Issue: PATHNAME-PRINT-READ (Version 1)

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

 From my list of "pending issues" distributed at the last X3J13 meeting...

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Issue:        PATHNAME-PRINT-READ
References:   File System Interface (pp409-427)
Category:     CHANGE/ADDITION
Edit history: 21-Oct-88, Version 1 by Pitman
Status:	      For Internal Discussion

Problem Description:

  Although pathnames are required to print re-readably, there is no
  standardized representation for pathnames and so no standardized
  way in which they should print.

  Further, it is common in programs to want pathnames to print in
  their file-system specific format.

Proposal (PATHNAME-PRINT-READ:SHARPSIGN-P):

  Define the reader syntax #P"..." to be equivalent to 
  #.(PARSE-NAMESTRING "...").

  Define that when *PRINT-ESCAPE* is T, the syntax #P"..." is
  how a pathname should be printed by WRITE (and hence by PRIN1,
  PRINT, etc.). The "..." is the namestring representation of the
  pathname.

  Define that when *PRINT-ESCAPE* is NIL, WRITE writes a pathname
  object P by writing (NAMESTRING p) instead.

Test Case:

  (PARSE-NAMESTRING "foo.lisp")
  => #P"foo.lisp"

  (FORMAT NIL "Written to ~A." #P"foo.bin")
  => "Written to foo.bin."

  (TYPEP #P"foo.bin" 'PATHNAME)
  => T

Rationale:

  This satisfies the stated goals.

  [For :ESCAPE T] It will not be possible to make the printed
  pathname printed representation totally portable because of
  variations in file systems, but for different Common Lisp
  implementations on the same file system, or for Common Lisp
  systems running on file systems having compatible syntax,
  portability would be improved by this specification.

  Also, some implementations (eg, Symbolics Genera) use
  specialized representations for pathnames on different file
  systems. Eg, an MSDOS pathname is of type MSDOS-PATHNAME,
  not just type PATHNAME. #S(PATHNAME ...) is not only more
  verbose than necessary but might be misleading to some users
  because the object created will not have a TYPE-OF PATHNAME.

  [For :ESCAPE NIL] Printing the namestring of a pathname is
  a common operation and it is convenient to have a shorthand
  for doing it. Further, some implementations may be able to
  optimize the presentation of a pathname in this mode by
  printing it without actually consing the string.

Current Practice:

  Symbolics Genera implements the proposed behavior.

Cost to Implementors:

  Fairly minor changes to the readtable and the printer.

Cost to Users:

  Users who now use the non-portable syntax #S(...) in order
  to enter literal pathnames might have to change. [However,
  implementations would be free to continue to support this
  read syntax for compatibility.]

Cost of Non-Adoption:

  Portability of code and data involving pathnames within a
  given file system (or between suitably similar file systems)
  would be hampered needlessly.

Benefits:

  The cost of non-adoption would be avoided.

Aesthetics:

  The #P syntax is pretty and hides unimportant details.

Discussion:

  Pitman supports this change.