Re: [Haskell-cafe] Using a monad to decompose a function into functions

On Thursday 12 March 2009, you wrote:

2009/3/12 Marcin Kosiba :

...

Hi,        I'm doing a bit of research into mobility models and I'm currently exploring implementation language choices for the simulator (*snip*) The simulation algorithm requires expressing the node's mobility so that it is "stateless". The mobility model algorithm's type should be something like: mobility_model :: WorldState -> NodeState -> OtherInput -> (Action, NodeState)

where Action can alter WorldState and the second NodeState is an altered input NodeState. I perform a form of speculative execution on mobility_model so sometimes I need to backtrack to a previous world and node state. This is all fairly simple stuff, and was just an introduction. What I do now is store an enum in NodeState and implement mobility_model as one big case statement. Yes, this is very imperative of me, I know. What I'd like to do is to express mobility_model, so that the code would look like:

mobility_model world node input = do    do_calculus    emit_action    if something      then emit_action      else emit_action    do_calculus    emit_action    mobility_model world node input

Hi,

It seems you can use http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mon ad-State-Lazy.html Just have a look at the exemple :

tick :: State Int Int tick = do n <- get put (n+1) return n

your code would become something like mobility_model :: OtherInput -> State (WorldState,NodeState) () mobility_model input = do world <- gets fst node <- gets snd .... let (world',node') = ... put (world',node')

ok, that solves part of the problem. what this doesn't solve is that somewhere between these lines (which corespond to emit_action in my example)

let (world',node') = ... put (world',node') I need to return a value and an Action and NodeState to the simulation algorithm. and then, after the simulation algorithm calculates a new WorldState it will want the mobility_model to where it left off, but with a new WorldState.

I hope I'm clear about what I wish to achieve: each emit_action should return a value (Action, NodeState) and maybe a function mobility_model_cont which I then could call with the new WorldState to continue from where emit_action returned. Thanks, Marcin Kosiba