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diff --git a/doc/manual/expressions/language-values.xml b/doc/manual/expressions/language-values.xml deleted file mode 100644 index bb2090c88..000000000 --- a/doc/manual/expressions/language-values.xml +++ /dev/null @@ -1,313 +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-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>. <literal>$</literal> removes any special meaning - from the following <literal>$</literal>. Linefeed, carriage-return and tab - characters can be written as <literal>''\n</literal>, - <literal>''\r</literal>, <literal>''\t</literal>, and <literal>''\</literal> - escapes any other character. - - </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>Numbers, which can be <emphasis>integers</emphasis> (like - <literal>123</literal>) or <emphasis>floating point</emphasis> (like - <literal>123.43</literal> or <literal>.27e13</literal>).</para> - - <para>Numbers are type-compatible: pure integer operations will always - return integers, whereas any operation involving at least one floating point - number will have a floating point number as a result.</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> - - <para>If the first component of a path is a <literal>~</literal>, - it is interpreted as if the rest of the path were relative to the - user's home directory. e.g. <filename>~/foo</filename> would be - equivalent to <filename>/home/edolstra/foo</filename> for a user - whose home directory is <filename>/home/edolstra</filename>. - </para> - - <para>Paths can also be specified between angle brackets, e.g. - <literal><nixpkgs></literal>. This means that the directories - listed in the environment variable - <envar linkend="env-NIX_PATH">NIX_PATH</envar> will be searched - for the given file or directory name. - </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> - -<para>A set that has a <literal>__functor</literal> attribute whose value -is callable (i.e. is itself a function or a set with a -<literal>__functor</literal> attribute whose value is callable) can be -applied as if it were a function, with the set itself passed in first -, e.g., - -<programlisting> -let add = { __functor = self: x: x + self.x; }; - inc = add // { x = 1; }; -in inc 1 -</programlisting> - -evaluates to <literal>2</literal>. This can be used to attach metadata to a -function without the caller needing to treat it specially, or to implement -a form of object-oriented programming, for example. - -</para> - -</simplesect> - - -</section> |