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>Chapter 5. Complex packages</H1
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>The standard predicates</A
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>The standard predicates</A
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><P
>Settings in the META file have usually the form:</P
><PRE
CLASS="PROGRAMLISTING"
>varname ( predname1, predname2, ... ) = "value"</PRE
><P
>The names in the parantheses are called <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>formal
predicates</I
></SPAN
> and express a condition which must hold such
that the value on the right side is selected. When querying
information you can specify a set of <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
> actual
predicates</I
></SPAN
> that are assumed to be true. There are the
following standard predicates:</P
><P
></P
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><LI
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><P
>The "byte" predicate means that the bytecode compiler is used.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>The "native" predicate means that the native compiler is used.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>The "toploop" predicate means that the toploop is available in the
linked program.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>The "mt" predicate means that the program is multi-threaded.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>The "mt_posix" predicate means that in the case "mt" is set, too, the
POSIX libraries are used to implement threads.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>The "mt_vm" predicate means that in the case "mt" is set, too, the
VM-based libraries are used to implement threads.</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>The "gprof" predicate means that in the case "native" is set, too, the
program is compiled for profiling</P
></LI
><LI
STYLE="list-style-type: disc"
><P
>The "autolink" predicate indicates that ocamlc is able to perform
automatic linking.</P
></LI
></UL
><P
>It is possible to have more predicates indicating different
environments; just use them, it is not necessary to declare them
anywhere.</P
><P
>The value which is selected for a variable depends on the set of
assumed predicates. In order to get selected, all formal predicates
must be included in the set of actual predicates; if still
multiple values fulfill this condition, the value with the maximum
number of formal predicates is used; and if still in doubt, the first
of these in the META file is taken.</P
><P
>It is possible to negate a formal predicate: Just prepend a minus
sign to it, e.g. var(p,-q). In this case, it is required that the
negated predicates are false (not contained in the set of actual predicates).</P
><P
>For all variables that are evaluated after the dependency analysis,
findlib adds the so-called package predicates to the set of actual
predicates. For every selected package p the predicate pkg_p 
(with the fixed prefix "pkg_") is added. One application of this
are compatibility checks: The special <TT
CLASS="LITERAL"
>error</TT
> variable
is evaluated after dependency analysis, but before anything else
is done. For example, to state that package p is incompatible with
package q one can add to the META file of p:

<PRE
CLASS="PROGRAMLISTING"
>error(pkg_q) = "Package p is incompatible with q"</PRE
>

The value of <TT
CLASS="LITERAL"
>error</TT
> is printed as message in the
case the condition is true.</P
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libfindlib-ocaml-dev 1.7.3-2 / usr / share / doc / libfindlib-ocaml-dev / guide-html / c278.html
