Title

Vicinity

Author

Aubrey Jaffer

Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. You can access previous messages via the archive of the mailing list.

Abstract

A vicinity is a descriptor for a place in the file system. Vicinities hide from the programmer the concepts of host, volume, directory, and version. Vicinities express only the concept of a file environment where a file name can be resolved to a file in a system independent manner.

All of these procedures are file-system dependent. Use of these vicinity procedures can make programs file-system independent.

Rationale

Most computer languages expose the syntax of pathnames of the host file-system when dealing with files. But a great many programs require read access to data, text, or library files they were installed with.

Some programs use literal strings to locate accessory files, breaking on installations with different destinations. More savvy coders will construct pathnames from environment variables or compile-time definitions.

In most languages, programs intended for portability must condition all manipulations of pathnames to the syntax and capabilities of the host file-system. Inconsistent conditioning is a common cause of porting failures.

Common-Lisp attacks the general problem of naming files anywhere in any file system. It has a six-component pathname datatype to represent names in the most complicated file-system imaginable; names in simpler file systems use fewer components.

In this arrangement, portable file-handling programs must be capable of handling pathnames with 6 components, and those employing fewer. But which component will be used is not obvious. Is a ".txt" filename suffix a type or part of the name?

Vicinities attack a smaller problem, that of describing pathnames in 5 predefined locations, and their sub-vicinities. Those predefined locations cover the usual areas for ancillary and configuration files used by Scheme implementations and programs. The program-vicinity is particularly useful as it is the directory where the currently loading file is located. This is captured by redefining load to fluid-let a top-level variable with its argument.

The make-vicinity and pathname->vicinity procedures provide means to create new base vicinities. Base vicinities should generally be absolute pathnames.

Vicinities need not be tied to individual files in a file system. The files named could be members of a zip archive, as Java does. Vicinities can even be used on flat file systems (which have no directory structure) by having the vicinity express constraints on the file name. On most systems a vicinity is a string.

vicinity procedures are supported by all implementations in SLIB.

Specification

Function: program-vicinity
Returns the vicinity of the currently loading Scheme code. For an interpreter this would be the directory containing source code. For a compiled system (with multiple files) this would be the directory where the object or executable files are. If no file is currently loading, then the result is undefined. Warning: program-vicinity can return incorrect values if your program escapes back into a load continuation.

Function: library-vicinity
Returns the vicinity of the shared Scheme library.

Function: implementation-vicinity
Returns the vicinity of the underlying Scheme implementation. This vicinity will likely contain startup code and messages and a compiler.

Function: user-vicinity
Returns the vicinity of the current directory of the user. On most systems this is `""' (the empty string).

Function: home-vicinity
Returns the vicinity of the user's HOME directory, the directory which typically contains files which customize a computer environment for a user. If scheme is running without a user (eg. a daemon) or if this concept is meaningless for the platform, then home-vicinity returns #f.

Function: in-vicinity vicinity filename
Returns a filename suitable for use by load, open-input-file, open-output-file, etc. The returned filename is filename in vicinity. in-vicinity should allow filename to override vicinity when filename is an absolute pathname and vicinity is equal to the value of (user-vicinity). The behavior of in-vicinity when filename is absolute and vicinity is not equal to the value of (user-vicinity) is unspecified. For most systems in-vicinity can be string-append.

Function: sub-vicinity vicinity name
Returns the vicinity of vicinity restricted to name. This is used for large systems where names of files in subsystems could conflict. On systems with directory structure sub-vicinity will return a pathname of the subdirectory name of vicinity.

Function: make-vicinity dirpath
Returns dirpath as a vicinity for use as first argument to in-vicinity.

