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>Appending to variables</A
></H1
><P
>The syntax
<PRE
CLASS="PROGRAMLISTING"
>varname ( predname1, predname2, ... ) += "value"</PRE
>
(note the "+=" operator) can be used to append values to variables
depending on whether predicates are true. The "+=" lines in the META
file are evaluated one after the other, and <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>every</I
></SPAN
>
line is selected for which the formal predicates are satisfied,
and these lines are added to the current value of the variable. This is
different from the "=" operator where only the most specific
assignment is taken.</P
><P
>The values are considered as space-separated words when they
are appended, i.e. the new words are added to the list of current
words.</P
><P
>For example, in the hypothetic META file
<PRE
CLASS="PROGRAMLISTING"
>var(p) += "a"
var(p,q) += "b"</PRE
>
the value of var is "a b" if both p and q are true, and the value is
only "a" is only p is true (and the value is empty otherwise). If
the operator "=" had been used, the value would have been only "b"
when both p and q are true.</P
><P
>In the case that there both "=" and "+=" settings for
the same variable, a special algorithm is used: First the most specific
"=" setting is determined, and second all matching "+=" settings are
appended in the order they appear in the file.</P
><P
>Finally, here is a real-world example. Imagine you have an
archive p.cma and a special extension for the toploop, p_top.cma.
Furthermore, there is a special debugging version p_dev.cma that
replaces p.cma during development. You can now simply write
<PRE
CLASS="PROGRAMLISTING"
>archive(byte) = "p.cma"
archive(byte,development) = "p_dev.cma"
archive(byte,toploop) += "p_top.cma"</PRE
>
to select <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>either</I
></SPAN
> p.cma or p_dev.cma, and append
p_top.cma for toploops to whatever was selected in the first step.</P
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