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Issue: STRUCTURE-INFO (Version 1)
- To: CL-Cleanup@SAIL.STANFORD.EDU
- Subject: Issue: STRUCTURE-INFO (Version 1)
- From: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>
- Date: Fri, 24 Jun 88 13:59 EDT
- Cc: DICK@WHEATIES.AI.MIT.EDU
Please be sure to cc Dick@WHEATIES.AI.MIT.EDU in any correspondence
on this issue since he's not on CL-Cleanup.
References: Structures (pp305-320)
Edit history: 24-Jun-88, Version 1 by Pitman and Waters
Status: For Internal Discussion
There is no portable way to determine whether something is a structure
and no portable way to ask abstractly what the contents of a structure
are without understanding that particular structure.
Introduce these new functions:
STRUCTUREP thing [Function]
Predicate returns true if the given object is represented as
a structure (without the use of DEFSTRUCT's :TYPE option).
Otherwise, returns false.
STRUCTURE-CONTENTS structure [Function]
Returns an alist of field names (as keywords) and field values showing
the contents of the STRUCTURE. The STRUCTURE may be any object
for which STRUCTUREP returns true.
(DEFSTRUCT (FRED (:TYPE VECTOR)) (A 1) (B 2) (C 3))
(SETQ FRED (MAKE-FRED))
(STRUCTUREP FRED) => NIL
(VECTORP FRED) => T
(STRUCTURE-CONTENTS FRED) is undefined
(DEFSTRUCT WILMA (A 1) (B 2) (C 3))
(SETQ WILMA (MAKE-WILMA))
(STRUCTUREP WILMA) => T
(STRUCTURE-CONTENTS WILMA) => ((:A . 1) (:B . 2) (:C . 3))
STRUCTUREP is important for implementing a portable pretty printer in
a number of ways. STRUCTURE-CONTENTS is less important, but is necessary
if a portable pretty printer is to print structures in #S notation.
Some implementations, such as Symbolics Genera, provide
(TYPEP x 'STRUCTURE).
Most implementations do not go so far as to provide STRUCTUREP, however.
Probably no implementations offer STRUCTURE-CONTENTS in any exported form.
Cost to Implementors:
Since the standard printer must access this same information when
printing structures, it must be very trivial to provide this functionality.
Cost to Users:
None. This is an upward compatible change.
Cost of Non-Adoption:
Portable pretty printers would not be able to pretty-print structure
objects in #S notation.
In addition to pretty printers, this might be of some use to programmers
customizing the generic function DESCRIBE, or to people writing
No major aesthetic impact.
Dick Waters submitted a request for changes of this kind in a letter to
X3J13 dated June 14, 1988. Pitman wrote up the request formally.
If the CLOS meta-object protocol becomes a standard part of CL, this
facility would not be necessary. However, if that protocol is in any way
optional, it would be useful to have this interface since it can be
implemented using considerably less powerful primitives than that