I used the State monad to implement a Brainfuck [1] interpreter a few months ago. It stored the program counter, pointer and the memory of the machine. There might have been a different (better?) way, but as I was trying to learn more about monads, it was an obvious choice. Thomas [1] http://www.muppetlabs.com/~breadbox/bf/ On Mon, Jul 6, 2009 at 18:54, Thomas Hartman wrote:

Can someone give some simple common scenarios where the state monad is useful, besides labeling trees?

References to puzzles like those in project Euler or similar would be nice.

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Off the top of my head state is important when getting from A to B depends on the path you took. As such a common scenario I find myself in all the time is not having a good CLI craps game. (And which I resolve by rewriting in every language I learn.) Stake, current bet, bets outstanding, point. Lots of state. Also user interaction, varying output, error conditions, etc. depending on how complex you want. A much simpler problem is to model some large number of throws using different play strategies. Removes all the icky user interaction. Alternately you can just abuse toy problems. import Control.Monad.State fac n = execState (facs n) 1 facs n = do y <- get if n == 0 then return y else do put (y*n) facs (n-1) Enjoy, -ljr Thomas Hartman wrote:

Can someone give some simple common scenarios where the state monad is useful, besides labeling trees?

References to puzzles like those in project Euler or similar would be nice.

Thanks! _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe