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Lifting and unlifting IO actions

The unliftio library

Many actions in common Haskell libraries are tied to the IO type. For example:

readFile :: FilePath -> IO ByteString

There are times when we want to use these actions in monad transformer stacks that sit on top of IO. For many cases, we can use the MonadIO class and its liftIO method to make this possible:

readFileLifted :: MonadIO m => FilePath -> m ByteString
readFileLifted fp = liftIO (readFile fp)

However, there are two problems with this:

  1. There are some classes of functions for which liftIO is insufficient. For example, try to convert timeout :: Int -> IO a -> IO (Maybe a) into timeout :: MonadIO m => Int -> m a -> m (Maybe a). It can’t be done.
  2. It adds a lot of tedium to our code to insert liftIO calls where needed

The unliftio library solves both of these problems, as well as a few others we’ll get to. It also serves as the basis for the rio standard library.

Let’s start with a high-level approach to the first problem, explain the solution to the second problem, and then give some more details.

Unlifting

MonadIO works for readFile, but doesn’t work for timeout. The reason is that readFile has IO in only positive position, whereas timeout has IO in both positive and negative position. If those terms are unfamiliar and you’d like to learn more, check out our blog post on Covariance and Contravariance. In short: readFile only has IO as output, while timeout takes IO as input as well.

liftIO lets us convert an IO a to an m a. We need to do the opposite: convert an m a into an IO a. And that’s precisely what the MonadUnliftIO class provides:

timeoutUnlifted :: MonadUnliftIO m => Int -> m a -> m (Maybe a)
timeoutUnlifted duration action = withRunInIO $ \run -> timeout duration (run action)

For minimal dependency footprint, the MonadUnliftIO typeclass and a few helper functions are defined in the unliftio-core library. But we’re going to be talking about the batteries-loaded unliftio library, which is where we solve the second problem.

Batteries loaded

Using liftIO and withRunInIO directly is possible, but tedious. The goal of the unliftio library is to instead collect together common functionality and lift/unlift it from IO to either MonadIO or MonadUnliftIO.

If you check out the module list for the unliftio library, you’ll see a collection of functionality available. The tutorials on this site use these unlifted variants in place of the originals wherever possible. Some related tutorials you may be interested in:

That last bullet brings us to our final caveat.

Exception handling semantics

Unlike the other modules in this library, the UnliftIO.Exception module not only applies the MonadIO and MonadUnliftIO typeclasses to existing functions. It also changes the semantics of functions available in Control.Exception. This is to simplify proper exception handling, especially in the presence of asynchronous exceptions. Please see the exception handling tutorial for more information.

The UnliftIO module

As a convenience, the UnliftIO module reexports a lot of common functionality from the other modules in this library. You may find it convenient to add import UnliftIO to your code. Alternatively, consider using rio, which also reexports the UnliftIO module.

Limitations

This tutorial has intentionally avoided discussing the design decisions in the unliftio library and the MonadUnliftIO typeclass. There are limitations versus alternatives. If you’d like more information, please read the unliftio README.