Don't think of it as being all pure code, think of it as tracking in the type system which parts of your code may launch the missiles and which parts can't. Given the following program,

    main :: IO ()
    main = do
      coordinates <- getCoords
      launch trajectory
      where
        trajectory = calcTrajectory coordinates
    
    getCoords :: IO Coordinates
    getCoords = -- TODO
    
    launch :: Trajectory -> IO  ()
    launch = -- TODO
    
    calcTrajectory :: Coordinates -> Trajectory
    calcTrajectory = -- TODO
    

I can look at the types and be reasonably certain that calcTrajectory does no reads/writes to disk or the network or anything of that sort (the part after the last arrow isn't `IO something`), the only side effect is perhaps to heat up the CPU a bit.

This also nudges you in the direction of an Functional Core, Imperative Shell architecture https://www.destroyallsoftware.com/screencasts/catalog/funct...