undo (was Re: Why does Dynamic Typing really matter?!?)

[David Eppstein]

In article <rGt1a.1528$6a2.545590 at kent.svc.tds.net>,
 "Edward K. Ream" <edream at tds.net> wrote:

> Doesn't Photoshop have some kind of flexible undo?  If so, I would study
> their model carefully.

As far as I can tell as a user of Photoshop, it has the usual linear 
undo history, with two exceptions:
(1) Normally, as with most linear undo history systems, a new operation 
clears out the redo part of the history.  But if you immediately undo an 
operation, it vanishes from the undo history and the previous redo 
history is restored.
(2) You can assign names to states in the history, and return to those 
named states.  But these states do not carry any history with them, so 
when you return to them, the undo/redo history is cleared out.

-- 
David Eppstein       UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/