Issue: PUSH-EVALUATION-ORDER (Version 2)

[Masinter.pa@Xerox.COM]

I rewrote this proposal extensively; I removed all but the :ITEM-FIRST
proposal, extended it to include INCF, DECF, PUSHNEW, and macros which
are created with DEFINE-MODIFY-MACRO, and tried to write it as changes
to the language rather than changes to CLtL, etc. 

Is this ready for release? Moon was the only one who responded.

!

Issue:         PUSH-EVALUATION-ORDER
References:    CLtL p. 99 (generalized variables), p 270 (PUSH)
               p. 201 (INCF, DECF).
Category:      	CLARIFICATION
Edit History:  Jeff Peck, 15-Oct-1987, version 1.
               Larry Masinter, 23-Oct-87, version 2.

Problem Description:

In the form: (PUSH (ref1) (CAR (ref2)))
It is unclear whether (ref1) should be evaluated before (ref2). 

CLtL, page 99, in a discussion of generalized variable macros, states:
"Other macros that manipulate generalized variables include ... PUSH....
 Macros that manipulate generalized variables must guarentee the
"obvious"
 semantics: subforms of generalized-variable references are evaluated
...
 in exactly the same order as they appear in the *source* program."

That is, the sub-forms of Place should be evaluated once, left to right.

"The expansion of these macros must consist of code that follows these
 rules or has the same effect as such code.  This is accomplished by
 introducing temporary variables bound to the subforms of the
reference."
                                               â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??â??

This paragraph and a discussion of SETF on the previous pages may also
be
interpreted as requiring that *all* argument forms of such macros should
be evaluated once, in source order, left to right.


However, CLtL, page 270 states:
 "The effect of (PUSH Item Place) is roughly equivalent to
    (SETF Place (CONS Item Place))
  except that the latter would evaluate any subforms of Place twice
  while PUSH takes care to evaluate them only once."

That is, the effect of the form (PUSH Item Place) is to evaluate 
(SETF Place (CONS Item Place)) but with subforms of Place only evaluated
once.

Place and Item appear in different order in the PUSH form and the
indicated equivalent SETF form.  Should the PUSH form have primacy over
the
obvious SETF form with respect to the left-to-right evaluation?

Are all subforms in a macro argument list guarenteed to be evaluated in
order, or only those subforms representing generalized variable
references?

The same question arises for other forms which manipulate generalized
variables, e.g., PUSHNEW, INCF, DECF, and those defined with
DEFINE-MODIFY-MACRO.


Test Case:

(LET ((REF2 (LIST '())))
 (PUSH (PROGN (PRINC "1") 'REF-1)
       (CAR (PROGN (PRINC "2") REF2))))

If the subforms evaluate in left-to-right order, this will print 12
rather than 21.

Proposal: PUSH-EVALUATION-ORDER:ITEM-FIRST

Explicitly state that for the macros that manipulate generalized
variables (PUSH, PUSHNEW, INCF, DECF, SHIFTF, ROTATEF, PSETF, SETF and
those defined with DEFINE-MODIFY-MACRO) the subforms of the macro
(including but not limited to those of the generalized variable
reference) are evaluated exactly as many times as they appear in the
source program, and in exactly the same order as they appear in the
source program.

For example, PUSH is expected to behave as if described as:

(PUSH Item Place) is generally equivalent to (SETF Place (CONS Item
Place))
  except that the subforms of Place are evaluated only once, and Item
  is evaluated before Place."

The phase "subforms of the reference" which appears several times in
CLtL should be made more specific to be "subforms of the
[generalized-variable manipulating macro] arguments".

Rationale:

This is the unstated intention of the page 97-100 discussion of
generalized-variable referencing macros, and indeed the intended
definition of "obvious semantics" for all macros.

Current practice:

Many implementations do not currently follow this evaluation order. In
the form (PUSH Item Place), Lucid, Franz, Kyoto and Xerox evaluate Place
then Item. Symbolics evaluates Item then Place.


For example, in Franz:

(macroexpand '(push (ref1) (car (ref2))))

    (LET* ((#:G8 (REF2))
	   (#:G7 (CONS (REF1) (CAR #:G8))))
      (EXCL::.INV-CAR #:G8 #:G7)) 
    
In Symbolics Common Lisp, it returns:
    
    (LET* ((#:G5 (REF1))
	   (#:G4 (REF2)))
      NIL
      (SYS:RPLACA2 #:G4 (VALUES (CONS #:G5 (CAR #:G4)))))


Adoption Cost:

Minimal, PUSH could simply be defined by the appropriate macro.

Cost of non-adoption:

Obvious programs may be non-portable, although it should be rare that
order of evaluation will effect actual operation. 

Benefits:

The implementation and semantics of PUSH become obvious to all.  

Esthetics:

Common Lisp defines order of evaluation as left-to-right; this
clarification ensures consistency across the language. 


Discussion:

David Moon (Symbolics) argues that the unstated intention of page 99
is the definition of the language, while admitting that:

"The quoted paragraphs could be taken to restrict order of evaluation
only
   of the subforms of (CAR (ref2)), not all of the subforms of the PUSH
form."

No performance impact is expected; while some macro expansions may
appear to be more verbose, most compilers deal reasonably with the
required order of evaluation.