# Simple StateT use

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DonStewart (Talk | contribs) (document StateT example) |
m (Added type signatures) |

## Revision as of 04:35, 11 February 2007

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 a 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