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><A
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>Defining additional predicates</A
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><P
>Additional predicates can be simply used without any declaration,
i.e. you can freely invent new predicates. One application of them
is to express conditional linkage. For example, one may provide archives
for various conditions, such as a production archive, and a
development archive. A single new predicate, "development", would be
sufficient to express this:</P
><PRE
CLASS="PROGRAMLISTING"
>archive(byte) = "p_production.cma"
archive(byte,development) = "p_development.cma"</PRE
><P
>When linking, include a "-predicates development" argument in the
compiler command to select the development archive, otherwise the
production archive is taken.</P
><P
>Predicates could also be used to select among several implementations
of the same module. Define simply one predicate per implementation,
e.g. "p_impl1", "p_impl2", and "p_impl3", and specify the archives for
every implementation separately:</P
><PRE
CLASS="PROGRAMLISTING"
>archive(byte,p_impl1) = "p_impl1.cma"
archive(byte,p_impl2) = "p_impl2.cma"
archive(byte,p_impl3) = "p_impl3.cma"</PRE
><P
>In the case that the implementations have different dependencies,
simply provide multiple "requires" variables:</P
><PRE
CLASS="PROGRAMLISTING"
>requires(p_impl1) = "q"
requires(p_impl2) = "q,r"
requires(p_impl3) = "r,s"</PRE
><P
>Sometimes, the implementations require different linker options. In
this case, define several versions of the "linkopts" variable just
like in the "requires" example.</P
><P
>Note that predicates are global identifiers that can be potentially
applied to every selected package. In the case that a predicate is
only meaningful for a single package, it is common practice to
choose the package name as prefix of the predicate name (e.g.
"netstring_minimum" is a predicate usually only applied to the "netstring"
package).</P
><P
></P
><P
>Predicates could be used to select which features of a library are
linked. For example,
<PRE
CLASS="PROGRAMLISTING"
>archive(byte) = "p_basic.cma"
archive(byte,p_extension) = "p_basic.cma p_ext.cma"</PRE
>
would add the p_ext.cma archive only if the p_extension predicate
were set. <SPAN
CLASS="emphasis"
><I
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>It is considered as bad practice to select
extensions of libraries by predicates. Findlib provides the construct
of subpackages for this purpose.</I
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