# unliftio ![Tests](https://github.com/fpco/unliftio/workflows/Tests/badge.svg) Provides the core `MonadUnliftIO` typeclass, a number of common instances, and a collection of common functions working with it. Not sure what the `MonadUnliftIO` typeclass is all about? Read on! __NOTE__ This library is young, and will likely undergo some serious changes over time. It's also very lightly tested. That said: the core concept of `MonadUnliftIO` has been refined for years and is pretty solid, and even though the code here is lightly tested, the vast majority of it is simply apply `withUnliftIO` to existing functionality. Caveat emptor and all that. __NOTE__ The `UnliftIO.Exception` module in this library changes the semantics of asynchronous exceptions to be in the style of the `safe-exceptions` package, which is orthogonal to the "unlifting" concept. While this change is an improvment in most cases, it means that `UnliftIO.Exception` is not always a drop-in replacement for `Control.Exception` in advanced exception handling code. See [Async exception safety](#async-exception-safety) for details. ## Quickstart * Replace imports like `Control.Exception` with `UnliftIO.Exception`. Yay, your `catch` and `finally` are more powerful and safer (see [Async exception safety](#async-exception-safety))! * Similar with `Control.Concurrent.Async` with `UnliftIO.Async` * Or go all in and import `UnliftIO` * Naming conflicts: let `unliftio` win * Drop the deps on `monad-control`, `lifted-base`, and `exceptions` * Compilation failures? You may have just avoided subtle runtime bugs Sound like magic? It's not. Keep reading! ## Unlifting in 2 minutes Let's say I have a function: ```haskell readFile :: FilePath -> IO ByteString ``` But I'm writing code inside a function that uses `ReaderT Env IO`, not just plain `IO`. How can I call my `readFile` function in that context? One way is to manually unwrap the `ReaderT` data constructor: ```haskell myReadFile :: FilePath -> ReaderT Env IO ByteString myReadFile fp = ReaderT $ \_env -> readFile fp ``` But having to do this regularly is tedious, and ties our code to a specific monad transformer stack. Instead, many of us would use `MonadIO`: ```haskell myReadFile :: MonadIO m => FilePath -> m ByteString myReadFile = liftIO . readFile ``` But now let's play with a different function: ```haskell withBinaryFile :: FilePath -> IOMode -> (Handle -> IO a) -> IO a ``` We want a function with signature: ```haskell myWithBinaryFile :: FilePath -> IOMode -> (Handle -> ReaderT Env IO a) -> ReaderT Env IO a ``` If I squint hard enough, I can accomplish this directly with the `ReaderT` constructor via: ```haskell myWithBinaryFile fp mode inner = ReaderT $ \env -> withBinaryFile fp mode (\h -> runReaderT (inner h) env) ``` I dare you to try and accomplish this with `MonadIO` and `liftIO`. It simply can't be done. (If you're looking for the technical reason, it's because `IO` appears in [negative/argument position](https://www.fpcomplete.com/blog/2016/11/covariance-contravariance) in `withBinaryFile`.) However, with `MonadUnliftIO`, this is possible: ```haskell import Control.Monad.IO.Unlift myWithBinaryFile :: MonadUnliftIO m => FilePath -> IOMode -> (Handle -> m a) -> m a myWithBinaryFile fp mode inner = withRunInIO $ \runInIO -> withBinaryFile fp mode (\h -> runInIO (inner h)) ``` That's it, you now know the entire basis of this library. ## How common is this problem? This pops up in a number of places. Some examples: * Proper exception handling, with functions like `bracket`, `catch`, and `finally` * Working with `MVar`s via `modifyMVar` and similar * Using the `timeout` function * Installing callback handlers (e.g., do you want to do [logging](https://www.stackage.org/package/monad-logger) in a signal handler?). This also pops up when working with libraries which are monomorphic on `IO`, even if they could be written more extensibly. ## Examples Reading through the codebase here is likely the best example to see how to use `MonadUnliftIO` in practice. And for many cases, you can simply add the `MonadUnliftIO` constraint and then use the pre-unlifted versions of functions (like `UnliftIO.Exception.catch`). But ultimately, you'll probably want to use the typeclass directly. The type class has only one method -- `withRunInIO`: ```haskell class MonadIO m => MonadUnliftIO m where withRunInIO :: ((forall a. m a -> IO a) -> IO b) -> m b ``` `withRunInIO` provides a function to run arbitrary computations in `m` in `IO`. Thus the "unlift": it's like `liftIO`, but the other way around. Here are some sample typeclass instances: ```haskell instance MonadUnliftIO IO where withRunInIO inner = inner id instance MonadUnliftIO m => MonadUnliftIO (ReaderT r m) where withRunInIO inner = ReaderT $ \r -> withRunInIO $ \run -> inner (run . flip runReaderT r) instance MonadUnliftIO m => MonadUnliftIO (IdentityT m) where withRunInIO inner = IdentityT $ withRunInIO $ \run -> inner (run . runIdentityT) ``` Note that: * The `IO` instance does not actually do any lifting or unlifting, and therefore it can use `id` * `IdentityT` is essentially just wrapping/unwrapping its data constructor, and then recursively calling `withRunInIO` on the underlying monad. * `ReaderT` is just like `IdentityT`, but it captures the reader environment when starting. We can use `withRunInIO` to unlift a function: ```haskell timeout :: MonadUnliftIO m => Int -> m a -> m (Maybe a) timeout x y = withRunInIO $ \run -> System.Timeout.timeout x $ run y ``` This is a common pattern: use `withRunInIO` to capture a run function, and then call the original function with the user-supplied arguments, applying `run` as necessary. `withRunInIO` takes care of invoking `unliftIO` for us. We can also use the run function with different types due to `withRunInIO` being higher-rank polymorphic: ```haskell race :: MonadUnliftIO m => m a -> m b -> m (Either a b) race a b = withRunInIO $ \run -> A.race (run a) (run b) ``` And finally, a more complex usage, when unlifting the `mask` function. This function needs to unlift values to be passed into the `restore` function, and then `liftIO` the result of the `restore` function. ```haskell mask :: MonadUnliftIO m => ((forall a. m a -> m a) -> m b) -> m b mask f = withRunInIO $ \run -> Control.Exception.mask $ \restore -> run $ f $ liftIO . restore . run ``` ## Limitations Not all monads which can be an instance of `MonadIO` can be instances of `MonadUnliftIO`, due to the `MonadUnliftIO` laws (described in the Haddocks for the typeclass). This prevents instances for a number of classes of transformers: * Transformers using continuations (e.g., `ContT`, `ConduitM`, `Pipe`) * Transformers with some monadic state (e.g., `StateT`, `WriterT`) * Transformers with multiple exit points (e.g., `ExceptT` and its ilk) In fact, there are two specific classes of transformers that this approach does work for: * Transformers with no context at all (e.g., `IdentityT`, `NoLoggingT`) * Transformers with a context but no state (e.g., `ReaderT`, `LoggingT`) This may sound restrictive, but this restriction is fully intentional. Trying to unlift actions in stateful monads leads to unpredictable behavior. For a long and exhaustive example of this, see [A Tale of Two Brackets](https://www.fpcomplete.com/blog/2017/06/tale-of-two-brackets), which was a large motivation for writing this library. ## Comparison to other approaches You may be thinking "Haven't I seen a way to do `catch` in `StateT`?" You almost certainly have. Let's compare this approach with alternatives. (For an older but more thorough rundown of the options, see [Exceptions and monad transformers](http://www.yesodweb.com/blog/2014/06/exceptions-transformers).) There are really two approaches to this problem: * Use a set of typeclasses for the specific functionality we care about. This is the approach taken by the `exceptions` package with `MonadThrow`, `MonadCatch`, and `MonadMask`. (Earlier approaches include `MonadCatchIO-mtl` and `MonadCatchIO-transformers`.) * Define a generic typeclass that allows any control structure to be unlifted. This is the approach taken by the `monad-control` package. (Earlier approaches include `monad-peel` and `neither`.) The first style gives extra functionality in allowing instances that have nothing to do with runtime exceptions (e.g., a `MonadCatch` instance for `Either`). This is arguably a good thing. The second style gives extra functionality in allowing more operations to be unlifted (like threading primitives, not supported by the `exceptions` package). Another distinction within the generic typeclass family is whether we unlift to just `IO`, or to arbitrary base monads. For those familiar, this is the distinction between the `MonadIO` and `MonadBase` typeclasses. This package's main objection to all of the above approaches is that they work for too many monads, and provide difficult-to-predict behavior for a number of them (arguably: plain wrong behavior). For example, in `lifted-base` (built on top of `monad-control`), the `finally` operation will discard mutated state coming from the cleanup action, which is usually not what people expect. `exceptions` has _different_ behavior here, which is arguably better. But we're arguing here that we should disallow all such ambiguity at the type level. So comparing to other approaches: ### monad-unlift Throwing this one out there now: the `monad-unlift` library is built on top of `monad-control`, and uses fairly sophisticated type level features to restrict it to only the safe subset of monads. The same approach is taken by `Control.Concurrent.Async.Lifted.Safe` in the `lifted-async` package. Two problems with this: * The complicated type level functionality can confuse GHC in some cases, making it difficult to get code to compile. * We don't have an ecosystem of functions like `lifted-base` built on top of it, making it likely people