emphasis>path</emphasis> as a set mapping directory entries - to the corresponding file type. For instance, if directory - <filename>A</filename> contains a regular file - <filename>B</filename> and another directory - <filename>C</filename>, then <literal>builtins.readDir - ./A</literal> will return the set - -<programlisting> -{ B = "regular"; C = "directory"; }</programlisting> - - The possible values for the file type are - <literal>"regular"</literal>, <literal>"directory"</literal>, - <literal>"symlink"</literal> and - <literal>"unknown"</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-readFile'> - <term><function>builtins.readFile</function> - <emphasis>path</emphasis></term> - - <listitem><para>Return the contents of the file - <emphasis>path</emphasis> as a string.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-removeAttrs'> - <term><function>removeAttrs</function> - <emphasis>set</emphasis> <emphasis>list</emphasis></term> - <term><function>builtins.removeAttrs</function> - <emphasis>set</emphasis> <emphasis>list</emphasis></term> - - <listitem><para>Remove the attributes listed in - <emphasis>list</emphasis> from - <emphasis>set</emphasis>. The attributes don’t have to - exist in <emphasis>set</emphasis>. For instance, - -<programlisting> -removeAttrs { x = 1; y = 2; z = 3; } [ "a" "x" "z" ]</programlisting> - - evaluates to <literal>{ y = 2; }</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-replaceStrings'> - <term><function>builtins.replaceStrings</function> - <emphasis>from</emphasis> <emphasis>to</emphasis> <emphasis>s</emphasis></term> - - <listitem><para>Given string <emphasis>s</emphasis>, replace - every occurrence of the strings in <emphasis>from</emphasis> - with the corresponding string in - <emphasis>to</emphasis>. For example, - -<programlisting> -builtins.replaceStrings ["oo" "a"] ["a" "i"] "foobar" -</programlisting> - - evaluates to <literal>"fabir"</literal>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-seq'> - <term><function>builtins.seq</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Evaluate <emphasis>e1</emphasis>, then - evaluate and return <emphasis>e2</emphasis>. This ensures - that a computation is strict in the value of - <emphasis>e1</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-sort'> - <term><function>builtins.sort</function> - <emphasis>comparator</emphasis> <emphasis>list</emphasis></term> - - <listitem><para>Return <emphasis>list</emphasis> in sorted - order. It repeatedly calls the function - <emphasis>comparator</emphasis> with two elements. The - comparator should return <literal>true</literal> if the first - element is less than the second, and <literal>false</literal> - otherwise. For example, - -<programlisting> -builtins.sort builtins.lessThan [ 483 249 526 147 42 77 ] -</programlisting> - - produces the list <literal>[ 42 77 147 249 483 526 - ]</literal>.</para> - - <para>This is a stable sort: it preserves the relative order of - elements deemed equal by the comparator.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-split'> - <term><function>builtins.split</function> - <emphasis>regex</emphasis> <emphasis>str</emphasis></term> - - <listitem><para>Returns a list composed of non matched strings interleaved - with the lists of the <link - xlink:href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04">extended - POSIX regular expression</link> <emphasis>regex</emphasis> matches - of <emphasis>str</emphasis>. Each item in the lists of matched - sequences is a regex group. - -<programlisting> -builtins.split "(a)b" "abc" -</programlisting> - -Evaluates to <literal>[ "" [ "a" ] "c" ]</literal>. - -<programlisting> -builtins.split "([ac])" "abc" -</programlisting> - -Evaluates to <literal>[ "" [ "a" ] "b" [ "c" ] "" ]</literal>. - -<programlisting> -builtins.split "(a)|(c)" "abc" -</programlisting> - -Evaluates to <literal>[ "" [ "a" null ] "b" [ null "c" ] "" ]</literal>. - -<programlisting> -builtins.split "([[:upper:]]+)" " FOO " -</programlisting> - -Evaluates to <literal>[ " " [ "FOO" ] " " ]</literal>. - - </para></listitem> - </varlistentry> - - - <varlistentry xml:id='builtin-splitVersion'> - <term><function>builtins.splitVersion</function> - <emphasis>s</emphasis></term> - - <listitem><para>Split a string representing a version into its - components, by the same version splitting logic underlying the - version comparison in <link linkend="ssec-version-comparisons"> - <command>nix-env -u</command></link>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-stringLength'> - <term><function>builtins.stringLength</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return the length of the string - <emphasis>e</emphasis>. If <emphasis>e</emphasis> is - not a string, evaluation is aborted.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-sub'> - <term><function>builtins.sub</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Return the difference between the numbers - <emphasis>e1</emphasis> and - <emphasis>e2</emphasis>.