Function Erroneously Returning Nil
I'm trying to learn Lisp now, as a supplement to my CS1 course because the class was moving too slow for me. I picked up "Practical Common Lisp," which so far has turned out to be a great book, but I'm having some trouble getting some examples to work. For instance, if I load the following file into the REPL:
;;;; Created on 2010-09-01 19:44:03
(defun makeCD (title artist rating ripped)
(list :title title :artist artist :rating rating :ripped ripped))
(defvar *db* nil)
(defun addRecord (cd)
(push cd *db*))
(defun dumpDB ()
(dolist (cd *db*)
(format t "~{~a:~10t~a~%~}~%" cd)))
(defun promptRead (prompt)开发者_如何学C
(format *query-io* "~a: " prompt)
(force-output *query-io*)
(read-line *query-io*))
(defun promptForCD ()
(makeCD
(promptRead "Title")
(promptRead "Artist")
(or (parse-integer (promptRead "Rating") :junk-allowed t) 0)
(y-or-n-p "Ripped [y/n]: ")))
(defun addCDs ()
(loop (addRecord (promptForCD))
(if (not (y-or-n-p "Another? [y/n]: ")) (return))))
(defun saveDB (fileName)
(with-open-file (out fileName
:direction :output
:if-exists :supersede)
(with-standard-io-syntax
(print *db* out))))
(defun loadDB (fileName)
(with-open-file (in fileName)
(with-standard-io-syntax
(setf *db* (read in)))))
(defun select (selectorFn)
(remove-if-not selectorFn *db*))
(defun artistSelector (artist)
#'(lambda (cd) (equal (getf cd :artist) artist)))
And query the 'database' using (select (artistSelector "The Beatles"))
, even if I do indeed have an entry in the database where :artist
is equal to "The Beatles"
, the function returns NIL
.
What am I doing incorrectly here?
Nothing, AFAICT:
$ sbcl This is SBCL 1.0.34.0... [[pasted in code above verbatim, then:]] * (addRecord (makeCD "White Album" "The Beatles" 5 t)) ((:TITLE "White Album" :ARTIST "The Beatles" :RATING 5 :RIPPED T)) * (select (artistSelector "The Beatles")) ((:TITLE "White Album" :ARTIST "The Beatles" :RATING 5 :RIPPED T))
CL-USER 18 > (addcds)
Title: Black Album
Artist: Prince
Rating: 10
Title: White Album
Artist: The Beatles
Rating: 10
NIL
CL-USER 19 > (select (artistSelector "The Beatles"))
((:TITLE "White Album" :ARTIST "The Beatles" :RATING 10 :RIPPED T))
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