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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

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.


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.

Test Case:


=> #P"foo.lisp"

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

=> "Written to foo.bin."

(TYPEP #P"foo.bin" 'PATHNAME)

=> T


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.


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.

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