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><A
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>Dependencies</A
></H1
><P
>Often packages use other packages themselves. Let q be another package
consisting of the single module M3 which contains references to M1 and
M2. At the first glance, we can ignore this when describing the
package, as we could always add "-package p" and "-package q" options
when compiling programs using q. This solution is not optimal, as the
user of a package must have knowledge about details of the package that
should normally be hidden. When we introduced the notion of packages,
one of the most important properties was that we can replace packages
by improved versions. Imagine that q is replaced by q', but q' uses
not only p but also r. Every program that used q and is now forced to
use q' must be changed (at least in the Makefile) in order to link r,
too. This is clearly not what is intended by packages.</P
><P
>The far better solution is to store dependency information in the META
files. The "requires" variable can list names of packages that are
direct ancestors, i.e. referred directly. (Do not put indirect
ancestors into this variable, for example further packages required by
r when describing q - these are found out automatically.) The META
file of q looks like:</P
><PRE
CLASS="PROGRAMLISTING"
>description = "Something that needs p"
requires = "p"
version = "1"
archive(byte) = q.cma
archive(native) = q.cmxa</PRE
><P
>If you want to put several package names into "requires", separate
them with commas or spaces.</P
><P
>The "ocamlfind query" command ignores the "requires" value by default,
you must add the -recursive option. In this case, all direct or
indirect ancestors of the packages given on the command line are
selected, too, and printed in topological order. For example, the
command</P
><PRE
CLASS="PROGRAMLISTING"
>ocamlfind query -recursive -long-format -predicate byte q</PRE
><P
>prints two records:</P
><PRE
CLASS="PROGRAMLISTING"
>package: p
description: Our super-duper package
version: 1
archive(s): p.cma
linkopts:
location: /usr/local/lib/ocaml/site-lib/p
package: q
description: Something that needs p
version: 1
archive(s): q.cma
linkopts:
location: /usr/local/lib/ocaml/site-lib/p</PRE
><P
>Without -recursive, only q would have been printed.</P
><P
>The compiler frontend provided with ocamlfind always works
recursively. In order to compile and link another.ml that uses q, the
following command is sufficient:</P
><PRE
CLASS="PROGRAMLISTING"
>ocamlfind ocamlc -o another -package q -linkpkg another.ml</PRE
><P
>It is not necessary to specify -package p in this statement as the
dependency relation is always used to find out the actually meant
set of packages.</P
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