Section 3.4. The Object Inspector

A facility for noninteractive inspection is also provided to allow
construction of different inspection interfaces. Like the interactive
facility, it allows objects to be examined in ways not ordinarily
possible. The noninteractive system follows a simple, object-oriented
protocol. Ordinary Scheme objects are encapsulated in procedures, or
inspector objects, that take symbolic messages and return either
information about the encapsulated object or new inspector objects
that encapsulate pieces of the object.


procedure: (inspect/object object)
returns: an inspector object procedure
libraries: (scheme)

inspect/object is used to turn an ordinary Scheme object into an
inspector object. All inspector objects accept the messages type,
print, and write. The type message returns a symbolic representation
of the type of the object. The print and write messages must be
accompanied by a port parameter. They cause a representation of the
object to be written to the port, using the Scheme procedures
pretty-print and write. All inspector objects except for variable
inspector objects accept the message value, which returns the actual
object encapsulated in the inspector object.

(define x (inspect/object '(1 2 3)))
(x 'type) <graphic> pair
(define p (open-output-string))
(x 'write p)
(get-output-string p) <graphic> "(1 2 3)"
(x 'length) <graphic> (proper 3)
(define y (x 'car))
(y 'type) <graphic> simple
(y 'value) <graphic> 1 


Procedure inspector objects. Procedure inspector objects contain
Scheme procedures.

(procedure-object 'type) returns the symbol procedure.

(procedure-object 'length) returns the number of free variables.

(procedure-object 'ref n) returns an inspector object containing the
nth free variable of the procedure. See the description below of
variable inspector objects. n must be nonnegative and less than the
length of the procedure.

(procedure-object 'eval expr) evaluates expr and returns its
value. The values of the procedure's free variables are bound within
the evaluated expression to identifiers of the form %n, where n is the
location number displayed by the inspector. The values of named
variables are also bound to their names.

(procedure-object 'code) returns an inspector object containing the
procedure's code object. See the description below of code inspector
objects.

Code inspector objects. Code inspector objects contain Chez Scheme code objects.

(code-object 'type) returns the symbol code.

(code-object 'name) returns a string or #f. The name associated with a
code inspector object is the name of the variable to which the
procedure was originally bound or assigned. Since the binding of a
variable can be changed, this name association may not always be
accurate. #f is returned if the inspector cannot determine a name for
the procedure.

(code-object 'source) returns an inspector object containing the
source information attached to the code object or #f if no source
information is attached.

(code-object 'source-path) attempts to find the pathname of the file
containing the source for the lambda expression that produced the code
object. If successful, three values are returned to identify the file
and position of the application within the file: path, line, and
char. Two values, a file name and an absolute character position, are
returned if the file name is known but the named file cannot be
found. The search may be unsuccessful even if a file by the expected
name is found in the path if the file has been modified since the
source code was compiled. If no file name is known, no values are
returned. The parameter source-directories (Section 12.5) determines
the set of directories searched for source files identified by
relative path names.

(code-object 'free-count) returns the number of free variables in any
procedure for which this is the corresponding code.

(code-object 'info) returns an inspector object containing the
system's internal info structure for the code object.

(code-object 'reloc) returns an inspector object containing the
system's internal relocation entries for the code object.