Simple StateT use

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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 = runStateT code [1..] >> return ()

-- layer a infinite list of uniques over the IO monad
code = do
    x <- pop
    io $ print x
    y <- pop
    io $ print y
    return ()

-- pop the next unique off the stack
pop = do
    (x:xs) <- get
    put xs
    return x

io = liftIO