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<TITLE>CLHS: Issue PATHNAME-PRINT-READ Writeup</TITLE>
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<H2>Issue PATHNAME-PRINT-READ Writeup</H2>
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<PRE><B>Status:</B> Passed, as amended, Jun89 X3J13<P>
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<P>
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<B>Issue:</B> <A HREF="iss260.htm">PATHNAME-PRINT-READ</A><P>
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<B>References:</B> File System Interface (pp409-427)<P>
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<B>Category:</B> CHANGE/ADDITION<P>
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<B>Edit history:</B> 21-Oct-88, Version 1 by Pitman<P>
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3-Jul-89, 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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Although pathnames are required to print re-readably, there is no<P>
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standardized representation for pathnames and so no standardized<P>
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way in which they should print.<P>
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<P>
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Further, it is common in programs to want pathnames to print in<P>
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their file-system specific format.<P>
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<P>
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<B>Proposal (PATHNAME-PRINT-READ:SHARPSIGN-P):<P>
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</B><P>
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Define the reader syntax #P"..." to be equivalent to <P>
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#.(<A REL=DEFINITION HREF="../Body/f_pars_1.htm#parse-namestring"><B>PARSE-NAMESTRING</B></A> "...").<P>
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<P>
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Define that when <A REL=DEFINITION HREF="../Body/v_pr_esc.htm#STprint-escapeST"><B>*PRINT-ESCAPE*</B></A> is true, the syntax #P"..." is<P>
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how a <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A> should be printed by <A REL=DEFINITION HREF="../Body/f_wr_pr.htm#write"><B>WRITE</B></A> (and hence by <A REL=DEFINITION HREF="../Body/f_wr_pr.htm#prin1"><B>PRIN1</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_wr_pr.htm#print"><B>PRINT</B></A>, etc.). The "..." is the <A REL=DEFINITION HREF="../Body/f_namest.htm#namestring"><B>namestring</B></A> representation of the<P>
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<A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A>.<P>
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<P>
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Define that when <A REL=DEFINITION HREF="../Body/v_pr_esc.htm#STprint-escapeST"><B>*PRINT-ESCAPE*</B></A> is <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, <A REL=DEFINITION HREF="../Body/f_wr_pr.htm#write"><B>WRITE</B></A> writes a <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A><P>
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object P by writing (<A REL=DEFINITION HREF="../Body/f_namest.htm#namestring"><B>NAMESTRING</B></A> p) instead.<P>
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<P>
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<B>Test Case:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/f_pars_1.htm#parse-namestring"><B>PARSE-NAMESTRING</B></A> "foo.lisp")<P>
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=> #P"foo.lisp"<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/f_format.htm#format"><B>FORMAT</B></A> <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> "Written to ~A." #P"foo.bin")<P>
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=> "Written to foo.bin."<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/f_typep.htm#typep"><B>TYPEP</B></A> #P"foo.bin" '<A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A>)<P>
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=> T<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This satisfies the stated goals.<P>
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<P>
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[For :ESCAPE T] It will not be possible to make the printed<P>
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<A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A> printed representation totally portable because of<P>
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variations in file systems, but for different Common Lisp<P>
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implementations on the same file system, or for Common Lisp<P>
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systems running on file systems having compatible syntax,<P>
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portability would be improved by this specification.<P>
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<P>
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Also, some implementations (eg, Symbolics Genera) use<P>
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specialized representations for pathnames on different file<P>
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systems. Eg, an MSDOS <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A> is of type MSDOS-PATHNAME,<P>
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not just type <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A>. #S(<A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A> ...) is not only more<P>
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verbose than necessary but might be misleading to some users<P>
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because the object created will not have a <A REL=DEFINITION HREF="../Body/f_tp_of.htm#type-of"><B>TYPE-OF</B></A> <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A>.<P>
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<P>
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[For :ESCAPE NIL] Printing the <A REL=DEFINITION HREF="../Body/f_namest.htm#namestring"><B>namestring</B></A> of a <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A> is<P>
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a common operation and it is convenient to have a shorthand<P>
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for doing it. Further, some implementations may be able to<P>
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optimize the presentation of a <A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A> in this mode by<P>
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printing it without actually consing the string.<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 implements the proposed behavior.<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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Fairly minor changes to the <A REL=DEFINITION HREF="../Body/t_rdtabl.htm#readtable"><B>readtable</B></A> and the printer.<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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Users who now use the non-portable syntax #S(...) in order<P>
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to enter literal pathnames might have to change. [However,<P>
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implementations would be free to continue to support this<P>
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read syntax for compatibility.]<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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Portability of code and data involving pathnames within a<P>
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given file system (or between suitably similar file systems)<P>
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would be hampered needlessly.<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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The #P syntax is pretty and hides unimportant details.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Pitman supports this change.<P>
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<P>
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-----<P>
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Summary of discussion on CL-Cleanup:<P>
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<P>
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EB noted that Lucid CL implements the proposed behavior and that there<P>
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is cost to users who define their own #P read macro. He weakly supports<P>
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the proposal but wishes someone had pursued a `generic pathnames' proposal.<P>
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<P>
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Pierson noted that KCL uses #"...", but that this collides with proposed<P>
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syntax for Dick Waters' pretty printer. He also thinks #P is better<P>
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because it is already more widely used for that purpose.<P>
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<P>
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Masinter noted that Envos Medley prints pathnames with the syntax<P>
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#.(<A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>pathname</B></A> "asdf"), which he thinks is not as pretty as #P"asdf"<P>
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but currently more portable.<P>
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<P>
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KMP and JonL raised the issues that #. has the disadvantage that it must<P>
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be parsed by the full Lisp engine, while #P can be parsed by something<P>
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simpler. Permitting #. leaves a gaping hole for trojan horses, and<P>
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also requires the presence of the evaluator in a delivery system.<P>
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<P>
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MLY, GSB, Peirson, and IIM argued for not using up an extra dispatch<P>
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character.<P>
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<P>
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MLY suggested #S(<A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A> <A REL=DEFINITION HREF="../Body/f_namest.htm#namestring"><B>namestring</B></A> [optional-host]).<P>
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<P>
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IIM noted they use #.(<A REL=DEFINITION HREF="../Body/a_pn.htm#pathname"><B>PATHNAME</B></A> <A REL=DEFINITION HREF="../Body/f_namest.htm#namestring"><B>namestring</B></A> host) because different file<P>
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systems have different parsing conventions. <P>
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<P>
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</PRE>
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