.\" ------------------------------ sec9.t ------------------------------
.sh 1 "Modules"
.pp
To describe how modules (or maybe more properly, just libraries) are
currently supported in our system, we must describe the \fI_\|$undefined_\|pred\fR/1
interrupt handler.  The system keeps a table of modules and routines that
are needed from each.  When a predicate is found to be undefined, the table
is searched to see if it is defined by some module.  If so, that module is
loaded (if it hasn't been previously loaded) and the association is made
between the routine name as defined in the module, and the routine name as
used by the invoker.  
.pp
The table of modules and needed routines is:
.(l C
defined_\|mods(\fIModname\fR, [\fIpred\fR\*<1\*>/\fIarity\fR\*<1\*>, ..., \fIpred\*<n\*>\fR/\fIarity\*<n\*>\fR]).
.)l
where \fIModname\fR is the name of the module.  The module exports \fIn\fR
predicate definitions.  The first exported pred is of arity \fIarity\fR\*<\*>1,
and needs to be invoked by the name of \fIpred\fR\*<1\*>.  
.pp
The table of modules that have already been loaded is given by
.(l
	loaded_\|mods(\fIModname\fR).
.)l
A module is a file of predicate definitions, together with a fact defining a list
of predicates exported by that module, and a set of facts, each of which specifies, for some
other module, the predicates imported from that module.  For example, consider a module
name `\fIp\fR'. It contains a single fact, named \fIp_\|export\fR, that is true of the
list of predicate/arity's that are exported.  E.g.
.(l
	p_\|export([p1/2, p2/4])
.)l
indicates that the module \fIp\fR exports the predicates \fIp1\fR/2 and \fIp2\fR/4.
For each module \fIm\fR which contains predicates needed by the module \fIp\fR, there
is a fact for \fIp_use\fR, describing what module is needed and the names of the
predicates defined there that are needed.  For example, if module \fIp\fR needs
to import predicates \fIip1\fR/2 and \fIip2\fR/3 from module \fIq\fR, there would be a fact
.(l
	p_\|use(q,[ip1/2, ip2/3])
.)l
where \fIq\fR is a module that exports two predicates: one 2-ary and one
3-ary.  This list corresponds to the export list of module \fIq\fR.  
.pp
The correspondence between the predicates in the export list of an exporting
module, and those in the import or \fIuse\fR list of a module which imports one or
more of them, is by position, i.e. the predicate names at the exporting and importing names may be different, and
the association between names in the two lists is by the position in the list.  If the importing
module does not wish to import one or more of the predicates exported by the
exporting module, it may put an anonymous variable in the corresponding position in
its \fIuse\fR list.  Thus, for example, if module \fIp\fR above had wished to import only
the predicate \fIip2\fR/3 from module \fIq\fR, the corresponding use fact would
be
.(l
	p_\|use(q, [_\|\|, ip2/3]).
.)l
.pp
The initial set of predicates and the modules from which they are to be
loaded is set up by an initial call to \fI$prorc\fR/0 (see the SB-Prolog
system file \fImodlib/src/$prorc.P\fR).
This predicate makes initial calls to the predicate $define_\|mod which set up
the tables described above so that the use of standard predicates will cause
the correct modules to be loaded in the \fI_\|$undefined_\|pred\fR routine, and the
correct names to be used.
.\" ---------------------- end of section sec9.t --------------------