313 lines
11 KiB
XML
313 lines
11 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id='ssec-values'>
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<title>Values</title>
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<simplesect><title>Simple Values</title>
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<para>Nix has the following basic data types:
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<itemizedlist>
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<listitem>
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<para><emphasis>Strings</emphasis> can be written in three
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ways.</para>
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<para>The most common way is to enclose the string between double
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quotes, e.g., <literal>"foo bar"</literal>. Strings can span
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multiple lines. The special characters <literal>"</literal> and
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<literal>\</literal> and the character sequence
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<literal>${</literal> must be escaped by prefixing them with a
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backslash (<literal>\</literal>). Newlines, carriage returns and
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tabs can be written as <literal>\n</literal>,
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<literal>\r</literal> and <literal>\t</literal>,
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respectively.</para>
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<para>You can include the result of an expression into a string by
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enclosing it in
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<literal>${<replaceable>...</replaceable>}</literal>, a feature
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known as <emphasis>antiquotation</emphasis>. The enclosed
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expression must evaluate to something that can be coerced into a
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string (meaning that it must be a string, a path, or a
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derivation). For instance, rather than writing
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<programlisting>
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"--with-freetype2-library=" + freetype + "/lib"</programlisting>
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(where <varname>freetype</varname> is a derivation), you can
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instead write the more natural
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<programlisting>
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"--with-freetype2-library=${freetype}/lib"</programlisting>
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The latter is automatically translated to the former. A more
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complicated example (from the Nix expression for <link
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xlink:href='http://www.trolltech.com/products/qt'>Qt</link>):
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<programlisting>
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configureFlags = "
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-system-zlib -system-libpng -system-libjpeg
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${if openglSupport then "-dlopen-opengl
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-L${mesa}/lib -I${mesa}/include
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-L${libXmu}/lib -I${libXmu}/include" else ""}
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${if threadSupport then "-thread" else "-no-thread"}
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";</programlisting>
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Note that Nix expressions and strings can be arbitrarily nested;
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in this case the outer string contains various antiquotations that
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themselves contain strings (e.g., <literal>"-thread"</literal>),
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some of which in turn contain expressions (e.g.,
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<literal>${mesa}</literal>).</para>
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<para>The second way to write string literals is as an
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<emphasis>indented string</emphasis>, which is enclosed between
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pairs of <emphasis>double single-quotes</emphasis>, like so:
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<programlisting>
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''
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This is the first line.
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This is the second line.
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This is the third line.
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''</programlisting>
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This kind of string literal intelligently strips indentation from
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the start of each line. To be precise, it strips from each line a
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number of spaces equal to the minimal indentation of the string as
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a whole (disregarding the indentation of empty lines). For
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instance, the first and second line are indented two space, while
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the third line is indented four spaces. Thus, two spaces are
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stripped from each line, so the resulting string is
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<programlisting>
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"This is the first line.\nThis is the second line.\n This is the third line.\n"</programlisting>
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</para>
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<para>Note that the whitespace and newline following the opening
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<literal>''</literal> is ignored if there is no non-whitespace
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text on the initial line.</para>
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<para>Antiquotation
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(<literal>${<replaceable>expr</replaceable>}</literal>) is
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supported in indented strings.</para>
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<para>Since <literal>${</literal> and <literal>''</literal> have
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special meaning in indented strings, you need a way to quote them.
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<literal>$</literal> can be escaped by prefixing it with
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<literal>''</literal> (that is, two single quotes), i.e.,
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<literal>''$</literal>. <literal>''</literal> can be escaped by
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prefixing it with <literal>'</literal>, i.e.,
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<literal>'''</literal>. <literal>$</literal> removes any special meaning
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from the following <literal>$</literal>. Linefeed, carriage-return and tab
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characters can be written as <literal>''\n</literal>,
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<literal>''\r</literal>, <literal>''\t</literal>, and <literal>''\</literal>
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escapes any other character.
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</para>
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<para>Indented strings are primarily useful in that they allow
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multi-line string literals to follow the indentation of the
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enclosing Nix expression, and that less escaping is typically
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necessary for strings representing languages such as shell scripts
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and configuration files because <literal>''</literal> is much less
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common than <literal>"</literal>. Example:
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<programlisting>
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stdenv.mkDerivation {
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<replaceable>...</replaceable>
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postInstall =
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''
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mkdir $out/bin $out/etc
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cp foo $out/bin
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echo "Hello World" > $out/etc/foo.conf
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${if enableBar then "cp bar $out/bin" else ""}
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'';
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<replaceable>...</replaceable>
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}
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</programlisting>
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</para>
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<para>Finally, as a convenience, <emphasis>URIs</emphasis> as
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defined in appendix B of <link
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xlink:href='http://www.ietf.org/rfc/rfc2396.txt'>RFC 2396</link>
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can be written <emphasis>as is</emphasis>, without quotes. For
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instance, the string
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<literal>"http://example.org/foo.tar.bz2"</literal>
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can also be written as
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<literal>http://example.org/foo.tar.bz2</literal>.</para>
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</listitem>
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<listitem><para>Numbers, which can be <emphasis>integers</emphasis> (like
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<literal>123</literal>) or <emphasis>floating point</emphasis> (like
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<literal>123.43</literal> or <literal>.27e13</literal>).</para>
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<para>Numbers are type-compatible: pure integer operations will always
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return integers, whereas any operation involving at least one floating point
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number will have a floating point number as a result.</para></listitem>
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<listitem><para><emphasis>Paths</emphasis>, e.g.,
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<filename>/bin/sh</filename> or <filename>./builder.sh</filename>.
