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108 lines
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<H2>Issue DRIBBLE-TECHNIQUE Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss140.htm">DRIBBLE-TECHNIQUE</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> (p443)<P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> 06-Dec-87, Version 1 by Pitman<P>
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14-Feb-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 intent and effect of <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> is not very clearly specified. Users have<P>
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complained that <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> behaves quite differently in different implementations.<P>
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<P>
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<B>Proposal (DRIBBLE-TECHNIQUE:MAKE-EXPLICITLY-VAGUE):<P>
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</B><P>
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Clarify that <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> is intended primarily for interactive debugging and that<P>
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its effect cannot be relied upon from programs.<P>
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<P>
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<B>Test Case:<P>
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</B><P>
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#1: (<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>PROGN</B></A> (<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> "temp")<P>
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(<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'FOO)<P>
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(<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A>))<P>
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<P>
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#2: (<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> "temp")<P>
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(<A REL=DEFINITION HREF="../Body/s_progn.htm#progn"><B>PROGN</B></A> (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'FOO)<P>
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(<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A>)<P>
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(<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#princ"><B>PRINC</B></A> 'BAR)) <P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Users of <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> have been surprised about how differently <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> behaves in<P>
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different implementations. This makes the status quo much more explicit and will<P>
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lead to less surprise.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Some implementations implement <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> as a function which simply assigns<P>
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streams such as <A REL=DEFINITION HREF="../Body/v_debug_.htm#STstandard-outputST"><B>*STANDARD-OUTPUT*</B></A> to broadcast streams (or the approximate<P>
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equivalent thereof). Even within this camp, there is not widespread agreement<P>
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about which streams are affected. However, typically test case #1 will capture<P>
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the output of (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'FOO) in the file "temp" and will execute (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'BAR) but<P>
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not capture its output.<P>
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<P>
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Some implementations (eg, Symbolics) push to a recursive command loop when<P>
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<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> is called with an argument, and return from that command loop when<P>
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<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> is called with no argument. In these implementations, test case #1 will<P>
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enter a recursive command loop upon the first call to <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> and will not<P>
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execute the (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'FOO) until (<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A>) is done interactively. When the second<P>
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(<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A>) in test case #1 is executed, <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> will complain that output is not<P>
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currently being recorded. In test case #2, the output of (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'FOO) will be<P>
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captured, but the form (<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A> 'BAR) will never be executed because (<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A>)<P>
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does not return normally (it throws).<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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None. This is just a clarification.<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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Anyone who currently uses <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> in code that is believed to be portable is<P>
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kidding himself. If such code works in some implementations, it can continue to<P>
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work.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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Users will be aware that they cannot rely on <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> in portable code.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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No apparent effect.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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<A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A>, like other environment features, is a <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> interface to a<P>
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non-standard feature. As such, there is some question as to its place in the<P>
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ANSI <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>. However, if <A REL=DEFINITION HREF="../Body/f_dribbl.htm#dribble"><B>DRIBBLE</B></A> remains in the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>, its behavior is made<P>
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explicitly vague by this proposal.<P>
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<P>
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</PRE>
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