ftrain

;; simplest fold definition
(define (fold f init l)
  (if (null? l) init (fold f (f init (car l)) (cdr l))))

;; by this time you are almost certainly comfortable with signatures like:

;; Int -> Int
(fold + 0 '[1 2 3 4 5])
;; => 15

ftrain

(def date-parsers [nil (java.text.SimpleDateFormat. "yyyy") (java.text.SimpleDateFormat. "yyyy-M") (java.text.SimpleDateFormat. "yyyy-M-dd")])

(def grains [:decade :year :month :day]) ;; what's decade doing?

(map #(let [[_ year month day] (re-find #"(\d{2,4})[-_]([\dx]{2})[-_]([\dx]{2})" %)
            date (first (split-with (partial not= "xx") [(if (= (count year) 2) (str "19" year) year) month day]))]
        [(.parse (parsers (count date)) (clojure.string/join "-" date)) (grains (count date))])
     ["thefilehasaname__1939-xx-01.mp3"
      "thefilehasaname__39_01_01.mp3"
      "thefilehasaname__39-01-01.mp3"