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<h5 class="subsubsection">23.3.3.10 Writing a Guile Pretty-Printer</h5>

<p><a name="index-writing-a-Guile-pretty_002dprinter-2605"></a>
A pretty-printer consists of two basic parts: a lookup function to determine
if the type is supported, and the printer itself.

   <p>Here is an example showing how a <code>std::string</code> printer might be
written.  See <a href="Guile-Pretty-Printing-API.html#Guile-Pretty-Printing-API">Guile Pretty Printing API</a>, for details.

<pre class="smallexample">     (define (make-my-string-printer value)
       "Print a my::string string"
       (make-pretty-printer-worker
        "string"
        (lambda (printer)
          (value-field value "_data"))
        #f))
</pre>
   <p>And here is an example showing how a lookup function for the printer
example above might be written.

<pre class="smallexample">     (define (str-lookup-function pretty-printer value)
       (let ((tag (type-tag (value-type value))))
         (and tag
              (string-prefix? "std::string&lt;" tag)
              (make-my-string-printer value))))
</pre>
   <p>Then to register this printer in the global printer list:

<pre class="smallexample">     (append-pretty-printer!
      (make-pretty-printer "my-string" str-lookup-function))
</pre>
   <p>The example lookup function extracts the value's type, and attempts to
match it to a type that it can pretty-print.  If it is a type the
printer can pretty-print, it will return a &lt;gdb:pretty-printer-worker&gt; object. 
If not, it returns <code>#f</code>.

   <p>We recommend that you put your core pretty-printers into a Guile
package.  If your pretty-printers are for use with a library, we
further recommend embedding a version number into the package name. 
This practice will enable <span class="sc">gdb</span> to load multiple versions of
your pretty-printers at the same time, because they will have
different names.

   <p>You should write auto-loaded code (see <a href="Guile-Auto_002dloading.html#Guile-Auto_002dloading">Guile Auto-loading</a>) such that it
can be evaluated multiple times without changing its meaning.  An
ideal auto-load file will consist solely of <code>import</code>s of your
printer modules, followed by a call to a register pretty-printers with
the current objfile.

   <p>Taken as a whole, this approach will scale nicely to multiple
inferiors, each potentially using a different library version. 
Embedding a version number in the Guile package name will ensure that
<span class="sc">gdb</span> is able to load both sets of printers simultaneously. 
Then, because the search for pretty-printers is done by objfile, and
because your auto-loaded code took care to register your library's
printers with a specific objfile, <span class="sc">gdb</span> will find the correct
printers for the specific version of the library used by each
inferior.

   <p>To continue the <code>my::string</code> example,
this code might appear in <code>(my-project my-library v1)</code>:

<pre class="smallexample">     (use-modules (gdb))
     (define (register-printers objfile)
       (append-objfile-pretty-printer!
        (make-pretty-printer "my-string" str-lookup-function)))
</pre>
   <p class="noindent">And then the corresponding contents of the auto-load file would be:

<pre class="smallexample">     (use-modules (gdb) (my-project my-library v1))
     (register-printers (current-objfile))
</pre>
   <p>The previous example illustrates a basic pretty-printer. 
There are a few things that can be improved on. 
The printer only handles one type, whereas a library typically has
several types.  One could install a lookup function for each desired type
in the library, but one could also have a single lookup function recognize
several types.  The latter is the conventional way this is handled. 
If a pretty-printer can handle multiple data types, then its
<dfn>subprinters</dfn> are the printers for the individual data types.

   <p>The <code>(gdb printing)</code> module provides a formal way of solving this
problem (see <a href="Guile-Printing-Module.html#Guile-Printing-Module">Guile Printing Module</a>). 
Here is another example that handles multiple types.

   <p>These are the types we are going to pretty-print:

<pre class="smallexample">     struct foo { int a, b; };
     struct bar { struct foo x, y; };
</pre>
   <p>Here are the printers:

<pre class="smallexample">     (define (make-foo-printer value)
       "Print a foo object"
       (make-pretty-printer-worker
        "foo"
        (lambda (printer)
          (format #f "a=&lt;~a&gt; b=&lt;~a&gt;"
                  (value-field value "a") (value-field value "a")))
        #f))
     
     (define (make-bar-printer value)
       "Print a bar object"
       (make-pretty-printer-worker
        "foo"
        (lambda (printer)
          (format #f "x=&lt;~a&gt; y=&lt;~a&gt;"
                  (value-field value "x") (value-field value "y")))
        #f))
</pre>
   <p>This example doesn't need a lookup function, that is handled by the
<code>(gdb printing)</code> module.  Instead a function is provided to build up
the object that handles the lookup.

<pre class="smallexample">     (use-modules (gdb printing))
     
     (define (build-pretty-printer)
       (let ((pp (make-pretty-printer-collection "my-library")))
         (pp-collection-add-tag-printer "foo" make-foo-printer)
         (pp-collection-add-tag-printer "bar" make-bar-printer)
         pp))
</pre>
   <p>And here is the autoload support:

<pre class="smallexample">     (use-modules (gdb) (my-library))
     (append-objfile-pretty-printer! (current-objfile) (build-pretty-printer))
</pre>
   <p>Finally, when this printer is loaded into <span class="sc">gdb</span>, here is the
corresponding output of &lsquo;<samp><span class="samp">info pretty-printer</span></samp>&rsquo;:

<pre class="smallexample">     (gdb) info pretty-printer
     my_library.so:
       my-library
         foo
         bar
</pre>
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