Contents|Symbol Index|Up: Issues|Prev: STRUCTURE-DEFINITION-ACCESS|Next: STRUCTURE-READ-PRINT-SYNTAX

Cleanup Issue `STRUCTURE-INFO`

Status: For Internal Discussion
Category: ADDITION
References: Structures (pp305-320)

Problem Description

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.

Proposal (NEW-FUNCTIONS)

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.

Test Cases

  (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))

Rationale

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.

Current Practice

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.

Benefits

In addition to pretty printers, this might be of some use to programmers customizing the generic function DESCRIBE, or to people writing DESCRIBE-like facilities.

Aesthetics

No major aesthetic impact.

Discussion

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 protocol offers.

Edit History

24-Jun-88, Version 1 by Pitman and Waters