Issue DESCRIBE-UNDERSPECIFIED Writeup

Status:		Passed, as amended, Mar 89 X3J13

Forum:		Cleanup

Issue:		DESCRIBE-UNDERSPECIFIED

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)


Problem description:


 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.


Proposal DESCRIBE-UNDERSPECIFIED:DESCRIBE-OBJECT:


 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

  method.


  ARGUMENTS:


   The first argument is any Lisp object.  The second argument is a stream; it

   cannot be T or NIL.


  VALUES:


   The values returned by DESCRIBE-OBJECT are unspecified.


  REMARKS:


   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.


Rationale:


 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.


Current practice:


 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

 problem.


 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

 the case.


 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.


Benefits:


 The constraints on DESCRIBE methods are better specified, making it easier to

 write such methods properly.


Aesthetics:


 Minimal.


Discussion:


 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*.

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