will revert to the less safe cousin functions. ### monad-control The main contention until now is that unlifting in a transformer like `StateT` is unsafe. This is not universally true: if only one action is being unlifted, no ambiguity exists. So, for example, `try :: IO a -> IO (Either e a)` can safely be unlifted in `StateT`, while `finally :: IO a -> IO b -> IO a` cannot. `monad-control` allows us to unlift both styles. In theory, we could write a variant of `lifted-base` that never does state discards, and let `try` be more general than `finally`. In other words, this is an advantage of `monad-control` over `MonadUnliftIO`. We've avoided providing any such extra typeclass in this package though, for two reasons: * `MonadUnliftIO` is a simple typeclass, easy to explain. We don't want to complicated matters (`MonadBaseControl` is a notoriously difficult to understand typeclass). This simplicity is captured by the laws for `MonadUnliftIO`, which make the behavior of the run functions close to that of the already familiar `lift` and `liftIO`. * Having this kind of split would be confusing in user code, when suddenly `finally` is not available to us. We would rather encourage [good practices](https://www.fpcomplete.com/blog/2017/06/readert-design-pattern) from the beginning. Another distinction is that `monad-control` uses the `MonadBase` style, allowing unlifting to arbitrary base monads. In this package, we've elected to go with `MonadIO` style. This limits what we can do (e.g., no unlifting to `STM`), but we went this way because: * In practice, we've found that the vast majority of cases are dealing with `IO` * The split in the ecosystem between constraints like `MonadBase IO` and `MonadIO` leads to significant confusion, and `MonadIO` is by far the more common constraints (with the typeclass existing in `base`) ### exceptions One thing we lose by leaving the `exceptions` approach is the ability to model both pure and side-effecting (via `IO`) monads with a single paradigm. For example, it can be pretty convenient to have `MonadThrow` constraints for parsing functions, which will either return an `Either` value or throw a runtime exception. That said, there are detractors of that approach: * You lose type information about which exception was thrown * There is ambiguity about _how_ the exception was returned in a constraint like `(MonadIO m, MonadThrow m`) The latter could be addressed by defining a law such as `throwM = liftIO . throwIO`. However, we've decided in this library to go the route of encouraging `Either` return values for pure functions, and using runtime exceptions in `IO` otherwise. (You're of course free to also return `IO (Either e a)`.) By losing `MonadCatch`, we lose the ability to define a generic way to catch exceptions in continuation based monads (such as `ConduitM`). Our argument here is that those monads can freely provide their own catching functions. And in practice, long before the `MonadCatch` typeclass existed, `conduit` provided a `catchC` function. In exchange for the `MonadThrow` typeclass, we provide helper functions to convert `Either` values to runtime exceptions in this package. And the `MonadMask` typeclass is now replaced fully by `MonadUnliftIO`, which like the `monad-control` case limits which monads we can be working with. ## Async exception safety The [`safe-exceptions`](https://hackage.haskell.org/package/safe-exceptions) package builds on top of the `exceptions` package and provides intelligent behavior for dealing with asynchronous exceptions, a common pitfall. This library provides a set of exception handling functions with the same async exception behavior as that library. You can consider this library a drop-in replacement for `safe-exceptions`. In the future, we may reimplement `safe-exceptions` to use `MonadUnliftIO` instead of `MonadCatch` and `MonadMask`. ## Package split The `unliftio-core` package provides just the typeclass with minimal dependencies (just `base` and `transformers`). If you're writing a library, we recommend depending on that package to provide your instances. The `unliftio` package is a "batteries loaded" library providing a plethora of pre-unlifted helper functions. It's a good choice for importing, or even for use in a custom prelude. ## Orphans The `unliftio` package currently provides orphan instances for types from the `resourcet` and `monad-logger` packages. This is not intended as a long-term solution; once `unliftio` is deemed more stable, the plan is to move those instances into the respective libraries and remove the dependency on them here. If there are other temporary orphans that should be added, please bring it up in the issue tracker or send a PR, but we'll need to be selective about adding dependencies. ## Future questions * Should we extend the set of functions exposed in `UnliftIO.IO` to include things like `hSeek`? * Are there other libraries that deserve to be unlifted here?