Type Casting in Lisp

indienick · 01-17-2007, 10:10 AM

Is it possible to re-cast a variable type to another? I'm trying to cast the returned value of GET-UNIVERSAL-TIME to a string.

I am also trying to use CONCATENATE to...well...concatenate two pathnames, but I can't remember what the type is that I need to specify; 'STRING signals an error, and 'PATH or 'PATHNAME don't exist. I've already tried looking this up, and there doesn't seem to be any PATHNAME or PATH type or subtype.

Here's an example:

Code:

(defparameter *current-log-file* nil)
(setq *current-log-file*
      (make-pathname
       :directory '(:absolute "etc" "pkgsd" "logs")
       :name (get-universal-time) ; This is one of the offending lines
       :type "txt"))
...
(defparameter *base-dir* #p"/etc/pkgsd")
(defparameter *temp-dir* #p"/tmp")

; This is the other offending line
(defparameter *temp-path* (concatenate 'string *base-dir* *temp-dir*))

Should I perhaps just leave the two directory parameters as strings and work with them thusly?

taylor_venable · 01-17-2007, 10:57 AM

The first error results from GET-UNIVERSAL-TIME which returns an integer. To get this into a string, you can use WRITE-TO-STRING. The latter problem is because pathnames aren't sequences and thus can't be concatenated. But you can convert the pathnames into strings, then concatenate them, and convert them back into a pathname again. Here's some code:

Code:

(setq *current-log-file*
      (make-pathname
       :directory '(:absolute "etc" "pkgsd" "logs")
       :name (write-to-string (get-universal-time)) ; convert int -> string
       :type "txt"))
...
(defparameter *base-dir* #p"/etc/pkgsd")
(defparameter *temp-dir* #p"/tmp")

; convert pathname -> string -> pathname
(defparameter *temp-path*
  (parse-namestring
   (concatenate 'string
                (namestring *base-dir*)
                (namestring *temp-dir*))))

indienick · 01-17-2007, 11:51 AM

Thank you very much.

I've been doing some concurrent python programming, and it's totally messing up my Lisp abilities.