Issue: Pathname-subdirectory-list

[Masinter.pa@Xerox.COM]

This arrived in my mailbox before the X3J13 meeting. While I suspect
there may be some alternative proposals, and perhaps some other
important related issues, this seems like a good way to introduce the
topic.

!

ISSUE: (PATHNAME-SUBDIRECTORY-LIST)

REFERENCES: CLtL pages 409 - 418

CATEGORY: ADDITION
  
EDIT HISTORY: Version 1 by Ghenis.pasa@Xerox.com, 06/18/87

PROBLEM DESCRIPTION:

It is impossible to write PORTABLE code that can produce a pathname
based on directory plus SUBDIRECTORY information. If the directory used
is not a root, then the string provided must contain OS-specific
separators. This defeats the purpose of having an abstraction like
pathname. Specifying a subdirectory RELATIVE to the current default is
possible but also inconvenient and non-portable.

This problem is even worse for programs running on machines on a network
that can retrieve files from multiple hosts, each using a different OS
and thus a different subdirectory delimiter.


PROPOSAL (PATHNAME-SUBDIRECTORY-LIST:ALLOW): 

Add a :SUBDIRECTORIES field to pathnames, to store a list of strings.

The string form of a pathname can be obtained by using the appropriate
OS-specific separator and end-delimiters.
Require global variables called LISP:*HOST-OS-ALIST* and
LISP:*DEFAULT-OS* to provide the information needed to assemble
namestrings correctly


TEST CASE (desired behavior): 

	>(defparameter LISP:*HOST-OS-ALIST*
		'(("vmsvax" . "vms") ("unixvax" . "unix"))

	>(defparameter LISP:*DEFAULT-OS* "msdos")

	>(defvar vmspath
		(make-pathname :host "vmsvax"
		 			:directory "smith"
		 			:sudirectories '("lisp") 
					:name "test"
		 			:type "lsp"))

	>(defvar localpath 
		(make-pathname :directory "smith" 			
					:sudirectories '("lisp")
 					:name "test"
 					:type "lsp"))

	>(namestring vmspath)
	"{vmsvax}[smith.lisp]test.lsp"

	>(namestring localpath)
	"c:\smith\lisp\test.lsp"


RATIONALE:

Pathnames are an abstraction meant to deal with the common notions in
file systems. Subdirectories exist in most operating systems. Common
Lisp must provide a standard way of dealing with subdirectories for
pathnames to be truly useful.


CURRENT PRACTICE:

CLtL acknowledges this problem and declares it to be a system dependent
issue.


ADOPTION COST:

This should be a simple addition to implement.


BENEFITS: 

Adding a :SUBDIRECTORIES field to pathnames would make the abstraction
completely system-independent. Relative pathnames could be trivially
specified by pathnames lacking a :DIRECTORY field.


CONVERSION COST: This is an upwards-compatible addition.


AESTHETICS:

Adding a :SUBDIRECTORIES field to pathnames would make the abstraction
completely system-independent.

DISCUSSION: >>Additional arguments, discussions, endorsements,
  testimonials, etc. should go here.<<