widget =
attach: ->
@el.bind 'click', @handler
handler: (event) =>
doStuffWithThis()
Handler is bound to whatever scope widget is in. http://jashkenas.github.com/coffee-script/#try:widget%20%3D%...
See how confusing fat arrow is?
Edit: Okay, I see what you're doing; attach is supposed to be invoked with a bound scope, which does change the binding. So yeah, my code won't work in your example. I'd argue that that's more of a lack-of-context problem in the example though, since it's dependent on knowledge of the usage of the attach() function to know that that binding behavior isn't desired. Given the context of the example, I contend my solution was acceptable. :)
Fat arrow there is still functionally equivalent to $.proxy.
To see what scope the fat arrow is bound to, all you need to do is scan up the parent scope in the file and look for a thin arrow (or a class declaration).
Your "whitespace problem" is my solution.
Mixins... Can you show me an example?
Times when you need a reference to a canonical function object, like with $.proxy and $.unbind. (CoffeeScript could implement => to set the original function as a property of the bound function)
Added a question on github: https://github.com/jashkenas/coffee-script/issues/1934