Moduleeffectful-core-2.3.0.1Haskell2010

Effectful.Dispatch.Dynamic

Dynamically dispatched effects.

3 types
1 class
15 values

Packageeffectful-core-2.3.0.1
Exports19
LanguageHaskell2010
LicenceBSD-3-Clause
SourceDynamic.hs

Introduction

0 declarations

A dynamically dispatched effect is a collection of operations that can be interpreted in different ways at runtime, depending on the handler that is used to run the effect.

This allows a programmer to separate the what from the how, i.e. define effects that model what the code should do, while providing handlers that determine how it should do it later. Moreover, different environments can use different handlers to change the behavior of specific parts of the application if appropriate.

An example

Let's create an effect for basic file access, i.e. writing and reading files.

First, we need to define a generalized algebraic data type of kind Effect, where each constructor corresponds to a specific operation of the effect in question.

Example1 expression

>>> :{  data FileSystem :: Effect where    ReadFile  :: FilePath -> FileSystem m String    WriteFile :: FilePath -> String -> FileSystem m ():}

Example1 expression

>>> type instance DispatchOf FileSystem = Dynamic

The FileSystem effect has two operations:

ReadFile, which takes a FilePath and returns a String in the monadic context.
WriteFile, which takes a FilePath, a String and returns a () in the monadic context.

For people familiar with mtl style effects, note that the syntax looks very similar to defining an appropriate type class:

class FileSystem m where
  readFile  :: FilePath -> m String
  writeFile :: FilePath -> String -> m ()

The biggest difference between these two is that the definition of a type class gives us operations as functions, while the definition of an effect gives us operations as data constructors. They can be turned into functions with the help of send:

Example1 expression

>>> :{  readFile :: (HasCallStack, FileSystem :> es) => FilePath -> Eff es String  readFile path = send (ReadFile path):}

Example1 expression

>>> :{  writeFile :: (HasCallStack, FileSystem :> es) => FilePath -> String -> Eff es ()  writeFile path content = send (WriteFile path content):}

Note: the above functions and the DispatchOf instance can also be automatically generated by the makeEffect function from the effectful-th package.

The following defines an EffectHandler that reads and writes files from the drive:

Example3 expressions

>>> import Control.Exception (IOException)>>> import Control.Monad.Catch (catch)>>> import qualified System.IO as IO

Example1 expression

>>> import Effectful.Error.Static

Example1 expression

>>> newtype FsError = FsError String deriving Show

Example1 expression

>>> :{ runFileSystemIO   :: (IOE :> es, Error FsError :> es)   => Eff (FileSystem : es) a   -> Eff es a runFileSystemIO = interpret $ \_ -> \case   ReadFile path           -> adapt $ IO.readFile path   WriteFile path contents -> adapt $ IO.writeFile path contents   where     adapt m = liftIO m `catch` \(e::IOException) -> throwError . FsError $ show e:}

Here, we use interpret and simply execute corresponding IO actions for each operation, additionally doing a bit of error management.

On the other hand, maybe there is a situation in which instead of interacting with the outside world, a pure, in-memory storage is preferred:

Example1 expression

>>> import qualified Data.Map.Strict as M

Example1 expression

>>> import Effectful.State.Static.Local

Example1 expression

>>> :{  runFileSystemPure    :: Error FsError :> es    => M.Map FilePath String    -> Eff (FileSystem : es) a    -> Eff es a  runFileSystemPure fs0 = reinterpret (evalState fs0) $ \_ -> \case    ReadFile path -> gets (M.lookup path) >>= \case      Just contents -> pure contents      Nothing       -> throwError . FsError $ "File not found: " ++ show path    WriteFile path contents -> modify $ M.insert path contents:}

Here, we use reinterpret and introduce a State effect for the storage that is private to the effect handler and cannot be accessed outside of it.

Let's compare how these differ.

Example1 expression

>>> :{  action = do    file <- readFile "effectful-core.cabal"    pure $ length file > 0:}

Example1 expression

>>> :t actionaction :: (FileSystem :> es) => Eff es Bool

Example1 expression

>>> runEff . runError @FsError . runFileSystemIO $ actionRight True

Example1 expression

>>> runPureEff . runErrorNoCallStack @FsError . runFileSystemPure M.empty $ actionLeft (FsError "File not found: \"effectful-core.cabal\"")

First order and higher order effects

Note that the definition of the FileSystem effect from the previous section doesn't use the m type parameter. What is more, when the effect is interpreted, the LocalEnv argument of the EffectHandler is also not used. Such effects are first order.

