(defmacro with-post-params (params &body body)
  (let ((payload-assoc (gensym)))
    `(let ((,payload-assoc (url-decode-params (payload-as-string))))
       (let ,(loop for param in params
           collect `(,param (cdr (assoc (symbol-name ',param)
                        ,payload-assoc
                        :test #'string-equal))))
     ,@body))))

(with-post-params (name age sex)
  (format t "L'age de ~a (~a) est ~a ans" name age sex))

;; Si j'expand ma macro, j'obtiens:
(LET ((#:G831 (URL-DECODE-PARAMS (PAYLOAD-AS-STRING))))
  (LET ((NAME (CDR (ASSOC (SYMBOL-NAME 'NAME) #:G831 :TEST #'STRING-EQUAL)))
        (AGE (CDR (ASSOC (SYMBOL-NAME 'AGE) #:G831 :TEST #'STRING-EQUAL)))
        (SEX (CDR (ASSOC (SYMBOL-NAME 'SEX) #:G831 :TEST #'STRING-EQUAL))))
    (FORMAT T "L'age de ~a (~a) est ~a ans" NAME AGE SEX)))