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

[Marcin Kosiba]

On Thursday 12 March 2009, you wrote:
> 2009/3/12 Marcin Kosiba <marcin.kosiba at gmail.com>:
> > 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
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