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

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.

Not quite... remember the IO monad is a function which returns an IO program... as long as none of the IO program has been executed it is possible to 'edit' the program to remove parts... The problem only really occurs in interactive programs, where input forces partial evaluation of the function result... once the function has been evaluated up to the input the output so far cannot be retracted. Keean.