I find very strange that the ANSI specification defines the function DIGIT-CHAR, but doesn't also define CHAR-DIGIT. It would be nice to have it in alexandria.

One can use char-code to implement it, but only assuming that the char-code of alphabetic characters are sequential (which happens very oftenly, I don't know any implementation in which this may not work, but this behaviour is not required by the specification).

So, here is my suggestion:

(defun char-digit (char &optional (radix 10)) "Takes a character and returns its weight. Signals an error if char is not a digit character in the given radix." (assert (digit-char-p char radix) (char) "Character ~a is not a digit character." char) (let ((char (char-upcase char))) #+(or sbcl clisp ecl cmucl openmcl allegro lispworks corman gcl abcl unicode) (cond ((char<= #\0 char #\9) (- (char-code char) (char-code #\0))) ((char<= #\A char #\Z) (+ 10 (- (char-code char) (char-code #\A)))) (t nil)) #-(or sbcl clisp ecl cmucl openmcl allegro lispworks corman gcl abcl unicode) (parse-integer (string char) :radix radix)))

I tested and it works. Well, I am open to suggestions, and feel free to include this code in alexandria as well.

Gustavo.