Issue PATHNAME-PRINT-READ Writeup

Status:		Passed, as amended, Jun89 X3J13


Issue:        PATHNAME-PRINT-READ

References:   File System Interface (pp409-427)

Category:     CHANGE/ADDITION

Edit history: 21-Oct-88, Version 1 by Pitman

		3-Jul-89, Version 2 by Masinter


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 true, 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.


-----

Summary of discussion on CL-Cleanup:


 EB noted that Lucid CL implements the proposed behavior and that there

  is cost to users who define their own #P read macro. He weakly supports

  the proposal but wishes someone had pursued a `generic pathnames' proposal.


 Pierson noted that KCL uses #"...", but that this collides with proposed

  syntax for Dick Waters' pretty printer. He also thinks #P is better

  because it is already more widely used for that purpose.


 Masinter noted that Envos Medley prints pathnames with the syntax

  #.(pathname "asdf"), which he thinks is not as pretty as #P"asdf"

  but currently more portable.


 KMP and JonL raised the issues that #. has the disadvantage that it must

  be parsed by the full Lisp engine, while #P can be parsed by something

  simpler.  Permitting #. leaves a gaping hole for trojan horses, and

  also requires the presence of the evaluator in a delivery system.


 MLY, GSB, Peirson, and IIM argued for not using up an extra dispatch

 character.


 MLY suggested #S(PATHNAME namestring [optional-host]).


 IIM noted they use #.(PATHNAME namestring host) because different file

  systems have different parsing conventions.