I would like to start fiddling with ClojureScript, but I am puzzled about some points. My problem is what is a good way to deal with state changes coming from user interaction, when you trying to work functionally.
Let me give a couple of examples. I have in mind applications which run in the browser, but I think the issue is more general. Of course something will be changing - the DOM at least. But I would like to find out how to organize the rest of the code to work with immutable data structures.
1) Say I want to attach some events to some DOM object. This is not hard to do in a mostly functional way: when you create the node, you attach it a hash map with the various event handlers. But consider the case where you are using event delegation. Then when you create a new node, you may attach a event handler to some parent node that probably already exists. So you would have to change the hash associated to the already existing node.
2) Say I am designing an autocomplete module for an input field. Every time the user presses a key, I can make a call to a server to get the suggestions. This is easy. But now suppose I want to optimize it a bit. If I know all results matching
foo there is no point in asking again for all results matching
foobar; I can just filter the former. So I need to build some kind of cache. This cache will be updated every time the user inserts a new word which is not a superset of the words previously entered. Again: how do I model the cache? The most reasonable way seems to be a hash map mapping words to results, but it should be mutable.
Can you suggest some patterns that would make it easier to incorporate changes due to user interaction into a functional design?