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diff --git a/doc/manual/expressions/language-values.xml b/doc/manual/expressions/language-values.xml new file mode 100644 index 000000000000..c3514e58f0de --- /dev/null +++ b/doc/manual/expressions/language-values.xml @@ -0,0 +1,271 @@ +<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-values'> + +<title>Values</title> + + +<simplesect><title>Simple Values</title> + +<para>Nix has the following basic data types: + +<itemizedlist> + + <listitem> + + <para><emphasis>Strings</emphasis> can be written in three + ways.</para> + + <para>The most common way is to enclose the string between double + quotes, e.g., <literal>"foo bar"</literal>. Strings can span + multiple lines. The special characters <literal>"</literal> and + <literal>\</literal> and the character sequence + <literal>${</literal> must be escaped by prefixing them with a + backslash (<literal>\</literal>). Newlines, carriage returns and + tabs can be written as <literal>\n</literal>, + <literal>\r</literal> and <literal>\t</literal>, + respectively.</para> + + <para>You can include the result of an expression into a string by + enclosing it in + <literal>${<replaceable>...</replaceable>}</literal>, a feature + known as <emphasis>antiquotation</emphasis>. The enclosed + expression must evaluate to something that can be coerced into a + string (meaning that it must be a string, a path, or a + derivation). For instance, rather than writing + +<programlisting> +"--with-freetype2-library=" + freetype + "/lib"</programlisting> + + (where <varname>freetype</varname> is a derivation), you can + instead write the more natural + +<programlisting> +"--with-freetype2-library=${freetype}/lib"</programlisting> + + The latter is automatically translated to the former. A more + complicated example (from the Nix expression for <link + xlink:href='http://www.trolltech.com/products/qt'>Qt</link>): + +<programlisting> +configureFlags = " + -system-zlib -system-libpng -system-libjpeg + ${if openglSupport then "-dlopen-opengl + -L${mesa}/lib -I${mesa}/include + -L${libXmu}/lib -I${libXmu}/include" else ""} + ${if threadSupport then "-thread" else "-no-thread"} +";</programlisting> + + Note that Nix expressions and strings can be arbitrarily nested; + in this case the outer string contains various antiquotations that + themselves contain strings (e.g., <literal>"-thread"</literal>), + some of which in turn contain expressions (e.g., + <literal>${mesa}</literal>).</para> + + <para>The second way to write string literals is as an + <emphasis>indented string</emphasis>, which is enclosed between + pairs of <emphasis>double single-quotes</emphasis>, like so: + +<programlisting> +'' + This is the first line. + This is the second line. + This is the third line. +''</programlisting> + + This kind of string literal intelligently strips indentation from + the start of each line. To be precise, it strips from each line a + number of spaces equal to the minimal indentation of the string as + a whole (disregarding the indentation of empty lines). For + instance, the first and second line are indented two space, while + the third line is indented four spaces. Thus, two spaces are + stripped from each line, so the resulting string is + +<programlisting> +"This is the first line.\nThis is the second line.\n This is the third line.\n"</programlisting> + + </para> + + <para>Note that the whitespace and newline following the opening + <literal>''</literal> is ignored if there is no non-whitespace + text on the initial line.</para> + + <para>Antiquotation + (<literal>${<replaceable>expr</replaceable>}</literal>) is + supported in indented strings.</para> + + <para>Since <literal>${</literal> and <literal>''</literal> have + special meaning in indented strings, you need a way to quote them. + <literal>${</literal> can be escaped by prefixing it with + <literal>''</literal> (that is, two single quotes), i.e., + <literal>''${</literal>. <literal>''</literal> can be escaped by + prefixing it with <literal>'</literal>, i.e., + <literal>'''</literal>. Finally, linefeed, carriage-return and + tab characters can be written as <literal>''\n</literal>, + <literal>''\r</literal>, <literal>''\t</literal>.</para> + + <para>Indented strings are primarily useful in that they allow + multi-line string literals to follow the indentation of the + enclosing Nix expression, and that less escaping is typically + necessary for strings representing languages such as shell scripts + and configuration files because <literal>''</literal> is much less + common than <literal>"</literal>. Example: + +<programlisting> +stdenv.mkDerivation { + <replaceable>...</replaceable> + postInstall = + '' + mkdir $out/bin $out/etc + cp foo $out/bin + echo "Hello World" > $out/etc/foo.conf + ${if enableBar then "cp bar $out/bin" else ""} + ''; + <replaceable>...