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><P
>These expressions
(named after similar things in Unix shells)
are another way of writing string literals, often useful for large strings.
Everything from <TT
CLASS="LITERAL"
>``</TT
> to <TT
CLASS="LITERAL"
>''</TT
>
(including newlines and backslashes, but not <TT
CLASS="LITERAL"
>$</TT
> characters)
is treated as literal text, and layout is ignored.
The exception is the <TT
CLASS="LITERAL"
>$</TT
> character,
so that you can embed the value of the variable <TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
>
in the string by writing
<TT
CLASS="LITERAL"
>$(</TT
><TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
><TT
CLASS="LITERAL"
>)</TT
>.
To get a literal <TT
CLASS="LITERAL"
>$</TT
> character,
write <TT
CLASS="LITERAL"
>$$</TT
> —
single <TT
CLASS="LITERAL"
>$</TT
> characters are not allowed.</P
><P
>When the
<A
HREF="options.html#OPTION-HERE-DOCUMENTS"
><CODE
CLASS="OPTION"
>+H</CODE
></A
> option
is given, the following
<PRE
CLASS="PROGRAMLISTING"
>letter name = ``Dear $(name),
Here are some characters: \ ' ` ".
To learn more, send $$10 to the address below.''</PRE
>
is equivalent the Haskell 98 declaration
<PRE
CLASS="PROGRAMLISTING"
>letter name = "Dear " ++ quote name ++ ",\n\
\Here are some characters: \\ ' ` \".\n\
\To learn more, send $10 to the address below."</PRE
>
The function
<PRE
CLASS="PROGRAMLISTING"
>class Quote where
quote :: a -> String</PRE
>
(basically no change for <TT
CLASS="LITERAL"
>String</TT
> and <TT
CLASS="LITERAL"
>Char</TT
>,
and <CODE
CLASS="FUNCTION"
>show</CODE
> for everything else)
comes from the <TT
CLASS="LITERAL"
>Hugs.Quote</TT
> module,
which also defines several common instances,
and should be imported if you use the
<TT
CLASS="LITERAL"
>$(</TT
><TT
CLASS="REPLACEABLE"
><I
>var</I
></TT
><TT
CLASS="LITERAL"
>)</TT
> form.
(This module also requires the <CODE
CLASS="OPTION"
>-98</CODE
> option.)</P
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