usage help string generator

(define (usage specs lastrun)
  (string-join
   (map
    (lambda (spec)
      (let* ((long-name (car spec))
         (props (cdr spec))
         (single-char (assoc-ref props 'single-char))
         (description (assoc-ref props 'description))
         (value (assoc-ref props 'value))
         (value-desc (assoc-ref props 'value-desc))
         (default (assoc-ref props 'default))
         (lastrun (hash-ref lastrun long-name)))
    (string-append
     (if single-char
         (string #\- (car single-char)))
     " "
     (string-append "--" (symbol->string long-name))
     (if value
         (if value-desc
         (string-append " " (car value-desc) "\n")
         " ARG\n")
         "\n")
     (if description (car description) "NO DESCRIPTION")
     (if value
         (cond
          (lastrun (string-append " (default " lastrun ")"))
          (default (string-append " (default " (car default) ")"))
          (else ""))
         (if (or lastrun default) " (default)" "")))))
    specs)
   "\n\n"))