If an effect makes use of the m parameter, it is a higher order effect.

Interpretation of higher order effects is slightly more involving. To see why, let's consider the Profiling effect for logging how much time a specific action took to run:

Example1 expression

>>> :{  data Profiling :: Effect where    Profile :: String -> m a -> Profiling m a:}

Example1 expression

>>> type instance DispatchOf Profiling = Dynamic

Example1 expression

>>> :{  profile :: (HasCallStack, Profiling :> es) => String -> Eff es a -> Eff es a  profile label action = send (Profile label action):}

If we naively try to interpret it, we will run into trouble:

Example1 expression

>>> import GHC.Clock (getMonotonicTime)

Example1 expression

>>> :{ runProfiling :: IOE :> es => Eff (Profiling : es) a -> Eff es a runProfiling = interpret $ \_ -> \case   Profile label action -> do     t1 <- liftIO getMonotonicTime     r <- action     t2 <- liftIO getMonotonicTime     liftIO . putStrLn $ "Action '" ++ label ++ "' took " ++ show (t2 - t1) ++ " seconds."     pure r:}...... Couldn't match type ‘localEs’ with ‘es’...

The problem is that action has a type Eff localEs a, while the monad of the effect handler is Eff es. localEs represents the local environment in which the Profile operation was called, which is opaque as the effect handler cannot possibly know how it looks like.

The solution is to use the LocalEnv that an EffectHandler is given to run the action using one of the functions from the localUnlift family:

Example1 expression

>>> :{ runProfiling :: IOE :> es => Eff (Profiling : es) a -> Eff es a runProfiling = interpret $ \env -> \case   Profile label action -> localSeqUnliftIO env $ \unlift -> do     t1 <- getMonotonicTime     r <- unlift action     t2 <- getMonotonicTime     putStrLn $ "Action '" ++ label ++ "' took " ++ show (t2 - t1) ++ " seconds."     pure r:}

In a similar way we can define a dummy interpreter that does no profiling:

Example1 expression

>>> :{ runNoProfiling :: Eff (Profiling : es) a -> Eff es a runNoProfiling = interpret $ \env -> \case   Profile label action -> localSeqUnlift env $ \unlift -> unlift action:}

...and it's done.

Example1 expression

>>> action = profile "greet" . liftIO $ putStrLn "Hello!"

Example1 expression

>>> :t actionaction :: (Profiling :> es, IOE :> es) => Eff es ()

Example1 expression

>>> runEff . runProfiling $ actionHello!Action 'greet' took ... seconds.

Example1 expression

>>> runEff . runNoProfiling $ actionHello!

Integration with `mtl` style effects

There exists a lot of libraries that provide their functionality as an mtl style effect, which generally speaking is a type class that contains core operations of the library in question.

Such effects are quite easy to use with the Eff monad. As an example, consider the mtl style effect for generation of random numbers:

Example1 expression

>>> :{  class Monad m => MonadRNG m where    randomInt :: m Int:}

Let's say the library also defines a helper function for generation of random strings:

Example2 expressions

>>> import Control.Monad>>> import Data.Char

Example1 expression

>>> :{ randomString :: MonadRNG m => Int -> m String randomString n = map chr <$> replicateM n randomInt:}

To make it possible to use it with the Eff monad, the first step is to create an effect with operations that mirror the ones of a type class:

Example1 expression

>>> :{  data RNG :: Effect where    RandomInt :: RNG m Int:}

Example1 expression

>>> type instance DispatchOf RNG = Dynamic

If we continued as in the example above, we'd now create top level helper functions that execute effect operations using send, in this case randomInt tied to RandomInt. But this function is already declared by the MonadRNG type class! Therefore, what we do instead is provide an orphan, canonical instance of MonadRNG for Eff that delegates to the RNG effect:

Example1 expression

>>> :set -XUndecidableInstances

Example1 expression

>>> :{  instance RNG :> es => MonadRNG (Eff es) where    randomInt = send RandomInt:}

Now we only need an interpreter:

Example1 expression

>>> :{  runDummyRNG :: Eff (RNG : es) a -> Eff es a  runDummyRNG = interpret $ \_ -> \case    RandomInt -> pure 55:}

and we can use any function that requires a MonadRNG constraint with the Eff monad as long as the RNG effect is in place:

Example1 expression

>>> runEff . runDummyRNG $ randomString 3"777"

Functional dependencies

For dealing with classes that employ functional dependencies an additional trick is needed.

