I'm trying to develop a very simple simulation framework. A simulation consists
of a list of models. Models generate output events based on an input event. Here
is what I have currently (it works fine).
-----8<-----
import Data.List (foldl')
import Control.Monad.State ( get, gets, modify, put, runState, State )
type Event = Char
type Environment = String
-- | Given an event, a model generates zero or more new events
type Model = Event -> State Environment [Event]
modelA :: Model
modelA a = do
s <- get
put $ s ++ a:"A "
return $ a:"hip "
modelT :: Model
modelT a = do
s <- get
put $ s ++ a:"T "
return $ a:"hop "
-- | Process a sequence of events using one model
runModel :: Model -> [Event] -> State Environment [Event]
runModel m [] = return []
runModel m (e:es) = do
s <- get
let (es', s') = runState (m e) s
let (es'', s'') = runState (runModel m es) s'
put s''
return $ es' ++ es''
-- | Process a sequence of events using multiple models
runModels :: [Model] -> [Event] -> State Environment [Event]
runModels [] es = return []
runModels (m:ms) es = do
s <- get
let (es', s') = runState (runModel m es) s
let (es'', s'') = runState (runModels ms es) s'
put s''
return $ es' ++ es''
----->8-----
Now what I want to do is to give models the ability to generate random numbers.
So I thought I might change the definition of Model to:
type Model g = Event -> StateT Environment (Rand g) [Event]
And then I could alter one of the models to use random numbers like so:
modelT :: Model g
modelT _ = do
s <- get
put $ s ++ " R"
n <- HOW DO I GET A RANDOM NUMBER????
return [n]
I can't figure out how to get a random number from the inner monad. I assume I
use lift, but I can't get it to work. Would it be better to use RandT instead of
StateT? (And if so, how would I do the state operations?)
Thank you in advance for any help!
