# Simple StateT use

### From HaskellWiki

(Difference between revisions)

DonStewart (Talk | contribs) (document StateT example) |
m (Added type signatures) |
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import Control.Monad.State | import Control.Monad.State | ||

+ | main :: IO () | ||

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

− | |||

-- | -- | ||

-- layer a infinite list of uniques over the IO monad | -- layer a infinite list of uniques over the IO monad | ||

-- | -- | ||

+ | |||

+ | code :: StateT [Integer] IO () | ||

code = do | code = do | ||

x <- pop | x <- pop | ||

Line 22: | Line 24: | ||

-- pop the next unique off the stack | -- pop the next unique off the stack | ||

-- | -- | ||

+ | pop :: StateT [Integer] IO Integer | ||

pop = do | pop = do | ||

(x:xs) <- get | (x:xs) <- get | ||

Line 27: | Line 30: | ||

return x | return x | ||

+ | io :: IO a -> StateT [Integer] IO a | ||

io = liftIO | io = liftIO | ||

</haskell> | </haskell> | ||

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

## 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