Consider the following:

Example1 expression

>>> :set -XFunctionalDependencies

Example1 expression

>>> :{  class Monad m => MonadInput i m | m -> i where    input :: m i:}

An attempt to define the instance as in the example above leads to violation of the liberal coverage condition:

Example1 expression

>>> :{  instance Reader i :> es => MonadInput i (Eff es) where    input = ask:}......Illegal instance declaration for ‘MonadInput i (Eff es)’......  The liberal coverage condition fails in class ‘MonadInput’......    for functional dependency: ‘m -> i’......

However, there exists a dirty trick for bypassing the coverage condition, i.e. including the instance head in the context:

Example1 expression

>>> :{  instance (MonadInput i (Eff es), Reader i :> es) => MonadInput i (Eff es) where    input = ask:}

Now the MonadInput class can be used with the Eff monad:

Example1 expression

>>> :{  double :: MonadInput Int m => m Int  double = (+) <$> input <*> input:}

Example1 expression

>>> runPureEff . runReader @Int 3 $ double6

Sending operations to the handler

1 declaration

valuesend

:: (HasCallStack, DispatchOf e ~ 'Dynamic, e :> es)
=> e (Eff es) a
The operation.
-> Eff es a

Send an operation of the given effect to its handler for execution.

Handling effects

5 declarations

type

type EffectHandler (e :: Effect) (es :: [Effect]) = forall a (localEs :: [Effect]). (HasCallStack, e :> localEs) => LocalEnv localEs es -> e (Eff localEs) a -> Eff es a

Type signature of the effect handler.

valueinterpret

:: DispatchOf e ~ 'Dynamic
=> EffectHandler e es
The effect handler.
-> Eff (e ': es) a
-> Eff es a

Interpret an effect.

Note: interpret can be turned into a reinterpret with the use of inject.

valuereinterpret

:: DispatchOf e ~ 'Dynamic
=> (Eff handlerEs a -> Eff es b)
Introduction of effects encapsulated within the handler.
-> EffectHandler e handlerEs
The effect handler.
-> Eff (e ': es) a
-> Eff es b

Interpret an effect using other, private effects.

interpret ≡ reinterpret id

valueinterpose

:: (DispatchOf e ~ 'Dynamic, e :> es)
=> EffectHandler e es
The effect handler.
-> Eff es a
-> Eff es a

Replace the handler of an existing effect with a new one.

Note: this function allows for augmenting handlers with a new functionality as the new handler can send operations to the old one.

Example1 expression

>>> :{  data E :: Effect where    Op :: E m ()  type instance DispatchOf E = Dynamic:}

Example1 expression

>>> :{  runE :: IOE :> es => Eff (E : es) a -> Eff es a  runE = interpret $ \_ Op -> liftIO (putStrLn "op"):}

Example1 expression

>>> runEff . runE $ send Opop

Example1 expression

>>> :{  augmentE :: (E :> es, IOE :> es) => Eff es a -> Eff es a  augmentE = interpose $ \_ Op -> liftIO (putStrLn "augmented op") >> send Op:}

Example1 expression

>>> runEff . runE . augmentE $ send Opaugmented opop

valueimpose

:: (DispatchOf e ~ 'Dynamic, e :> es)
=> (Eff handlerEs a -> Eff es b)
Introduction of effects encapsulated within the handler.
-> EffectHandler e handlerEs
The effect handler.
-> Eff es a
-> Eff es b

Replace the handler of an existing effect with a new one that uses other, private effects.

interpose ≡ impose id

Handling local Eff computations

newtypenewtype LocalEnv (localEs :: [Effect]) (handlerEs :: [Effect])

Opaque representation of the Eff environment at the point of calling the send function, i.e. right before the control is passed to the effect handler.

The second type variable represents effects of a handler and is needed for technical reasons to guarantee soundness (see SharedSuffix for more information).

