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<H2>Issue DECODE-UNIVERSAL-TIME-DAYLIGHT Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss098.htm">DECODE-UNIVERSAL-TIME-DAYLIGHT</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_dec_un.htm#decode-universal-time"><B>DECODE-UNIVERSAL-TIME</B></A> (p445)<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> 20-Jun-88, Version 1 by Pitman<P>
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30-Sep-88, Version 2 by Masinter<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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The description of <A REL=DEFINITION HREF="../Body/f_dec_un.htm#decode-universal-time"><B>DECODE-UNIVERSAL-TIME</B></A> does not say what happens with<P>
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TIME-ZONE. Since the description of <A REL=DEFINITION HREF="../Body/f_encode.htm#encode-universal-time"><B>ENCODE-UNIVERSAL-TIME</B></A> mentions that<P>
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its behavior differs depending on whether a time zone is explicitly passed,<P>
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some implementors may have assumed that <A REL=DEFINITION HREF="../Body/f_dec_un.htm#decode-universal-time"><B>DECODE-UNIVERSAL-TIME</B></A> should do<P>
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likewise.<P>
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<P>
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Even if all implementations did the same thing, it should still be clarified<P>
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whether an implementation were permitted to return dsp=NIL tz=n rather than<P>
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dsp=T tz=n+1 for time zones in which daylight savings time was believed to<P>
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be (or known to be) in effect. Currently, you cannot tell whether "NIL" for<P>
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daylight-savings-time-p means "daylight savings time is in effect" or<P>
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just "daylight savings time is not known to not be in effect".<P>
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<P>
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These tools appear to be more portable than they are.<P>
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<P>
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<B>Proposal (DECODE-UNIVERSAL-TIME-DAYLIGHT:LIKE-ENCODE):<P>
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</B><P>
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Specify that, like <A REL=DEFINITION HREF="../Body/f_encode.htm#encode-universal-time"><B>ENCODE-UNIVERSAL-TIME</B></A>, <A REL=DEFINITION HREF="../Body/f_dec_un.htm#decode-universal-time"><B>DECODE-UNIVERSAL-TIME</B></A> ignores<P>
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daylight savings information if a timezone is explicitly specified.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This makes things consistent with <A REL=DEFINITION HREF="../Body/f_encode.htm#encode-universal-time"><B>ENCODE-UNIVERSAL-TIME</B></A>.<P>
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<P>
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<B>Test Case:<P>
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</B><P>
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;; ### This test case relies on time zone not changing in real<P>
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;; ### time, in defiance of warning in note at bottom<P>
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;; ### of p445.<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/s_let_l.htm#letST"><B>LET*</B></A> ((HERE (<A REL=DEFINITION HREF="../Body/f_nth.htm#nth"><B>NTH</B></A> 8 (<A REL=DEFINITION HREF="../Body/m_mult_1.htm#multiple-value-list"><B>MULTIPLE-VALUE-LIST</B></A> (<A REL=DEFINITION HREF="../Body/f_get_un.htm#get-decoded-time"><B>GET-DECODED-TIME</B></A>)))) ;Time zone<P>
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(RECENTLY (<A REL=DEFINITION HREF="../Body/f_get_un.htm#get-universal-time"><B>GET-UNIVERSAL-TIME</B></A>))<P>
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(A (<A REL=DEFINITION HREF="../Body/f_nthcdr.htm#nthcdr"><B>NTHCDR</B></A> 7 (<A REL=DEFINITION HREF="../Body/m_mult_1.htm#multiple-value-list"><B>MULTIPLE-VALUE-LIST</B></A> (<A REL=DEFINITION HREF="../Body/f_dec_un.htm#decode-universal-time"><B>DECODE-UNIVERSAL-TIME</B></A> RECENTLY))))<P>
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(B (<A REL=DEFINITION HREF="../Body/f_nthcdr.htm#nthcdr"><B>NTHCDR</B></A> 7 (<A REL=DEFINITION HREF="../Body/m_mult_1.htm#multiple-value-list"><B>MULTIPLE-VALUE-LIST</B></A> (<A REL=DEFINITION HREF="../Body/f_dec_un.htm#decode-universal-time"><B>DECODE-UNIVERSAL-TIME</B></A> RECENTLY HERE)))))<P>
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(<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> A B (<A REL=DEFINITION HREF="../Body/f_equal.htm#equal"><B>EQUAL</B></A> A B)))<P>
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<P>
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Under this proposal, this would return ((T 5) (<A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> 5) <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>) in EDT, for example. <P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Symbolics Genera, Symbolics Cloe, Lucid 3.0 and Envos Medley seem to <P>
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implement this proposal. Some other implementations do not.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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The cost of changing this should be trivial.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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This feature is already not well-defined since no portable program can rely<P>
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on the current behavior, so the cost is small.<P>
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<P>
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<B>Cost of Non-Adoption:<P>
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</B><P>
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The time primitives are considerably less useful if this point is not<P>
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clearly spelled out.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The cost of non-adoption would be avoided.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Anything that improves the intelligibility of language primitives improves<P>
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language aesthetics.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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An alternative would be to specify that, unlike <A REL=DEFINITION HREF="../Body/f_encode.htm#encode-universal-time"><B>ENCODE-UNIVERSAL-TIME</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_dec_un.htm#decode-universal-time"><B>DECODE-UNIVERSAL-TIME</B></A> treats daylight savings information the same<P>
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regardless of whether a time zone argument is explicitly or not. This seems <P>
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actually to be what was intended originally.<P>
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<P>
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This problem arose while trying to port Macsyma between different<P>
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Common Lisp implementations. The cleanup committee does not have<P>
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a strong opinion on this matter, as long as the behavior is specified.<P>
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<P>
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</PRE>
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