must clear insert buffer-string?

Question

This code

(defun insert-dateutc.* ()
  (interactive)
  (insert (with-temp-buffer
            (call-process "date" nil t nil "+%Y-%m-%dT%H:%M:%S")
            (buffer-substring-no-properties 1 (line-end-position 0)))))

(defun insert-random-uuid.* ()
  (interactive)
  (with-temp-buffer
    (call-process "uuidgen" nil t nil)
    (buffer-substring-no-properties 1 (line-end-position 0))))

which works stand-alone, but when stuck into this code

(defun foo ()
  (interactive)
  (insert (format "'(start-symbol :id %s
               :graph 
               :ts %s
               :0.0 
               :1.0 
               )" (insert-random-uuid.*) (insert-dateutc.*))))

produces this:

2015-11-19T16:07:38'(start-symbol :id 9c5a8a91-325a-44d1-8535-03afc10f405a
               :graph 
               :ts nil
               :0.0 
               :1.0 
               )nil

That is, it's sticking the formatted date outside the format output. I can only assume that I'm going to the buffer-substring-no-properties too many times, or it needs to be reset or something. An earlier version that simply did this

(defun foo ()
  (interactive)
  (insert (format "'(start-symbol :id %s
               :graph 
               :ts %s
               :0.0 
               :1.0 
               )" (insert-random-uuid.*) (format-time-string "%Y-%m-%dT%H:%M:%S"))))

played well with insert-random-uuid.*

See:

'(start-symbol :id 2bfebed2-82c3-4ccb-b9f3-84b82a421e06
               :graph 
               :ts 2015-11-19T16:21:39
               :0.0 
               :1.0 
               )nil

Not sure what the problem is. . . .

Answer 1

If your function inserts text into a buffer, it should be called insert-something and its return value should not be passed to format and friends.

I.e., rename insert-random-uuid.* to get-random-uuid.* and insert-dateutc.* to get-dateutc.* and remove the insert call from the latter.