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<h2 id="sec:whymodules"><a id="sec:5.1"><span class="sec-nr">5.1</span> <span class="sec-title">Why 
Use Modules?</span></a></h2>

<a id="sec:whymodules"></a>

<p>In classic Prolog systems, all predicates are organised in a single 
namespace and any predicate can call any predicate. Because each 
predicate in a file can be called from anywhere in the program, it 
becomes very hard to find the dependencies and enhance the 
implementation of a predicate without risking to break the overall 
application. This is true for any language, but even worse for Prolog 
due to its frequent need for `helper predicates'.

<p>A Prolog module encapsulates a set of predicates and defines an
<em>interface</em>. Modules can import other modules, which makes the 
dependencies explicit. Given explicit dependencies and a well-defined 
interface, it becomes much easier to change the internal organisation of 
a module without breaking the overall application.

<p>Explicit dependencies can also be used by the development 
environment. The SWI-Prolog library <code>library(prolog_xref)</code> 
can be used to analyse completeness and consistency of modules. This 
library is used by the built-in editor PceEmacs for syntax highlighting, 
jump-to-definition, etc.

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / whymodules.html
