Issue: DECODE-UNIVERSAL-DAYLIGHT (Version 1)

[Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>]

Issue:        DECODE-UNIVERSAL-TIME-DAYLIGHT
References:   DECODE-UNIVERSAL-TIME (p445)
Category:     CLARIFICATION
Edit history: 20-Jun-88, Version 1 by Pitman
Status:	      For Internal Discussion

Problem Description:

  The description of DECODE-UNIVERSAL-TIME does not say what happens with
  TIME-ZONE. Since the description of ENCODE-UNIVERSAL-TIME mentions that
  its behavior differs depending on whether a time zone is explicitly passed,
  some implementors may have assumed that DECODE-UNIVERSAL-TIME should do
  likewise.

  Even if all implementations did the same thing, it should still be clarified
  whether an implementation were permitted to return dsp=NIL tz=n rather than
  dsp=T tz=n+1 for time zones in which daylight savings time was believed to
  be (or known to be) in effect. Currently, you cannot tell whether "NIL" for
  daylight-savings-time-p means "daylight savings time is in effect" or
  just "daylight savings time is not known to not be in effect".

  These tools appear to be more portable than they are.

Proposal (DECODE-UNIVERSAL-TIME-DAYLIGHT:LIKE-ENCODE):

  Specify that, like ENCODE-UNIVERSAL-TIME, DECODE-UNIVERSAL-TIME ignores
  daylight savings information if a timezone is explicitly specified.

  Rationale:

    This makes things consistent with ENCODE-UNIVERSAL-TIME.

Proposal (DECODE-UNIVERSAL-TIME-DAYLIGHT:UNLIKE-ENCODE):

  Specify that, unlike ENCODE-UNIVERSAL-TIME, DECODE-UNIVERSAL-TIME treats
  daylight savings information the same regardless of whether a time zone
  argument is explicitly or not.

  Rationale:

    Although inconsistent with ENCODE-UNIVERSAL-TIME, this seems
    to be what was intended.

    [Maybe someone can suggest a better motivation? -kmp]

Test Case:

  ;; ### Kludge: This test case relies on time zone not changing in real
  ;; ###   time, in blatant defiance of warning in note at bottom
  ;; ###   of p445.
  (LET* ((HERE (NTH 8 (MULTIPLE-VALUE-LIST (GET-DECODED-TIME)))) ;Time zone
	 (RECENTLY (GET-UNIVERSAL-TIME))
	 (A (NTHCDR 7 (MULTIPLE-VALUE-LIST (DECODE-UNIVERSAL-TIME RECENTLY))))
	 (B (NTHCDR 7 (MULTIPLE-VALUE-LIST (DECODE-UNIVERSAL-TIME RECENTLY HERE)))))
    (LIST A B (EQUAL A B)))

Current Practice:

  Symbolics Genera and Symbolics Cloe return ((T 5) (NIL 5) NIL) in EDT.

Cost to Implementors:

  The cost of changing this should be trivial.

Cost to Users:

  This feature is already not well-defined since no portable program can rely
  on the current behavior, so the cost is effectively zero.

Cost of Non-Adoption:

  The time primitives are considerably less useful if this point is not
  clearly spelled out.

Benefits:

  The cost of non-adoption would be avoided.

Aesthetics:

  Anything that improves the intelligibility of language primitives improves
  language aesthetics.

Discussion:

  Pitman wrote this proposal after running into this problem while trying
  to port Macsyma between different Common Lisp implementations. He doesn't
  have a strong preference on which proposal should be adopted, but thinks
  this issue should be nailed down.