Unlifts

valuelocalSeqUnlift

:: (HasCallStack, SharedSuffix es handlerEs)
=> LocalEnv localEs handlerEs
Local environment.
-> ((forall r. Eff localEs r -> Eff es r) -> Eff es a)
Continuation with the unlifting function in scope.
-> Eff es a

Create a local unlifting function with the SeqUnlift strategy. For the general version see localUnlift.

valuelocalSeqUnliftIO

:: (HasCallStack, SharedSuffix es handlerEs, IOE :> es)
=> LocalEnv localEs handlerEs
Local environment.
-> ((forall r. Eff localEs r -> IO r) -> IO a)
Continuation with the unlifting function in scope.
-> Eff es a

Create a local unlifting function with the SeqUnlift strategy. For the general version see localUnliftIO.

valuelocalUnlift

:: (HasCallStack, SharedSuffix es handlerEs)
=> LocalEnv localEs handlerEs
Local environment.
-> UnliftStrategy
-> ((forall r. Eff localEs r -> Eff es r) -> Eff es a)
Continuation with the unlifting function in scope.
-> Eff es a

Create a local unlifting function with the given strategy.

valuelocalUnliftIO

:: (HasCallStack, SharedSuffix es handlerEs, IOE :> es)
=> LocalEnv localEs handlerEs
Local environment.
-> UnliftStrategy
-> ((forall r. Eff localEs r -> IO r) -> IO a)
Continuation with the unlifting function in scope.
-> Eff es a

Create a local unlifting function with the given strategy.

Lifts

valuelocalSeqLift

:: (HasCallStack, SharedSuffix es handlerEs)
=> LocalEnv localEs handlerEs
Local environment.
-> ((forall r. Eff es r -> Eff localEs r) -> Eff es a)
Continuation with the lifting function in scope.
-> Eff es a

Create a local lifting function with the SeqUnlift strategy. For the general version see localLift.

valuelocalLift

:: (HasCallStack, SharedSuffix es handlerEs)
=> LocalEnv localEs handlerEs
Local environment.
-> UnliftStrategy
-> ((forall r. Eff es r -> Eff localEs r) -> Eff es a)
Continuation with the lifting function in scope.
-> Eff es a

Create a local lifting function with the given strategy.

valuewithLiftMap

:: (HasCallStack, SharedSuffix es handlerEs)
=> LocalEnv localEs handlerEs
Local environment.
-> ((forall a b. (Eff es a -> Eff es b) -> Eff localEs a -> Eff localEs b) -> Eff es r)
Continuation with the lifting function in scope.
-> Eff es r

Utility for lifting Eff computations of type

Eff es a -> Eff es b

Eff localEs a -> Eff localEs b

Note: the computation must not run its argument in a different thread, attempting to do so will result in a runtime error.

valuewithLiftMapIO

:: (HasCallStack, SharedSuffix es handlerEs, IOE :> es)
=> LocalEnv localEs handlerEs
Local environment.
-> ((forall a b. (IO a -> IO b) -> Eff localEs a -> Eff localEs b) -> Eff es r)
Continuation with the lifting function in scope.
-> Eff es r

Utility for lifting IO computations of type

IO a -> IO b

Eff localEs a -> Eff localEs b

Note: the computation must not run its argument in a different thread, attempting to do so will result in a runtime error.

Useful e.g. for lifting the unmasking function in mask-like computations:

Example1 expression

>>> :{data Fork :: Effect where  ForkWithUnmask :: ((forall a. m a -> m a) -> m ()) -> Fork m ThreadIdtype instance DispatchOf Fork = Dynamic:}

Example1 expression

>>> :{runFork :: IOE :> es => Eff (Fork : es) a -> Eff es arunFork = interpret $ \env (ForkWithUnmask m) -> withLiftMapIO env $ \liftMap -> do  localUnliftIO env (ConcUnlift Ephemeral $ Limited 1) $ \unlift -> do    forkIOWithUnmask $ \unmask -> unlift $ m $ liftMap unmask:}

Bidirectional lifts

valuelocalLiftUnlift

:: (HasCallStack, SharedSuffix es handlerEs)
=> LocalEnv localEs handlerEs
Local environment.
-> UnliftStrategy
-> ((forall r. Eff es r -> Eff localEs r) -> (forall r. Eff localEs r -> Eff es r) -> Eff es a)
Continuation with the lifting and unlifting function in scope.
-> Eff es a

Create a local lifting and unlifting function with the given strategy.

