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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="../Issues/iss051_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss052.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss053_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue CLOSE-CONSTRUCTED-STREAM Writeup</H2>
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<PRE><B>Status:</B> DRAFT<P>
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<B>Forum:</B> Cleanup<P>
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<B>Issue:</B> <A HREF="iss052.htm">CLOSE-CONSTRUCTED-STREAM</A><P>
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<P>
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<B>References:</B> Close (CLtL p. 332), <A REL=DEFINITION HREF="../Body/m_w_op_1.htm#with-open-stream"><B>WITH-OPEN-STREAM</B></A> <P>
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(CLtL p 330), <A REL=DEFINITION HREF="../Body/f_mk_syn.htm#make-synonym-stream"><B>make-synonym-stream</B></A>, <A REL=DEFINITION HREF="../Body/f_mk_bro.htm#make-broadcast-stream"><B>make-broadcast-stream</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_mk_con.htm#make-concatenated-stream"><B>make-concatenated-stream</B></A>, <A REL=DEFINITION HREF="../Body/f_mk_two.htm#make-two-way-stream"><B>make-two-way-stream</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_mk_ech.htm#make-echo-stream"><B>make-echo-stream</B></A>, <A REL=DEFINITION HREF="../Body/f_mk_s_1.htm#make-string-input-stream"><B>make-string-input-stream</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_mk_s_2.htm#make-string-output-stream"><B>make-string-output-stream</B></A>, <A REL=DEFINITION HREF="../Body/m_w_in_f.htm#with-input-from-string"><B>with-input-from-string</B></A>,<P>
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with-output-from-string<P>
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<P>
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Related issues: <A HREF="iss053.htm">CLOSED-STREAM-OPERATIONS</A><P>
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<P>
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<B>Category:</B> Clarification/Change<P>
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<P>
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<B>Edit history:</B> Masinter, 6-Jan-89, Version 1<P>
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Masinter, 12-Jan-89, Version 2<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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First some terminology:<P>
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<P>
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A "composite" stream is one created with <A REL=DEFINITION HREF="../Body/f_mk_syn.htm#make-synonym-stream"><B>MAKE-SYNONYM-STREAM</B></A>, <P>
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<A REL=DEFINITION HREF="../Body/f_mk_bro.htm#make-broadcast-stream"><B>MAKE-BROADCAST-STREAM</B></A>, <A REL=DEFINITION HREF="../Body/f_mk_con.htm#make-concatenated-stream"><B>MAKE-CONCATENATED-STREAM</B></A>, <A REL=DEFINITION HREF="../Body/f_mk_two.htm#make-two-way-stream"><B>MAKE-TWO-WAY-STREAM</B></A>, <P>
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<A REL=DEFINITION HREF="../Body/f_mk_ech.htm#make-echo-stream"><B>MAKE-ECHO-STREAM</B></A>. <P>
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<P>
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The "constituents" of a composite stream are the streams it points to:<P>
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the <A REL=DEFINITION HREF="../Body/f_symb_5.htm#symbol-value"><B>SYMBOL-VALUE</B></A> of the symbol given to <A REL=DEFINITION HREF="../Body/f_mk_syn.htm#make-synonym-stream"><B>MAKE-SYNONYM-STREAM</B></A><P>
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the streams given to <A REL=DEFINITION HREF="../Body/f_mk_bro.htm#make-broadcast-stream"><B>MAKE-BROADCAST-STREAM</B></A> or <A REL=DEFINITION HREF="../Body/f_mk_con.htm#make-concatenated-stream"><B>MAKE-CONCATENATED-STREAM</B></A><P>
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the input-stream and output-stream given to <A REL=DEFINITION HREF="../Body/f_mk_two.htm#make-two-way-stream"><B>MAKE-TWO-WAY-STREAM</B></A> or <A REL=DEFINITION HREF="../Body/f_mk_ech.htm#make-echo-stream"><B>MAKE-ECHO-STREAM</B></A>.<P>
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<P>
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A "constructed" stream is either a composite stream or one created with <P>
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<A REL=DEFINITION HREF="../Body/f_mk_s_1.htm#make-string-input-stream"><B>MAKE-STRING-INPUT-STREAM</B></A>, <A REL=DEFINITION HREF="../Body/f_mk_s_2.htm#make-string-output-stream"><B>MAKE-STRING-OUTPUT-STREAM</B></A>, <A REL=DEFINITION HREF="../Body/m_w_in_f.htm#with-input-from-string"><B>WITH-INPUT-FROM-STRING</B></A>,<P>
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WITH-OUTPUT-FROM-STRING.<P>
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<P>
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The function "CLOSE" is currently described in 21.3, which starts "This<P>
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section contains discussion of only those operations that are common to all<P>
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streams." This would seem to imply that they apply to constructed streams. <P>
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<P>
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The definition of <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> "The argument must be a stream. No further input/output<P>
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operations may be performed on it. ... " It further states "... and it is <P>
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permissible to close an already closed stream."<P>
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<P>
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However, the behavior on the constructed streams is not well defined, and<P>
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implementations (apparently) differ.<P>
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<P>
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<B>Proposal (CLOSE-CONSTRUCTED-STREAM:IS-ERROR): <P>
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</B><P>
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It "is an error" to call <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> on a constructed stream. <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> might signal an<P>
