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<H2><A NAME="s2">2.</A> <A HREF="slshfun.html#toc2">The <CODE>require</CODE> Function</A></H2>

<P>The functions in <CODE>require.sl</CODE> facilitate the loading of <B>S-Lang</B>
code.</P>
<H2><A NAME="_featurep"></A> <A NAME="ss2.1">2.1</A> <A HREF="slshfun.html#toc2.1"><B>_featurep</B></A>
</H2>

<P>
<DL>
<DT><B> Synopsis </B><DD>
<P>Test whether or not a feature is present</P>
<DT><B> Usage </B><DD>
<P><CODE>Int_Type _featurep (String_Type feature [,String_Type namespace])</CODE></P>
<DT><B> Description </B><DD>
<P>The <CODE>_featurep</CODE> function returns a non-zero value if the specified 
feature is present.  Otherwise, it returns 0 to indicate that the feature
has not been loaded.</P>
<DT><B> See Also </B><DD>
<P><CODE>require, provide</CODE></P>
</DL>
</P>


<H2><A NAME="provide"></A> <A NAME="ss2.2">2.2</A> <A HREF="slshfun.html#toc2.2"><B>provide</B></A>
</H2>

<P>
<DL>
<DT><B> Synopsis </B><DD>
<P>Declare that a specified feature is available</P>
<DT><B> Usage </B><DD>
<P><CODE>provide (String_Type feature [,String_Type namespace])</CODE></P>
<DT><B> Description </B><DD>
<P>The <CODE>provide</CODE> function may be used to declare that a "feature" has 
been loaded into the specified namespace.  If the namespace argument is not
present, the current namespace will be used.
See the documentation for <CODE>require</CODE> for more information.</P>
<DT><B> See Also </B><DD>
<P><CODE>require, _featurep</CODE></P>
</DL>
</P>


<H2><A NAME="require"></A> <A NAME="ss2.3">2.3</A> <A HREF="slshfun.html#toc2.3"><B>require</B></A>
</H2>

<P>
<DL>
<DT><B> Synopsis </B><DD>
<P>Make sure a feature is present, and load it if not</P>
<DT><B> Usage </B><DD>
<P><CODE>require (feature [,namespace [,file]])</CODE>
<BLOCKQUOTE><CODE>
<PRE>
   String_Type feature, namespace, file;
</PRE>
</CODE></BLOCKQUOTE>
</P>
<DT><B> Description </B><DD>
<P>The <CODE>require</CODE> function ensures that a specified "feature" is present.
If the feature is not present, the <CODE>require</CODE> function will attempt to
load the feature from a file.  If the <CODE>namespace</CODE> argument is present
and non-NULL, the specified namespace will be used.  The default is to use
the current non-anonymous namespace. If called with three arguments, the 
feature will be loaded from the file specified by the third argument 
if it does not already exist in the namespace.  Otherwise, the feature 
will be loaded from a file given by the name of the feature, with 
".sl" appended.</P>
<P>If after loading the file, if the feature is not present, 
a warning message will be issued.</P>
<DT><B> Example </B><DD>
<P>
<BLOCKQUOTE><CODE>
<PRE>
    require ("histogram");
    require ("histogram", "foo");
    require ("histogram", "foo", "/home/bob/hist.sl");
    require ("histogram", ,"/home/bob/hist.sl");
</PRE>
</CODE></BLOCKQUOTE>
</P>
<DT><B> Notes </B><DD>
<P>"feature" is an abstract quantity that is undefined here.</P>
<P>A popular use of the <CODE>require</CODE> function is to ensure that a specified
file has already been loaded.  In this case, the feature is the 
filename itself.  The advantage of using this mechanism over using 
<CODE>evalfile</CODE> is that if the file has already been loaded, <CODE>require</CODE>
will not re-load it. </P>
<DT><B> See Also </B><DD>
<P><CODE>provide, _featurep, evalfile</CODE></P>
</DL>
</P>


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