Combine state with IO actions

Question

Suppose I have a state monad such as:

data Registers = Reg {...}

data ST = ST {registers :: Registers,
              memory    :: Array Int Int}

newtype Op a = Op {runOp :: ST -> (ST, a)}

instance Monad Op where
 return a    = Op $ \st -> (st, a)
 (>>=) stf f = Op $ \st -> let (st1, a1) = runOp stf st
                               (st2, a2) = runOp (f a1) st1
                            in (st2, a2)

with functions like

getState :: (ST -> a) -> Op a
getState g = Op (\st -> (st, g st)

updState :: (ST -> ST) -> Op ()
updState g = Op (\st -> (g st, ()))

and so forth. I want to combine various operations in this monad with IO actions. So I could either write an evaluation loop in which operations in this monad were performed and an IO action is executed with the result, or, I think, I should be able to do something like the following:

newtype Op a = Op {runOp :: ST -> IO (ST, a)}

Printing functions would have type Op () and other functions would have type Op a, e.g., I could read a character from the terminal using a function of type IO Char. However, I'm not sure what such a function would look like, since e.g., the following is not valid.

runOp (do x <- getLine; setMem 10 ... (read x :: Int) ... ) st

since getLine has type IO Char, but this expression would have type Op Char. In outline, how would I do this?

Combine state with IO actions

2 Answers