#### 23.3.3.10 Writing a Guile Pretty-Printer

A pretty-printer consists of two basic parts: a lookup function to determine if the type is supported, and the printer itself.

Here is an example showing how a `std::string` printer might be written. See Guile Pretty Printing API, for details.

    (define (make-my-string-printer value)
      "Print a my::string string"
      (make-pretty-printer-worker
       "string"
       (lambda (printer)
         (value-field value "_data"))
       #f))

And here is an example showing how a lookup function for the printer example above might be written.

    (define (str-lookup-function pretty-printer value)
      (let ((tag (type-tag (value-type value))))
        (and tag
             (string-prefix? "std::string<" tag)
             (make-my-string-printer value))))

Then to register this printer in the global printer list:

    (append-pretty-printer!
     (make-pretty-printer "my-string" str-lookup-function))

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 <gdb:pretty-printer-worker> object. If not, it returns `#f`.

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 GDB to load multiple versions of your pretty-printers at the same time, because they will have different names.

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

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 GDB 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, GDB will find the correct printers for the specific version of the library used by each inferior.

To continue the `my::string` example, this code might appear in `(my-project my-library v1)`:

    (use-modules (gdb))
    (define (register-printers objfile)
      (append-objfile-pretty-printer!
       (make-pretty-printer "my-string" str-lookup-function)))

And then the corresponding contents of the auto-load file would be:

    (use-modules (gdb) (my-project my-library v1))
    (register-printers (current-objfile))

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 subprinters are the printers for the individual data types.

The `(gdb printing)` module provides a formal way of solving this problem (see Guile Printing Module). Here is another example that handles multiple types.

These are the types we are going to pretty-print:

    struct foo { int a, b; };
    struct bar { struct foo x, y; };

Here are the printers:

    (define (make-foo-printer value)
      "Print a foo object"
      (make-pretty-printer-worker
       "foo"
       (lambda (printer)
         (format #f "a=<~a> b=<~a>"
                 (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=<~a> y=<~a>"
                 (value-field value "x") (value-field value "y")))
       #f))

This example doesn’t need a lookup function, that is handled by the `(gdb printing)` module. Instead a function is provided to build up the object that handles the lookup.

    (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))

And here is the autoload support:

    (use-modules (gdb) (my-library))
    (append-objfile-pretty-printer! (current-objfile) (build-pretty-printer))

Finally, when this printer is loaded into GDB, here is the corresponding output of ‘`info pretty-printer`’:

    (gdb) info pretty-printer
    my_library.so:
      my-library
        foo
        bar