Issue STRUCTURE-READ-PRINT-SYNTAX Writeup

Issue:              STRUCTURE-READ-PRINT-SYNTAX

References:         DEFSTRUCT

Related issues:     Issue DEFSTRUCT-SLOTS-CONSTRAINTS-NAME

Category:           CHANGE

Edit history:       V1, 11 May 90, Sandra Loosemore

                    V2, 29 May 90, Sandra Loosemore (cross-references)

  


Problem description:


The use of non-keywords for slot names in #S reader/printer

syntax is ugly.  The standard ought to encourage a cleaner

style.


Proposal (STRUCTURE-READ-PRINT-SYNTAX:KEYWORDS):


  (1) Deprecate the use of non-keywords to name slots in #S syntax

      for reading structures.


  (2) Require that #S syntax for printing structures print slot

      names as keywords.


Rationale:


  This encourages a cleaner style while maintaining backward

  compatibility for users.

   

Current Practice:


  Some implementations already use keywords for printing

  structures, while some use unqualified symbols.  Practice among

  users is also mixed.  Many users tend to follow the example of

  whatever implementation they use most frequently.


Cost to Implementors:


  The actual implementation cost of changing the #S printer 

  is trivial, but there may be more work involved in updating 

  documentation.


Cost to Users:


   None.


Cost of non-adoption:


  See "esthetics".


Performance impact:


  Using keywords is probably slightly more efficient than using

  unqualified symbols, at least on the reader end.


Benefits:


  See "esthetics".


Esthetics:


  The use of non-keywords to name slots in #s syntax is ugly.

  Technically, there is no reason now why both users and

  implementors cannot use keywords, but the fact that CLtL

  goes out of its way to explain that it's not necessary to

  do so, and that all of the examples of #s syntax use non-keywords, 

  has encouraged both users and implementors to follow this style.


Discussion:


  Proposal DEFSTRUCT-SLOTS-CONSTRAINTS-NAME:DUPLICATES-ERROR made

  it clear that DEFSTRUCT slot names must have unique

  symbol-names.  Therefore there is no possibility for conflicts

  if the slot names are printed as keyword symbols.


  Several people have voiced support for at least changing the

  examples in the standard to use keywords, to encourage both

  users and implementors to adopt the style.


  Dan Pierson says:

    Yes, YEs, YES!  I've been wanting this fix for bogus implementations

    since CLtL came out and ******lisp interpreted it the other way.