Mixing I / O with state computation

As an exercise, I am writing a command line RPN calculator in Haskell. The idea is that it will ask for input (number or operator) and print a new stack. My plan is to store a list of numbers in the state monad and perform calculations against that list. For example:

> 4
[4]
> 3
[3,2]
> 5
[5,3,2]
> +
[8, 2]

      

        
        
        
      

    

etc.

I start by just trying to create a list in a state monad with input and output on each record. I am already stuck due to the combination of IO and State in the same function. My problem is that I also need to overwrite the input to keep the prompt after entering the first number.

Here is my code:

module Main where

import Control.Monad.State

addEntry :: Int -> State [Int] Int
addEntry entry = do
  entries <- get
  put (entry : entries)
  return entry

mainLoop :: [Int] -> IO ()
mainLoop entries = do
  entry <- readLn
  newEntries <- execState (addEntry entry) entries
  print newEntries
  mainLoop newEntries

main :: IO ()
main = do
    print $ mainLoop []

      

        
        
        
      

    

and here is the compiler error I am currently getting:

src/Main.hs@14:28-14:42 Couldn't match type [Int] with ‘IO [Int]’
Expected type: State (IO [Int]) Int
  Actual type: State [Int] Int …
src/Main.hs@14:44-14:51 Couldn't match expected type ‘IO [Int]’ with actual type [Int] …

      

        
        
        
      

    

Any advice on how to structure these functions so that I don't mix IO and State?