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A path must contain at least one slash to be recognised as such; for
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instance, <filename>builder.sh</filename> is not a
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path<footnote><para>It's parsed as an expression that selects the
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attribute <varname>sh</varname> from the variable
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<varname>builder</varname>.</para></footnote>. If the file name is
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relative, i.e., if it does not begin with a slash, it is made
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absolute at parse time relative to the directory of the Nix
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expression that contained it. For instance, if a Nix expression in
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<filename>/foo/bar/bla.nix</filename> refers to
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<filename>../xyzzy/fnord.nix</filename>, the absolute path is
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<filename>/foo/xyzzy/fnord.nix</filename>.</para>
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<para>If the first component of a path is a <literal>~</literal>,
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it is interpreted as if the rest of the path were relative to the
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user's home directory. e.g. <filename>~/foo</filename> would be
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equivalent to <filename>/home/edolstra/foo</filename> for a user
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whose home directory is <filename>/home/edolstra</filename>.
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</para>
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<para>Paths can also be specified between angle brackets, e.g.
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<literal><nixpkgs></literal>. This means that the directories
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listed in the environment variable
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<envar linkend="env-NIX_PATH">NIX_PATH</envar> will be searched
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for the given file or directory name.
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</para>
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</listitem>
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<listitem><para><emphasis>Booleans</emphasis> with values
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<literal>true</literal> and
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<literal>false</literal>.</para></listitem>
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<listitem><para>The null value, denoted as
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<literal>null</literal>.</para></listitem>
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</itemizedlist>
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</para>
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</simplesect>
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<simplesect><title>Lists</title>
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<para>Lists are formed by enclosing a whitespace-separated list of
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values between square brackets. For example,
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<programlisting>
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[ 123 ./foo.nix "abc" (f { x = y; }) ]</programlisting>
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defines a list of four elements, the last being the result of a call
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to the function <varname>f</varname>. Note that function calls have
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to be enclosed in parentheses. If they had been omitted, e.g.,
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<programlisting>
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[ 123 ./foo.nix "abc" f { x = y; } ]</programlisting>
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the result would be a list of five elements, the fourth one being a
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function and the fifth being a set.</para>
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<para>Note that lists are only lazy in values, and they are strict in length.
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</para>
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</simplesect>
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<simplesect><title>Sets</title>
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<para>Sets are really the core of the language, since ultimately the
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Nix language is all about creating derivations, which are really just
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sets of attributes to be passed to build scripts.</para>
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<para>Sets are just a list of name/value pairs (called
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<emphasis>attributes</emphasis>) enclosed in curly brackets, where
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each value is an arbitrary expression terminated by a semicolon. For
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example:
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<programlisting>
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{ x = 123;
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text = "Hello";
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y = f { bla = 456; };
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}</programlisting>
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This defines a set with attributes named <varname>x</varname>,
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<varname>text</varname>, <varname>y</varname>. The order of the
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attributes is irrelevant. An attribute name may only occur
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once.</para>
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<para>Attributes can be selected from a set using the
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<literal>.</literal> operator. For instance,
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<programlisting>
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{ a = "Foo"; b = "Bar"; }.a</programlisting>
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evaluates to <literal>"Foo"</literal>. It is possible to provide a
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default value in an attribute selection using the
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<literal>or</literal> keyword. For example,
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<programlisting>
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{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"</programlisting>
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will evaluate to <literal>"Xyzzy"</literal> because there is no
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<varname>c</varname> attribute in the set.</para>
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<para>You can use arbitrary double-quoted strings as attribute
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names:
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<programlisting>
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{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"
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</programlisting>
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This will evaluate to <literal>123</literal> (Assuming
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<literal>bar</literal> is antiquotable). In the case where an
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attribute name is just a single antiquotation, the quotes can be
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dropped:
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<programlisting>
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{ foo = 123; }.${bar} or 456 </programlisting>
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This will evaluate to <literal>123</literal> if
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<literal>bar</literal> evaluates to <literal>"foo"</literal> when
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coerced to a string and <literal>456</literal> otherwise (again
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assuming <literal>bar</literal> is antiquotable).</para>
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<para>In the special case where an attribute name inside of a set declaration
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evaluates to <literal>null</literal> (which is normally an error, as
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<literal>null</literal> is not antiquotable), that attribute is simply not
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added to the set:
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<programlisting>
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{ ${if foo then "bar" else null} = true; }</programlisting>
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This will evaluate to <literal>{}</literal> if <literal>foo</literal>
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evaluates to <literal>false</literal>.</para>
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<para>A set that has a <literal>__functor</literal> attribute whose value
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is callable (i.e. is itself a function or a set with a
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<literal>__functor</literal> attribute whose value is callable) can be
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applied as if it were a function, with the set itself passed in first
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, e.g.,
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<programlisting>
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let add = { __functor = self: x: x + self.x; };
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inc = add // { x = 1; };
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in inc 1
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</programlisting>
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evaluates to <literal>2</literal>. This can be used to attach metadata to a
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function without the caller needing to treat it specially, or to implement
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a form of object-oriented programming, for example.
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</para>
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</simplesect>
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</section>
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