# Difference between revisions of "Simple StateT use"

From HaskellWiki

DonStewart (talk | contribs) (document StateT example) |
m (Added type signatures) |
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import Control.Monad.State |
import Control.Monad.State |
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+ | main :: IO () |
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main = runStateT code [1..] >> return () |
main = runStateT code [1..] >> return () |
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− | |||

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

+ | code :: StateT [Integer] IO () |
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code = do |
code = do |
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x <- pop |
x <- pop |
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Line 22: | Line 24: | ||

-- pop the next unique off the stack |
-- pop the next unique off the stack |
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-- |
-- |
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+ | pop :: StateT [Integer] IO Integer |
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pop = do |
pop = do |
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(x:xs) <- get |
(x:xs) <- get |
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Line 27: | Line 30: | ||

return x |
return x |
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+ | io :: IO a -> StateT [Integer] IO a |
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io = liftIO |
io = liftIO |
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</haskell> |
</haskell> |

## 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
```