# Simple StateT use

### From HaskellWiki

(Difference between revisions)

m |
(+cat) |
||

Line 35: | Line 35: | ||

[[Category:Code]] | [[Category:Code]] | ||

+ | [[Category:Monad]] |

## Revision as of 00:51, 28 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