So, I'm trying to clear up some flow control issues in my head, and I
want to confirm that my thinking is at least in the right direction.
Let's say I implement a very simple restart facility with the
following style:
(def current-handler)
(def current-restart nil)
In some code I want to handle I put:
(binding [current-handler (fn [e] (if current-restart (current-
restart) <some non-local exit>))]
<stuff>)
Later on in the same dynamic extent as <stuff> I put:
(binding [current-restart (fn [] 42)]
(try <dodgy-expr>
(catch Throwable t (current-handler t))))
What can <some non-local exit> be?
In common lisp (if it didn't have a condition system) I could do
something like:
(block my-block
(let ((*current-handler*
(lambda (e)
(if *current-restart* (funcall *current-restart*) (return-
from my-block e)))))
<stuff>))
of in scheme (with fluid-let)
(call/cc
(lambda (my-esc)
(fluid-let ((*current-handler*
(lambda (e) (if *current-restart* (*current-
restart*) (my-esc e)))))
<stuff>)))
Anyway you get the idea; I can unconditionally take an escape
continuation.
However, in clojure/java given:
(binding [current-handler (fn [e] (if current-restart (current-
restart) <some non-local exit>))]
<stuff>)
The only thing I can do for <some non-local exit> is to throw (or re-
throw):
(try (binding [current-handler (fn [e] (if current-restart (current-
restart) (throw e)))]
<stuff>)
;;However, even if there is something to catch it...
(catch Throwable t <more stuff>))
There is no guarantee the catch block will ever see it, and there is
no way to unconditionally abort the rest of the computation from the
point of <some non-local exit> to the return of <stuff>. Is that
correct?
Matt
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