Simple StateT use

A small example showing how to combine a State monad (in this case a unique supply), with the IO monad, via a monad transformer.

No need to resort to nasty mutable variables or globals!

import Control.Monad.State

main :: IO ()
main = runStateT code [1..] >> return ()
--
-- layer an infinite list of uniques over the IO monad
--

code :: StateT [Integer] IO ()
code = do
    x <- pop
    io $ print x
    y <- pop
    io $ print y
    return ()

--
-- pop the next unique off the stack
--
pop :: StateT [Integer] IO Integer
pop = do
    (x:xs) <- get
    put xs
    return x

io :: IO a -> StateT [Integer] IO a
io = liftIO