lazyio- Run IO actions lazily while respecting their order

Safe HaskellSafe-Infered




  • Although this module calls unsafeInterleaveIO for you, it cannot take the responsibility from you. Using this module is still as unsafe as calling unsafeInterleaveIO manually. Thus we recommend to wrap the lazy I/O monad into a custom newtype with a restricted set of operations which is considered safe for interleaving I/O actions.
  • Operations like hClose are usually not safe within this monad, since they will only get executed, if their result is consumed. Since this result is often () this is quite unusual. It will also often be the case, that not the complete output is read, and thus the closing action is never reached. It is certainly best to call a closing action after you wrote the complete result of the lazy I/O monad somewhere
  • return a :: LazyIO a is very different from liftIO (return a) :: LazyIO a. The first one does not trigger previous IO actions, whereas the second one does.
  • We advise to lift strict IO functions into the lazy IO monad. Lifting a function like readFile may lead to unintended interleaving.

Use it like

 import qualified System.IO.Lazy as LazyIO $
    do liftIO $ putStr "enter first line:"
       x <- liftIO getLine
       liftIO $ putStr "enter second line:"
       y <- liftIO getLine
       return x

Because only the first entered line is needed, only the first prompt and the first getLine is executed.


data T a Source

run :: T a -> IO aSource