# Simple StateT use

### From HaskellWiki

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

-- | -- | ||

− | -- layer | + | -- layer an infinite list of uniques over the IO monad |

-- | -- | ||

## Revision as of 21:53, 27 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 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