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##### 23.3.3.8 Guile Pretty Printing API

An example output is provided (see Pretty Printing).

A pretty-printer is represented by an object of type <gdb:pretty-printer>. Pretty-printer objects are created with `make-pretty-printer`.

The following pretty-printer-related procedures are provided by the `(gdb)` module:

— Scheme Procedure: make-pretty-printer name lookup-function

> Return a `<gdb:pretty-printer>` object named name.

> lookup-function is a function of one parameter: the value to be printed. If the value is handled by this pretty-printer, then lookup-function returns an object of type <gdb:pretty-printer-worker> to perform the actual pretty-printing. Otherwise lookup-function returns `#f`.

— Scheme Procedure: pretty-printer? object

> Return `#t` if object is a `<gdb:pretty-printer>` object. Otherwise return `#f`.

— Scheme Procedure: pretty-printer-enabled? pretty-printer

> Return `#t` if pretty-printer is enabled. Otherwise return `#f`.

— Scheme Procedure: set-pretty-printer-enabled! pretty-printer flag

> Set the enabled flag of pretty-printer to flag. The value returned is unspecified.

— Scheme Procedure: pretty-printers

> Return the list of global pretty-printers.

— Scheme Procedure: set-pretty-printers! pretty-printers

> Set the list of global pretty-printers to pretty-printers. The value returned is unspecified.

— Scheme Procedure: make-pretty-printer-worker display-hint to-string children

> Return an object of type `<gdb:pretty-printer-worker>`.

> This function takes three parameters:

> ‘`display-hint`’
>   display-hint provides a hint to gdb or gdb front end via MI to change the formatting of the value being printed. The value must be a string or `#f` (meaning there is no hint). Several values for display-hint are predefined by gdb:
>   ‘`array`’
>     Indicate that the object being printed is “array-like”. The CLI uses this to respect parameters such as `set print elements` and `set print array`.
>   ‘`map`’
>     Indicate that the object being printed is “map-like”, and that the children of this value can be assumed to alternate between keys and values.
>   ‘`string`’
>     Indicate that the object being printed is “string-like”. If the printer's `to-string` function returns a Guile string of some kind, then gdb will call its internal language-specific string-printing function to format the string. For the CLI this means adding quotation marks, possibly escaping some characters, respecting `set print elements`, and the like.
> ‘`to-string`’
>   to-string is either a function of one parameter, the `<gdb:pretty-printer-worker>` object, or `#f`.

>   When printing from the CLI, if the `to-string` method exists, then gdb will prepend its result to the values returned by `children`. Exactly how this formatting is done is dependent on the display hint, and may change as more hints are added. Also, depending on the print settings (see Print Settings), the CLI may print just the result of `to-string` in a stack trace, omitting the result of `children`.

>   If this method returns a string, it is printed verbatim.

>   Otherwise, if this method returns an instance of `<gdb:value>`, then gdb prints this value. This may result in a call to another pretty-printer.

>   If instead the method returns a Guile value which is convertible to a `<gdb:value>`, then gdb performs the conversion and prints the resulting value. Again, this may result in a call to another pretty-printer. Guile scalars (integers, floats, and booleans) and strings are convertible to `<gdb:value>`; other types are not.

>   Finally, if this method returns `#f` then no further operations are peformed in this method and nothing is printed.

>   If the result is not one of these types, an exception is raised.

>   to-string may also be `#f` in which case it is left to children to print the value.

> ‘`children`’
>   children is either a function of one parameter, the `<gdb:pretty-printer-worker>` object, or `#f`.

>   gdb will call this function on a pretty-printer to compute the children of the pretty-printer's value.

>   This function must return a <gdb:iterator> object. Each item returned by the iterator must be a tuple holding two elements. The first element is the “name” of the child; the second element is the child's value. The value can be any Guile object which is convertible to a gdb value.

>   If children is `#f`, gdb will act as though the value has no children.

gdb provides a function which can be used to look up the default pretty-printer for a `<gdb:value>`:

— Scheme Procedure: default-visualizer value

> This function takes a `<gdb:value>` object as an argument. If a pretty-printer for this value exists, then it is returned. If no such printer exists, then this returns `#f`.