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* Documented indented string literals.

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* Release notes.
This commit is contained in:
Eelco Dolstra 2007-12-30 23:36:09 +00:00
parent de012e76e2
commit 59707975a3
3 changed files with 193 additions and 43 deletions
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74
doc/manual/writing-nix-expressions.xml
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@ -612,7 +612,10 @@ language.</para>
<listitem>
<para><emphasis>Strings</emphasis> are enclosed between double
<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
@ -658,8 +661,73 @@ configureFlags = "
some of which in turn contain expressions (e.g.,
<literal>${mesa}</literal>).</para>
<para>As a convenience, <emphasis>URIs</emphasis> as defined in
appendix B of <link
<para>The second way to write string literals is as an
<emphasis>indented string</emphasis>, which is enclosed between
pairs of <emphasis>double single-quotes</emphasis>, like so:
<programlisting>
''
This is the first line.
This is the second line.
This is the third line.
''</programlisting>
This kind of string literal intelligently strips indentation from
the start of each line. To be precise, it strips from each line a
number of spaces equal to the minimal indentation of the string as
a whole (disregarding the indentation of empty lines). For
instance, the first and second line are indented two space, while
the third line is indented three spaces. Thus, two spaces are
stripped from each line, so the resulting string is
<programlisting>
"This is the first line.\nThis is the second line.\n This is the third line.\n"</programlisting>
</para>
<para>Note that the whitespace and newline following the opening
<literal>''</literal> is ignored if there is no non-whitespace
text on the initial line.</para>
<para>Antiquotation
(<literal>${<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>, 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 writted 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