On Sun, Nov 1, 2009 at 9:14 PM, Richard Newman <[email protected]> wrote:
>
> > I've given it some thought and it seems it has to be a compiler level
> > feature. Am I right?
>
> You could implement Common Lisp-style "return-from" with a custom
> exception class:
>
> public class BlockException extends Exception {
> String name;
> Object val;
> public BlockException(String name, Object val) {
> this.name = name;
> this.val = val;
> }
> public String name() { return this.name; }
> public Object value() { return this.val; }
> }
>
> (defmacro block [name & body]
> `(try
> (do ,@body)
> (catch BlockException e
> (if (= (.name e) (name ,name))
> (.value e)
> (throw e)))))
>
> (defmacro return-from [name expr]
> `(throw (BlockException. (name ,name) ,expr)))
>
>
> Then use it:
>
>
> (defn foo []
> (block outside
> (do-something
> ;; ...
> (return-from outside 5)
> ;; ...
> )))
For this to work, you'd first have to implement Common Lisp style unquoting.
:)
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