Function: pathname->vicinity path
Returns the vicinity containing path.
(pathname->vicinity "/usr/local/lib/scm/Link.scm")
                    => "/usr/local/lib/scm/"

Function: vicinity:suffix? chr
Returns the `#t' if chr is a vicinity suffix character; and #f otherwise. Typical vicinity suffixes are `/', `:', and `\',

Implementation

This code is taken from slib/Template.scm and slib/require.scm

;;@ (implementation-vicinity) should be defined to be the pathname of
;;; the directory where any auxillary files to your Scheme
;;; implementation reside.
(define (implementation-vicinity)
  (case (software-type)
    ((UNIX)	"/usr/local/src/scheme/")
    ((VMS)	"scheme$src:")
    ((MS-DOS)	"C:\\scheme\\")))

;;@ (library-vicinity) should be defined to be the pathname of the
;;; directory where files of Scheme library functions reside.
(define library-vicinity
  (let ((library-path
	 (or
	  ;; Use this getenv if your implementation supports it.
	  (getenv "SCHEME_LIBRARY_PATH")
	  ;; Use this path if your scheme does not support GETENV
	  ;; or if SCHEME_LIBRARY_PATH is not set.
	  (case (software-type)
	    ((UNIX) "/usr/local/lib/slib/")
	    ((VMS) "lib$scheme:")
	    ((MS-DOS) "C:\\SLIB\\")
	    (else "")))))
    (lambda () library-path)))

;;@ (home-vicinity) should return the vicinity of the user's HOME
;;; directory, the directory which typically contains files which
;;; customize a computer environment for a user.
(define (home-vicinity)
  (let ((home (getenv "HOME")))
    (and home
	 (case (software-type)
	   ((UNIX COHERENT MS-DOS)	;V7 unix has a / on HOME
	    (if (eqv? #\/ (string-ref home (+ -1 (string-length home))))
		home
		(string-append home "/")))
	   (else home)))))
;@
(define in-vicinity string-append)
;@
(define (user-vicinity)
  (case (software-type)
    ((VMS)	"[.]")
    (else	"")))
;@
(define vicinity:suffix?
  (let ((suffi
	 (case (software-type)
	   ((AMIGA)				'(#\: #\/))
	   ((MACOS THINKC)			'(#\:))
	   ((MS-DOS WINDOWS ATARIST OS/2)	'(#\\ #\/))
	   ((NOSVE)				'(#\: #\.))
	   ((UNIX COHERENT PLAN9)		'(#\/))
	   ((VMS)				'(#\: #\]))
	   (else
	    (slib:warn "require.scm" 'unknown 'software-type (software-type))
	    "/"))))
    (lambda (chr) (and (memv chr suffi) #t))))
;@
(define (pathname->vicinity pathname)
  (let loop ((i (- (string-length pathname) 1)))
    (cond ((negative? i) "")
	  ((vicinity:suffix? (string-ref pathname i))
	   (substring pathname 0 (+ i 1)))
	  (else (loop (- i 1))))))
(define (program-vicinity)
  (if *load-pathname*
      (pathname->vicinity *load-pathname*)
      (slib:error 'program-vicinity "called while not within load")))
;@
(define sub-vicinity
  (case (software-type)
    ((VMS) (lambda
	       (vic name)
	     (let ((l (string-length vic)))
	       (if (or (zero? (string-length vic))
		       (not (char=? #\] (string-ref vic (- l 1)))))
		   (string-append vic "[" name "]")
		   (string-append (substring vic 0 (- l 1))
				  "." name "]")))))
    (else (let ((*vicinity-suffix*
		 (case (software-type)
		   ((NOSVE) ".")
		   ((MACOS THINKC) ":")
		   ((MS-DOS WINDOWS ATARIST OS/2) "\\")
		   ((UNIX COHERENT PLAN9 AMIGA) "/"))))
	    (lambda (vic name)
	      (string-append vic name *vicinity-suffix*))))))
;@
(define (make-vicinity pathname) pathname)

Copyright

Copyright (C) Aubrey Jaffer (2004). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.


Editor: David Van Horn
Last modified: Sun Jan 28 13:40:18 MET 2007