Re: [Haskell-cafe] What is MonadPlus good for?

On Sun, Feb 13, 2005 at 06:05:23PM -0500, ajb@spamcop.net wrote:

G'day all.

Quoting David Roundy :

...

It might be interesting to write a "backtracking" IO-like monad which obeyed m >> mzero === mzero. I imagine you could do it for something like an ACID database, if you define === as meaning "has the same final result on the database", which of course would only be useful if the database had sufficient locking that it couldn't have been read between the original m and the later mzero.

You should talk to the logic programming community about this some time.

As Lee Naish has pointed out on many occasions, it would involve finding a way to insert the page back into the laser printer and lift the toner off.

That's why it would only be possible in a limited realm such as a database, where you could "quarantine" the changes using locking, and where you could define strictly define "equality" in the world in which the IO takes place. It basically would just require a journalling system, which I imagine most decent databases provide in order to give ACID behavior. I'm far from a database expert, but this is how I understand that these things work. -- David Roundy http://www.darcs.net