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

[Marcin Kosiba]

Hi,
	I'm doing a bit of research into mobility models and I'm currently exploring 
implementation language choices for the simulator (yes, sadly it needs to be 
a custom one).
	I've been using Haskell here and there for some small tasks, and thought I 
should consider it as an implementation language for the simulator.
	While I already have an working implementation in Haskell, there is one thing 
that I would like to express in a more elegant way, but just can't figure 
out. 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

but I'd like to be able to alter world and node state before continuing from 
emit_action. 

I've tried to get this working by using the idea from 
http://www.haskell.org/pipermail/haskell/2005-April/015684.html but couldn't 
get the state-altering behavior I was looking for. I've also taken a look at 
http://monadicheadaches.blogspot.com/2008/01/python-25s-iterators-in-haskell-sort-of.html, 
the unified concurrency model and Control.Coroutine, but couldn't get the 
behavior I was going for.

Thanks!
	Marcin Kosiba
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