Safe Haskell | None |
---|---|
Language | Haskell98 |
Caution:
- Although this module calls
interleaveIO
for you, it cannot take the responsibility from you. Using this module is still as unsafe as callinginterleaveIO
manually. Thus we recommend to wrap the lazy I/O monad into a customnewtype
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 fromliftIO (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 LazyIO.run $ 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.