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-substring'> - <term><function>builtins.substring</function> - <emphasis>start</emphasis> <emphasis>len</emphasis> - <emphasis>s</emphasis></term> - - <listitem><para>Return the substring of - <emphasis>s</emphasis> from character position - <emphasis>start</emphasis> (zero-based) up to but not - including <emphasis>start + len</emphasis>. If - <emphasis>start</emphasis> is greater than the length of the - string, an empty string is returned, and if <emphasis>start + - len</emphasis> lies beyond the end of the string, only the - substring up to the end of the string is returned. - <emphasis>start</emphasis> must be - non-negative. For example, - -<programlisting> -builtins.substring 0 3 "nixos" -</programlisting> - - evaluates to <literal>"nix"</literal>. - </para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-tail'> - <term><function>builtins.tail</function> - <emphasis>list</emphasis></term> - - <listitem><para>Return the second to last elements of a list; - abort evaluation if the argument isn’t a list or is an empty - list.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-throw'> - <term><function>throw</function> - <emphasis>s</emphasis></term> - <term><function>builtins.throw</function> - <emphasis>s</emphasis></term> - - <listitem><para>Throw an error message - <emphasis>s</emphasis>. This usually aborts Nix expression - evaluation, but in <command>nix-env -qa</command> and other - commands that try to evaluate a set of derivations to get - information about those derivations, a derivation that throws an - error is silently skipped (which is not the case for - <function>abort</function>).</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-toFile'> - <term><function>builtins.toFile</function> - <emphasis>name</emphasis> - <emphasis>s</emphasis></term> - - <listitem><para>Store the string <emphasis>s</emphasis> in a - file in the Nix store and return its path. The file has suffix - <emphasis>name</emphasis>. This file can be used as an - input to derivations. One application is to write builders - “inline”. For instance, the following Nix expression combines - <xref linkend='ex-hello-nix' /> and <xref - linkend='ex-hello-builder' /> into one file: - -<programlisting> -{ stdenv, fetchurl, perl }: - -stdenv.mkDerivation { - name = "hello-2.1.1"; - - builder = builtins.toFile "builder.sh" " - source $stdenv/setup - - PATH=$perl/bin:$PATH - - tar xvfz $src - cd hello-* - ./configure --prefix=$out - make - make install - "; - - src = fetchurl { - url = "http://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz"; - sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465"; - }; - inherit perl; -}</programlisting> - - </para> - - <para>It is even possible for one file to refer to another, e.g., - -<programlisting> - builder = let - configFile = builtins.toFile "foo.conf" " - # This is some dummy configuration file. - <emphasis>...</emphasis> - "; - in builtins.toFile "builder.sh" " - source $stdenv/setup - <emphasis>...</emphasis> - cp ${configFile} $out/etc/foo.conf - ";</programlisting> - - Note that <literal>${configFile}</literal> is an antiquotation - (see <xref linkend='ssec-values' />), so the result of the - expression <literal>configFile</literal> (i.e., a path like - <filename>/nix/store/m7p7jfny445k...-foo.conf</filename>) will be - spliced into the resulting string.</para> - - <para>It is however <emphasis>not</emphasis> allowed to have files - mutually referring to each other, like so: - -<programlisting> -let - foo = builtins.toFile "foo" "...${bar}..."; - bar = builtins.toFile "bar" "...${foo}..."; -in foo</programlisting> - - This is not allowed because it would cause a cyclic dependency in - the computation of the cryptographic hashes for - <varname>foo</varname> and <varname>bar</varname>.</para> - <para>It is also not possible to reference the result of a derivation. - If you are using Nixpkgs, the <literal>writeTextFile</literal> function is able to - do that.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-toJSON'> - <term><function>builtins.toJSON</function> <emphasis>e</emphasis></term> - - <listitem><para>Return a string containing a JSON representation - of <emphasis>e</emphasis>. Strings, integers, floats, booleans, - nulls and lists are mapped to their JSON equivalents. Sets - (except derivations) are represented as objects. Derivations are - translated to a JSON string containing the derivation’s output - path. Paths are copied to the store and represented as a JSON - string of the resulting store path.</para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-toPath'> - <term><function>builtins.toPath</function> <emphasis>s</emphasis></term> - - <listitem><para> DEPRECATED. Use <literal>/. + "/path"</literal> - to convert a string into an absolute path. For relative paths, - use <literal>./. + "/path"</literal>. - </para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-toString'> - <term><function>toString</function> <emphasis>e</emphasis></term> - <term><function>builtins.toString</function> <emphasis>e</emphasis></term> - - <listitem><para>Convert the expression - <emphasis>e</emphasis> to a string. - <emphasis>e</emphasis> can be:</para> - <itemizedlist> - <listitem><para>A string (in which case the string is returned unmodified).</para></listitem> - <listitem><para>A path (e.g., <literal>toString /foo/bar</literal> yields <literal>"/foo/bar"</literal>.</para></listitem> - <listitem><para>A set containing <literal>{ __toString = self: ...; }</literal>.</para></listitem> - <listitem><para>An integer.</para></listitem> - <listitem><para>A list, in which case the string representations of its elements are joined with spaces.</para></listitem> - <listitem><para>A Boolean (<literal>false</literal> yields <literal>""</literal>, <literal>true</literal> yields <literal>"1"</literal>).</para></listitem> - <listitem><para><literal>null</literal>, which yields the empty string.