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 (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

Hi, It seems you can use http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona... 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') HTH, Thu