2: 1978: The year 1978 is such that the sum of the first two digits and the latter two digits is equal to the middle two digits, i.e. 19 + 78 = 97. What is the next year (after 1978) for which this is true?

Programming Praxis 10-4-13

(defun validate-candidate (candidate)
  (let ((candidate (write-to-string candidate)))
    (unless (= (length candidate) 4)
      (error "Test valid only for 4 digit numbers"))
    (let ((initial-digits (parse-integer (subseq candidate 0 2)))
          (middle-digits (parse-integer (subseq candidate 1 3)))
          (final-digits (parse-integer (subseq candidate 2 4))))
      (if (= middle-digits (+ initial-digits final-digits))
          t
          nil))))

(loop with val = 1979
   do (format t "~a~%"  (incf val))
   until (validate-candidate val))

;; Answer 2307