</para></listitem> - </itemizedlist> - </listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-toXML'> - <term><function>builtins.toXML</function> <emphasis>e</emphasis></term> - - <listitem><para>Return a string containing an XML representation - of <emphasis>e</emphasis>. The main application for - <function>toXML</function> is to communicate information with the - builder in a more structured format than plain environment - variables.</para> - - <!-- TODO: more formally describe the schema of the XML - representation --> - - <para>Here is an example where this is - the case: - </para> - -<programlisting><![CDATA[ -{ stdenv, fetchurl, libxslt, jira, uberwiki }: - -stdenv.mkDerivation (rec { - name = "web-server"; - - buildInputs = [ libxslt ]; - - builder = builtins.toFile "builder.sh" " - source $stdenv/setup - mkdir $out - echo "$servlets" | xsltproc ${stylesheet} - > $out/server-conf.xml]]> ① <![CDATA[ - "; - - stylesheet = builtins.toFile "stylesheet.xsl"]]> ② <![CDATA[ - "<?xml version='1.0' encoding='UTF-8'?> - <xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'> - <xsl:template match='/'> - <Configure> - <xsl:for-each select='/expr/list/attrs'> - <Call name='addWebApplication'> - <Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg> - <Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg> - </Call> - </xsl:for-each> - </Configure> - </xsl:template> - </xsl:stylesheet> - "; - - servlets = builtins.toXML []]> ③ <![CDATA[ - { path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; } - { path = "/wiki"; war = uberwiki + "/uberwiki.war"; } - ]; -})]]></programlisting> - - <para>The builder is supposed to generate the configuration file - for a <link xlink:href='http://jetty.mortbay.org/'>Jetty servlet - container</link>. A servlet container contains a number of - servlets (<filename>*.war</filename> files) each exported under a - specific URI prefix. So the servlet configuration is a list of - sets containing the <varname>path</varname> and - <varname>war</varname> of the servlet (<xref - linkend='ex-toxml-co-servlets' />). This kind of information is - difficult to communicate with the normal method of passing - information through an environment variable, which just - concatenates everything together into a string (which might just - work in this case, but wouldn’t work if fields are optional or - contain lists themselves). Instead the Nix expression is - converted to an XML representation with - <function>toXML</function>, which is unambiguous and can easily be - processed with the appropriate tools. For instance, in the - example an XSLT stylesheet (at point ②) is applied to it (at point - ①) to generate the XML configuration file for the Jetty server. - The XML representation produced at point ③ by - <function>toXML</function> is as follows:</para> - -<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?> -<expr> - <list> - <attrs> - <attr name="path"> - <string value="/bugtracker" /> - </attr> - <attr name="war"> - <path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" /> - </attr> - </attrs> - <attrs> - <attr name="path"> - <string value="/wiki" /> - </attr> - <attr name="war"> - <path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" /> - </attr> - </attrs> - </list> -</expr>]]></programlisting> - - <para>Note that <xref linkend='ex-toxml' /> uses the <function - linkend='builtin-toFile'>toFile</function> built-in to write the - builder and the stylesheet “inline” in the Nix expression. The - path of the stylesheet is spliced into the builder using the - syntax <literal>xsltproc ${stylesheet}</literal>.</para> - - </listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-trace'> - <term><function>builtins.trace</function> - <emphasis>e1</emphasis> <emphasis>e2</emphasis></term> - - <listitem><para>Evaluate <emphasis>e1</emphasis> and print its - abstract syntax representation on standard error. Then return - <emphasis>e2</emphasis>. This function is useful for - debugging.</para></listitem> - - </varlistentry> - - <varlistentry xml:id='builtin-tryEval'> - <term><function>builtins.tryEval</function> - <emphasis>e</emphasis></term> - - <listitem><para>Try to shallowly evaluate <emphasis>e</emphasis>. - Return a set containing the attributes <literal>success</literal> - (<literal>true</literal> if <emphasis>e</emphasis> evaluated - successfully, <literal>false</literal> if an error was thrown) and - <literal>value</literal>, equalling <emphasis>e</emphasis> - if successful and <literal>false</literal> otherwise. Note that this - doesn't evaluate <emphasis>e</emphasis> deeply, so - <literal>let e = { x = throw ""; }; in (builtins.tryEval e).success - </literal> will be <literal>true</literal>. Using <literal>builtins.deepSeq - </literal> one can get the expected result: <literal>let e = { x = throw ""; - }; in (builtins.tryEval (builtins.deepSeq e e)).success</literal> will be - <literal>false</literal>. - </para></listitem> - - </varlistentry> - - - <varlistentry xml:id='builtin-typeOf'> - <term><function>builtins.typeOf</function> - <emphasis>e</emphasis></term> - - <listitem><para>Return a string representing the type of the value - <emphasis>e</emphasis>, namely <literal>"int"</literal>, - <literal>"bool"</literal>, <literal>"string"</literal>, - <literal>"path"</literal>, <literal>"null"</literal>, - <literal>"set"</literal>, <literal>"list"</literal>, - <literal>"lambda"</literal> or - <literal>"float"</literal>.</para></listitem> - - </varlistentry> - - -</variablelist> - - -</section> |