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Here is a simple set of path finding functions, with which I have
added path finding to the Info reader. Version 19 will have a similar
facility, but in the meantime here is mine.

Please note the unusual indentation style, and that I have provided
"executable" comments for exemplification and testing. This is a
smalltalky sort of thing, and should be more common.

;;; Piercarlo Grandi, Aberystwyth, 1992

;; This function will return a pathname if it exists with any of the
;; given suffixes. It does not check for existence of the pathname
;; without any suffix.

; EXAMPLES:
;  (path-suffixed-p "/usr/include/types" '(".h"))
;  (path-suffixed-p "/usr/include/tar" ".h")

(defun path-suffixed-p (pathname suffixes)
  (if (null suffixes) nil
    (if (atom suffixes) (setq suffixes (list suffixes)))
    (let ((suffixed (concat pathname (car suffixes))))
      (if (file-exists-p suffixed) suffixed
	(path-suffixed-p pathname (cdr suffixes))
      )
    )
  )
)

;; This function will return a complete pathname composed of the prefix
;; followed by the name, followed _optionally_ by one of the suffixes,
;; if such a pathname exist. Prefix and suffix lists are scanned strictly
;; left to right.

; EXAMPLES:
;  (path-exists-p "/usr/include" "tar.h" ".h")
;  (path-exists-p "/usr/include" "tar" ".h")
;  (path-exists-p "/usr/include" "types" ".h")

(defun path-exists-p (prefix name &optional suffixes)
  (let ((pathname (expand-file-name name prefix)))
    (if (file-exists-p pathname) pathname
      (if (null suffixes) nil (path-suffixed-p pathname suffixes))
    )
  )
)

;; This function will return a complete pathname built by concatenating
;; one of the suffixes with the name with one of the suffixes,
;; if it exists and the given test applied to it is satisfied.

; EXAMPLES:
;  (path-find-file '("/usr/include" "/usr/include/sys") "tar.h" ".h" nil)
;  (path-find-file '("/usr/include" "/usr/include/sys") "types" ".h" 'file-exists-p)

;  (path-find-file load-path "debug" ".elc" nil)
;  (path-find-file load-path "calc" '(".elc" ".el"))

(defun path-find-file (prefixes name &optional suffixes test)
  (if (null prefixes) nil
    (if (atom prefixes) (setq prefixes (list prefixes)))
    (let
      (
	(pathname
	  (path-suffixed-p
	    (expand-file-name name (car prefixes))
	    (or suffixes '(""))
	  )
	)
      )
      (if (and pathname (or (null test) (apply test (list pathname))))
	pathname
	(path-find-file (cdr prefixes) name suffixes test)
      )
    )
  )
)

(provide 'pathfind)

--
Piercarlo Grandi, Dept of CS, PC/UW@Aberystwyth <pcg@aber.ac.uk>
  E l'italiano cantava, cantava. E le sue disperate invocazioni giunsero
  alle orecchie del suo divino protettore, il dio della barzelletta