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When I add a watch to a variable in clojure and rebind it, the watch is updated dynamically.

(def x "jlkfds")
x

In the above example x will always reflect its value.

However, when I try and do this using an atom, I have no luck. I have to execute the whole thing again to get the changes to reflect in either the instarepl or the watch.

(defonce y (atom 10))
@y *38*
(swap! y inc) *80*

In the above example I have executed the swap without executing the deref, and they have thus become out of sync.

What confuses me is that I saw a Javascript demo where someone (Chris) was able to watch the coordinates of a mouse pointer dynamically change. I really like the idea of having this functionality. Is there a way to do the same thing in Clojure?

Like this?

http://youtube.com/watch?v=d8-b6QEN-rk

Thank you

rhlvora
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1 Answers1

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Watches are only updated when the thing they are watching is eval'd. Just make sure that you are watching somewhere the watch macro can be eval'd. Try this.

(defonce y (atom 10))

(do //Eval this sexp
  (swap! y inc)
    @y) //watch this j

I think you are also running into issues with the insta-repl (live mode), periodically it will evaluate the entire page and that is where you are seeing the watches get out of sync. For example, line 1 is evaluated and the atom is created. You add a watch to line 2, evaluating it. You increment the watch with line 3 28 times, then do something that forces line 2 to eval again. This updates the watch to the new value of y. Re-evaluating line 3 doesn't change the watch on line two but it changes the value of y.

gvickers
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  • I definitely get what you're saying that watches only get updated when the thing they are watching is eval'd, but I want something like this in clojure: from: http://www.chris-granger.com/2013/08/22/light-table-050/ https://www.youtube.com/watch?v=d8-b6QEN-rk – rhlvora Oct 24 '14 at 04:23
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    @rhlvora, ...you might find http://swannodette.github.io/2014/02/27/taking-off-the-blindfold/ interesting. – Charles Duffy Oct 24 '14 at 15:55
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    @rhlvora Light-table _does_ give you that. In that video, the `foo` function is being called by those event listeners at the bottom of the editor. To do something like that in plain Clojure, you will need some sort of mouse movement listener to continuously call your function that sets the watched values. – gvickers Oct 25 '14 at 15:50