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error, or the result of <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> could cause the constituent streams to be closed,<P>
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etc, but the effect is implementation-dependent.<P>
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<P>
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Proposal: (CLOSE-CONSTRUCTED-STREAM:SIGNALS-ERROR)<P>
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<P>
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Calling <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> on a constructed stream signals an error.<P>
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<P>
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<B>Proposal (CLOSE-CONSTRUCTED-STREAM:ARGUMENT-STREAM-ONLY):<P>
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</B><P>
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The effect of <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> on a constructed stream is to close the "argument" stream<P>
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only. No i/o operations are permitted after the call to <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> on the stream<P>
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given to CLOSE; There is no effect on the constituents of composite streams.<P>
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<P>
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For streams created with <A REL=DEFINITION HREF="../Body/f_mk_s_2.htm#make-string-output-stream"><B>MAKE-STRING-OUTPUT-STREAM</B></A>, the result of<P>
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<A REL=DEFINITION HREF="../Body/f_get_ou.htm#get-output-stream-string"><B>GET-OUTPUT-STREAM-STRING</B></A> is unspecified after CLOS. For composite streams,<P>
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the call to <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> has no effect on any of the constituent streams.<P>
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<P>
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The "links" to the constituents may be broken; if the proposal in <A HREF="iss327.htm">STREAM-ACCESS</A> <P>
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passes, the results of the accessor functions there are unspecified after the<P>
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call to <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A>.)<P>
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<P>
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Proposal: (CLOSE-CONSTRUCTED-STREAM:CLOSE-CONSTITUENTS)<P>
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<P>
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<A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> first closes its argument; it then operates recursively on the constituents<P>
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of composite streams.<P>
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<P>
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<B>Examples:<P>
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</B><P>
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>>not written; sorry<<<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Specifying seems better than not saying what happens, even if it is<P>
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"implementation-dependent".<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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Implementations seem to vary widely.<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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Varying, depending on the current level of conformance. Making the changes<P>
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themselves is probably trivial (to the "close" <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> for each kind of<P>
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constructed stream type) but it is possible that system code might depend<P>
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on the behavior.<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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Likely small; we've not found any good uses for <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> on composite streams.<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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Continued minor ambiguity<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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near zero<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The language would be more well specified.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Well-specified languages are usually easier to deal with.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Signalling an error is reasonable if no Common Lisp program ever needs to<P>
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call <A REL=DEFINITION HREF="../Body/f_close.htm#close"><B>CLOSE</B></A> on a composite stream. We could not come up with a legitimate<P>
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case where you wouldn't instead close the underlying stream if that's what<P>
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you wanted.<P>
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<P>
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Allowing the result to be implementation dependent is the "status quo". <P>
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<P>
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ARGUMENT-STREAM-ONLY is probably the "safest" in that it makes it<P>
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harder to accidentally close a stream that wasn't intended. It<P>
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seems counterintuitive and yet it probably wouldn't be harmful<P>
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if it were well-defined that this was what it did.<P>
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<P>
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CLOSE-CONSTITUENTS could be an incompatible change for some<P>
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implementations. It makes more sense for things like broadcast streams<P>
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(which are usually non-interactive) than it does for echo streams<P>
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(which are sometimes interactive).<P>
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<P>
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</PRE>
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