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<h5 class="subsubsection">23.3.3.25 Iterators In Guile</h5>

<p><a name="index-guile-iterators-2855"></a><a name="index-g_t_003cgdb_003aiterator_003e-2856"></a>
A simple iterator facility is provided to allow, for example,
iterating over the set of program symbols without having to first
construct a list of all of them.  A useful contribution would be
to add support for SRFI 41 and SRFI 45.

<div class="defun">
&mdash; Scheme Procedure: <b>make-iterator</b><var> object progress next!<a name="index-make_002diterator-2857"></a></var><br>
<blockquote><p>A <code>&lt;gdb:iterator&gt;</code> object is constructed with the <code>make-iterator</code>
procedure.  It takes three arguments: the object to be iterated over,
an object to record the progress of the iteration, and a procedure to
return the next element in the iteration, or an implementation chosen value
to denote the end of iteration.

        <p>By convention, end of iteration is marked with <code>(end-of-iteration)</code>,
and may be tested with the <code>end-of-iteration?</code> predicate. 
The result of <code>(end-of-iteration)</code> is chosen so that it is not
otherwise used by the <code>(gdb)</code> module.  If you are using
<code>&lt;gdb:iterator&gt;</code> in your own code it is your responsibility to
maintain this invariant.

        <p>A trivial example for illustration's sake:

     <pre class="smallexample">          (use-modules (gdb iterator))
          (define my-list (list 1 2 3))
          (define iter
            (make-iterator my-list my-list
                           (lambda (iter)
                             (let ((l (iterator-progress iter)))
                               (if (eq? l '())
                                   (end-of-iteration)
                                   (begin
                                     (set-iterator-progress! iter (cdr l))
                                     (car l)))))))
</pre>
        <p>Here is a slightly more realistic example, which computes a list of all the
functions in <code>my-global-block</code>.

     <pre class="smallexample">          (use-modules (gdb iterator))
          (define this-sal (find-pc-line (frame-pc (selected-frame))))
          (define this-symtab (sal-symtab this-sal))
          (define this-global-block (symtab-global-block this-symtab))
          (define syms-iter (make-block-symbols-iterator this-global-block))
          (define functions (iterator-filter symbol-function? syms-iter))
</pre>
        </blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator?</b><var> object<a name="index-iterator_003f-2858"></a></var><br>
<blockquote><p>Return <code>#t</code> if <var>object</var> is a <code>&lt;gdb:iterator&gt;</code> object. 
Otherwise return <code>#f</code>. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-object</b><var> iterator<a name="index-iterator_002dobject-2859"></a></var><br>
<blockquote><p>Return the first argument that was passed to <code>make-iterator</code>. 
This is the object being iterated over. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-progress</b><var> iterator<a name="index-iterator_002dprogress-2860"></a></var><br>
<blockquote><p>Return the object tracking iteration progress. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>set-iterator-progress!</b><var> iterator new-value<a name="index-set_002diterator_002dprogress_0021-2861"></a></var><br>
<blockquote><p>Set the object tracking iteration progress. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-next!</b><var> iterator<a name="index-iterator_002dnext_0021-2862"></a></var><br>
<blockquote><p>Invoke the procedure that was the third argument to <code>make-iterator</code>,
passing it one argument, the <code>&lt;gdb:iterator&gt;</code> object. 
The result is either the next element in the iteration, or an end
marker as implemented by the <code>next!</code> procedure. 
By convention the end marker is the result of <code>(end-of-iteration)</code>. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>end-of-iteration</b><var><a name="index-end_002dof_002diteration-2863"></a></var><br>
<blockquote><p>Return the Scheme object that denotes end of iteration. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>end-of-iteration?</b><var> object<a name="index-end_002dof_002diteration_003f-2864"></a></var><br>
<blockquote><p>Return <code>#t</code> if <var>object</var> is the end of iteration marker. 
Otherwise return <code>#f</code>. 
</p></blockquote></div>

   <p>These functions are provided by the <code>(gdb iterator)</code> module to
assist in using iterators.

<div class="defun">
&mdash; Scheme Procedure: <b>make-list-iterator</b><var> list<a name="index-make_002dlist_002diterator-2865"></a></var><br>
<blockquote><p>Return a <code>&lt;gdb:iterator&gt;</code> object that will iterate over <var>list</var>. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-&gt;list</b><var> iterator<a name="index-iterator_002d_003elist-2866"></a></var><br>
<blockquote><p>Return the elements pointed to by <var>iterator</var> as a list. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-map</b><var> proc iterator<a name="index-iterator_002dmap-2867"></a></var><br>
<blockquote><p>Return the list of objects obtained by applying <var>proc</var> to the object
pointed to by <var>iterator</var> and to each subsequent object. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-for-each</b><var> proc iterator<a name="index-iterator_002dfor_002deach-2868"></a></var><br>
<blockquote><p>Apply <var>proc</var> to each element pointed to by <var>iterator</var>. 
The result is unspecified. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-filter</b><var> pred iterator<a name="index-iterator_002dfilter-2869"></a></var><br>
<blockquote><p>Return the list of elements pointed to by <var>iterator</var> that satisfy
<var>pred</var>. 
</p></blockquote></div>

<div class="defun">
&mdash; Scheme Procedure: <b>iterator-until</b><var> pred iterator<a name="index-iterator_002duntil-2870"></a></var><br>
<blockquote><p>Run <var>iterator</var> until the result of <code>(pred element)</code> is true
and return that as the result.  Otherwise return <code>#f</code>. 
</p></blockquote></div>

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