Forum: Public Review
References: Loosemore's public review comment #25 (X3J13/92-1325)
Norvig's public review comment #8.1 (X3J13/92-2208)
Pitman's public review comment #7 (X3J13/92-2707)
DEFSTRUCT :PRINT-FUNCTION option (p 8-10 and 8-11)
PRINT-OBJECT (p 22-55)
Edit history: 24 Dec 1992, Version 1 by Loosemore
01 Jan 1993, Version 2 by Loosemore (fix references,
add method signature for print-object)
Status: Parts (2),(3),(4) only passed 6-1, March 1993
Supplying the :PRINT-FUNCTION option to DEFSTRUCT is supposed to
be equivalent to defining a PRINT-OBJECT method, but the two can't
be entirely equivalent since the :PRINT-FUNCTION takes a third
argument (the current recursive printing depth) that the PRINT-OBJECT
method does not.
It is also not clear whether the inheritance of structure
:PRINT-FUNCTIONs by the :INCLUDE option works in such a way that
one may use CALL-NEXT-METHOD and the like.
(1) Remove the :PRINT-FUNCTION option to DEFSTRUCT.
(2) Add a new DEFSTRUCT option, :PRINT-OBJECT, and document it
This option can be used only if :TYPE is not supplied. The
argument to :PRINT-OBJECT should be in a form acceptable to
FUNCTION, and is used when printing structures of this type.
The function is called with two arguments: the structure to
be printed and the stream to print to.
Supplying (:PRINT-OBJECT fn) in a DEFSTRUCT definition for
a structure named s is equivalent to executing
(defmethod print-object ((object s) stream)
(funcall (function fn) object stream))
The :PRINT-OBJECT option can also be supplied without an
argument. This is equivalent to defining a PRINT-OBJECT
method which calls a function that prints the structure in
the default #S notation. [Kent Pitman's comment #7 proposes
to name this function PRINT-OBJECT-USING-SLOTS.]
If no :PRINT-OBJECT option is provided, then DEFSTRUCT does
not generate a PRINT-OBJECT method specialized on the named
(3) Change places in chapter 22 that refer to "print functions"
to refer to "PRINT-OBJECT methods" instead.
(4) Add to the "Method Signatures" section for PRINT-OBJECT:
print-object (object structure-object) stream
In the "Description" section for PRINT-OBJECT, state that the
method on the class structure-object prints the object in the
default #S notation as described in section 220.127.116.11.
The addition of PRINT-OBJECT and the pretty-printing protocol makes
it clear that *PRINT-LEVEL* truncation is handled automatically by
the printer, so the third argument to :PRINT-FUNCTION functions is
Simply redefining :PRINT-FUNCTION to take functions of two arguments
rather than of three would break existing programs. Replacing it
with a new option with a different name permits implementations to
continue to support the old :PRINT-FUNCTION as an extension, and/or
issue diagnostics about obsolete usages.
Specifying the behavior of :PRINT-OBJECT entirely in terms of
PRINT-OBJECT and normal method definition means that the standard
doesn't have to discuss :PRINT-OBJECT functions as a special case.
This makes the standard conceptually simpler.
Nobody ever does anything useful with the third argument to
:PRINT-FUNCTION functions anyway.
Cost to implementors:
Some implementations will doubtless have to change, but the addition
of the PRINT-OBJECT and pretty-printer protocols already means that
many implementations will have to rework their printers anyway.
Cost to users:
This is an incompatible change, but one that can be detected
easily. Implementations may continue to support :PRINT-FUNCTION
as an extension for backward compatibility.
I like it.
The references to :PRINT-FUNCTION in the structures and printers
chapters will have to be changed.
Barrett notes that the description for the PRINT-OBJECT method on
standard-object doesn't say anything about how it actually prints
the objects. Presumably this is covered by section 18.104.22.168, which
says that "other objects are printed in an implementation-dependent