Useful for lifting complicated Eff computations where the monadic action shows in both positive (as a result) and negative (as an argument) position.

Note: depending on the computation you're lifting localUnlift along with withLiftMap might be enough and is more efficient.

valuelocalLiftUnliftIO

:: (HasCallStack, SharedSuffix es handlerEs, IOE :> es)
=> LocalEnv localEs handlerEs
Local environment.
-> UnliftStrategy
-> ((forall r. IO r -> Eff localEs r) -> (forall r. Eff localEs r -> IO r) -> IO a)
Continuation with the lifting and unlifting function in scope.
-> Eff es a

Create a local unlifting function with the given strategy along with an unrestricted lifting function.

Useful for lifting complicated IO computations where the monadic action shows in both positive (as a result) and negative (as an argument) position.

Note: depending on the computation you're lifting localUnliftIO along with withLiftMapIO might be enough and is more efficient.

Utils

classclass SharedSuffix (es1 :: [Effect]) (es2 :: [Effect])

Require that both effect stacks share an opaque suffix.

Functions from the localUnlift family utilize this constraint to guarantee sensible usage of unlifting functions.

As an example, consider the following higher order effect:

Example1 expression

>>> :{  data E :: Effect where    E :: m a -> E m a  type instance DispatchOf E = Dynamic:}

Running local actions in a more specific environment is fine:

Example1 expression

>>> :{ runE1 :: Eff (E ': es) a -> Eff es a runE1 = interpret $ \env -> \case   E m -> runReader () $ do     localSeqUnlift env $ \unlift -> unlift m:}

Running local actions in a more general environment is fine:

Example1 expression

>>> :{ runE2 :: Eff (E ': es) a -> Eff es a runE2 = reinterpret (runReader ()) $ \env -> \case   E m -> raise $ do     localSeqUnlift env $ \unlift -> unlift m:}

However, running local actions in an unrelated environment is not fine as this would make it possible to run anything within runPureEff:

Example1 expression

>>> :{ runE3 :: Eff (E ': es) a -> Eff es a runE3 = reinterpret (runReader ()) $ \env -> \case   E m -> pure . runPureEff $ do     localSeqUnlift env $ \unlift -> unlift m:}......Could not deduce ...SharedSuffix '[] es......

Running local actions in a monomorphic effect stack is also not fine as this makes a special case of the above possible:

Example1 expression

>>> :{ runE4 :: Eff '[E, IOE] a -> Eff '[IOE] a runE4 = interpret $ \env -> \case   E m -> pure . runPureEff $ do     localSeqUnlift env $ \unlift -> unlift m:}......Running local actions in monomorphic effect stacks is not supported......

Instances4SharedSuffix

SharedSuffix es esDefined in effectful-core-2.3.0.1 · Effectful.Dispatch.Dynamic
SharedSuffix es1 es2 => SharedSuffix es1 (e ': es2)Defined in effectful-core-2.3.0.1 · Effectful.Dispatch.Dynamic
TypeError ('Text "Running local actions in monomorphic effect stacks is not supported." ':$$: 'Text "As a solution simply change the stack to have a polymorphic suffix.") => SharedSuffix '[] '[]Defined in effectful-core-2.3.0.1 · Effectful.Dispatch.Dynamic
This is always preferred to SharedSuffix es es as it's not incoherent.
SharedSuffix es1 es2 => SharedSuffix (e ': es1) es2Defined in effectful-core-2.3.0.1 · Effectful.Dispatch.Dynamic

Re-exports

1 declaration

typetype HasCallStack = IP "callStack" CallStack

Request a CallStack.

NOTE: The implicit parameter ?callStack :: CallStack is an implementation detail and should not be considered part of the CallStack API, we may decide to change the implementation in the future.

Introduction

An example

First order and higher order effects

Integration with mtl style effects

Functional dependencies

Sending operations to the handler

Handling effects

Handling local Eff computations

Unlifts

Lifts

Bidirectional lifts

Utils

Re-exports

Integration with `mtl` style effects