While impletenting the live reload feature for Clay a few weeks ago, unfortunately, I was bitten by the laziness of map. It seems quite obvious we all know that its result is a lazy sequence, but I just can't help fall into the "trap".
At that time I was adding a vector to keep track of all beholder instances watching file changes in user specified directories. So then when it's time to stop all the watchers, I used something like to achieve the goal:
(defn stop-watchers
"Stop all directory watchers."
[]
(map beholder/stop
(:watchers @*dir-watchers))
(reset! *dir-watchers dir-watchers-initial))
But the problem was the callback function got called multiple times, when a file got changed. It did trap me for some time, what made me even confused was that when I eval'ed the map form via cider, it worked perfectly.
After realizing it was due to the laziness, the solution was simple:
(defn stop-watchers
"Stop all directory watchers."
[]
(doseq [w (:watchers @*dir-watchers)]
(beholder/stop w))
(reset! *dir-watchers dir-watchers-initial))