59: Vicinity59: Vicinityby Aubrey Jaffer
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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.
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.
program-vicinity can return incorrect values if your program
escapes back into a load continuation.
`""' (the empty string).
home-vicinity
returns #f.
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. sub-vicinity will
return a pathname of the subdirectory name of
vicinity.
in-vicinity.
(pathname->vicinity "/usr/local/lib/scm/Link.scm")
=> "/usr/local/lib/scm/"
#f otherwise. Typical vicinity suffixes are `/',
`:', and `\'.
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 (C) Aubrey Jaffer (2004). All Rights Reserved.
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