Status: Passed, as amended, Jun89 X3J13
References: File System Interface (pp409-427)
Edit history: 21-Oct-88, Version 1 by Pitman
3-Jul-89, Version 2 by Masinter
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.
Define the reader syntax #P"..." to be equivalent to
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
Define that when *PRINT-ESCAPE* is NIL, WRITE writes a pathname
object P by writing (NAMESTRING p) instead.
(FORMAT NIL "Written to ~A." #P"foo.bin")
=> "Written to foo.bin."
(TYPEP #P"foo.bin" 'PATHNAME)
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.
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.
The cost of non-adoption would be avoided.
The #P syntax is pretty and hides unimportant details.
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
MLY suggested #S(PATHNAME namestring [optional-host]).
IIM noted they use #.(PATHNAME namestring host) because different file
systems have different parsing conventions.