</replaceable> +} +</programlisting> + + </para> + + <para>Finally, as a convenience, <emphasis>URIs</emphasis> as + defined in appendix B of <link + xlink:href='http://www.ietf.org/rfc/rfc2396.txt'>RFC 2396</link> + can be written <emphasis>as is</emphasis>, without quotes. For + instance, the string + <literal>"http://example.org/foo.tar.bz2"</literal> + can also be written as + <literal>http://example.org/foo.tar.bz2</literal>.</para> + + </listitem> + + <listitem><para><emphasis>Integers</emphasis>, e.g., + <literal>123</literal>.</para></listitem> + + <listitem><para><emphasis>Paths</emphasis>, e.g., + <filename>/bin/sh</filename> or <filename>./builder.sh</filename>. + A path must contain at least one slash to be recognised as such; for + instance, <filename>builder.sh</filename> is not a + path<footnote><para>It's parsed as an expression that selects the + attribute <varname>sh</varname> from the variable + <varname>builder</varname>.</para></footnote>. If the file name is + relative, i.e., if it does not begin with a slash, it is made + absolute at parse time relative to the directory of the Nix + expression that contained it. For instance, if a Nix expression in + <filename>/foo/bar/bla.nix</filename> refers to + <filename>../xyzzy/fnord.nix</filename>, the absolute path is + <filename>/foo/xyzzy/fnord.nix</filename>.</para></listitem> + + <listitem><para><emphasis>Booleans</emphasis> with values + <literal>true</literal> and + <literal>false</literal>.</para></listitem> + + <listitem><para>The null value, denoted as + <literal>null</literal>.</para></listitem> + +</itemizedlist> + +</para> + +</simplesect> + + +<simplesect><title>Lists</title> + +<para>Lists are formed by enclosing a whitespace-separated list of +values between square brackets. For example, + +<programlisting> +[ 123 ./foo.nix "abc" (f { x = y; }) ]</programlisting> + +defines a list of four elements, the last being the result of a call +to the function <varname>f</varname>. Note that function calls have +to be enclosed in parentheses. If they had been omitted, e.g., + +<programlisting> +[ 123 ./foo.nix "abc" f { x = y; } ]</programlisting> + +the result would be a list of five elements, the fourth one being a +function and the fifth being a set.</para> + +<para>Note that lists are only lazy in values, and they are strict in length. +</para> + +</simplesect> + + +<simplesect><title>Sets</title> + +<para>Sets are really the core of the language, since ultimately the +Nix language is all about creating derivations, which are really just +sets of attributes to be passed to build scripts.</para> + +<para>Sets are just a list of name/value pairs (called +<emphasis>attributes</emphasis>) enclosed in curly brackets, where +each value is an arbitrary expression terminated by a semicolon. For +example: + +<programlisting> +{ x = 123; + text = "Hello"; + y = f { bla = 456; }; +}</programlisting> + +This defines a set with attributes named <varname>x</varname>, +<varname>text</varname>, <varname>y</varname>. The order of the +attributes is irrelevant. An attribute name may only occur +once.</para> + +<para>Attributes can be selected from a set using the +<literal>.</literal> operator. For instance, + +<programlisting> +{ a = "Foo"; b = "Bar"; }.a</programlisting> + +evaluates to <literal>"Foo"</literal>. It is possible to provide a +default value in an attribute selection using the +<literal>or</literal> keyword. For example, + +<programlisting> +{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"</programlisting> + +will evaluate to <literal>"Xyzzy"</literal> because there is no +<varname>c</varname> attribute in the set.</para> + +<para>You can use arbitrary double-quoted strings as attribute +names: + +<programlisting> +{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}" +</programlisting> + +This will evaluate to <literal>123</literal> (Assuming +<literal>bar</literal> is antiquotable). In the case where an +attribute name is just a single antiquotation, the quotes can be +dropped: + +<programlisting> +{ foo = 123; }.${bar} or 456 </programlisting> + +This will evaluate to <literal>123</literal> if +<literal>bar</literal> evaluates to <literal>"foo"</literal> when +coerced to a string and <literal>456</literal> otherwise (again +assuming <literal>bar</literal> is antiquotable).</para> + +<para>In the special case where an attribute name inside of a set declaration +evaluates to <literal>null</literal> (which is normally an error, as +<literal>null</literal> is not antiquotable), that attribute is simply not +added to the set: + +<programlisting> +{ ${if foo then "bar" else null} = true; }</programlisting> + +This will evaluate to <literal>{}</literal> if <literal>foo</literal> +evaluates to <literal>false</literal>.</para> + + +</simplesect> + + +</section> |