Status: Passed, as amended, Mar 89 X3J13
References: CLtL p441-2
88-002R, DESCRIBE function
Category: CHANGE, ADDITION
Edit history: Version 1, 10-Mar-89, Kim A. Barrett
Version 2, 9-Apr-89, Masinter (as per Mar 89 X3J13)
The CLOS Specification (X3J13 Document 88-002R) changes the definition of the
function DESCRIBE, making it a generic function. However, it does not specify
any of the protocol needed to make user-defined methods interact properly to
produce some of the effects mentioned in CLtL. For example, CLtL says that
sometimes the method for describing an object will involve describing
something that it finds inside the object, and that such recursive
descriptions are indented appropriately. How do user-written methods achieve
this indentation? Must they arrange for the indentation explicitly, or is
there some automatic mechanism that handles it?
The new specification does not easily lend itself to certain kinds of features
which some implementations have included in their versions of DESCRIBE, such
as analogues to the printer's depth limits (*PRINT-DEPTH*) and circular
structure detection during recursion (*PRINT-CIRCLE*).
In addition, DESCRIBE does not take a stream argument, instead always doing
output to *STANDARD-OUTPUT*. This means that a program which wants to use
DESCRIBE to output some information to a particular stream must rebind
*STANDARD-OUTPUT* around the call to DESCRIBE. This is a nuisance, and is
also potentially a bad idea in implementations which have interrupts and such.
Remove the section of 88-002R which specifies that DESCRIBE is a generic
function. Modify DESCRIBE to accept an optional second stream argument, which
defaults to *STANDARD-OUTPUT*, and which is handled in the same way as the
stream argument to PRINT (that is, permitting arguments of NIL and T).
The value of this argument is the stream which output will be directed to.
Specify that DESCRIBE is implemented in terms of the generic function
DESCRIBE-OBJECT, described below.
DESCRIBE-OBJECT object stream [Generic Function]
The generic function DESCRIBE-OBJECT writes a description of an object to a
stream. The function DESCRIBE-OBJECT is called by the DESCRIBE function; it
should not be called by the user.
Each implementation is required to provide a method on the class
STANDARD-OBJECT and methods on enough other classes so as to ensure that
there is always an applicable method. Implementations are free to add
methods for other classes. Users can write methods for DESCRIBE-OBJECT for
their own classes if they do not wish to inherit an implementation-supplied
The first argument is any Lisp object. The second argument is a stream; it
cannot be T or NIL.
The values returned by DESCRIBE-OBJECT are unspecified.
Methods on DESCRIBE-OBJECT may recursively call DESCRIBE. Indentation,
depth limits, and circularity detection are all taken care of automatically,
provided that each method handles exactly one level of structure and calls
DESCRIBE recursively if there are more structural levels.
If this rule is not obeyed, the results are undefined.
In some implementations the stream argument passed to a DESCRIBE-OBJECT
method is not the original stream, but is an intermediate stream that
implements parts of DESCRIBE. Methods should therefore not depend on the
identity of this stream.
This proposal was closely modeled on the CLOS description of PRINT-OBJECT,
which was well thought out and provides a great deal of functionality and
implementation freedom. The same implementation techniques applicable to
PRINT-OBJECT will be applicable to DESCRIBE-OBJECT.
The reason for making the return values for DESCRIBE-OBJECT unspecified is to
avoid forcing users to include explicit (VALUES) in all their methods.
DESCRIBE will take care of that.
Probably nobody does precisely what this proposal suggests.
Cost to Implementors:
A fair amount of work may be required, since every method/subfunction of
DESCRIBE in an implementation may need at least some fixing to be in line with
this proposal. On the other hand, that work may already be needed in order to
conform to 88-002R, and this proposal may make the conversion easier by
simplifying the translation of an existing implementation of DESCRIBE.
Cost to Users:
Any users who are using an implementation which supports the current CLOS
specification of DESCRIBE and have defined their own methods will have to
change them. CLOS is sufficiently recent that this probably isn't a big
Those users who have made use of implementation-specific hooks into DESCRIBE
to define their own methods will likely have to change, but that was already
Users who are currently binding *STANDARD-OUTPUT* around calls to DESCRIBE may
wish to change their code.
Cost of non-adoption:
Portable DESCRIBE methods may be difficult to write because the protocol they
must follow is insufficiently specified.
The constraints on DESCRIBE methods are better specified, making it easier to
write such methods properly.
An additional change which is not included in the present proposal would be to
make the syntax of DESCRIBE and DESCRIBE-OBJECT be
DESCRIBE object &optional stream &key
DESCRIBE-OBJECT object stream &key
allowing implementation-specific extensions to the arguments. A possible
standard keyword argument is :VERBOSE, which might be used to specify how much
output to produce.
It might be desirable to define some new describe control variables analogous
to the printer control variables, ie. *DESCRIBE-LEVEL* and *DESCRIBE-CIRCLE*,
and possibly *DESCRIBE-LENGTH*.