A fast, flexible, fused effect system for Haskell

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fused-effects is an effect system for Haskell emphasizing expressivity and efficiency. The former is achieved by encoding algebraic, higher-order effects, while the latter is the result of fusing effect handlers all the way through computations.

Readers already familiar with effect systems may wish to start with the usage instead.

Algebraic effects

In fused-effects and other systems with algebraic (or, sometimes, extensible) effects, effectful programs are split into two parts: the specification (or syntax) of the actions to be performed, and the interpretation (or semantics) given to them. Thus, a program written using the syntax of an effect can be given different meanings by using different effect handlers.

These roles are performed by the effect and carrier types, respectively. Effects are datatypes with one constructor for each action. Carriers are generally newtypes, with a Carrier instance specifying how an effect’s constructors should be interpreted. Each carrier handles one effect, but multiple carriers can be defined for the same effect, corresponding to different interpreters for the effect’s syntax.

Higher-order effects

Unlike most other effect systems, fused-effects offers higher-order (or scoped) effects in addition to first-order algebraic effects. In a strictly first-order algebraic effect system, operations (like local or catchError) which specify some action limited to a given scope must be implemented as interpreters, hard-coding their meaning in precisely the manner algebraic effects were designed to avoid. By specifying effects as higher-order functors, these operations are likewise able to be given a variety of interpretations. This means, for example, that you can introspect and redefine both the local and ask operations provided by the Reader effect, rather than solely ask (as is the case with certain formulations of algebraic effects).

As Nicolas Wu et al showed in Effect Handlers in Scope, this has implications for the expressiveness of effect systems. It also has the benefit of making effect handling more consistent, since scoped operations are just syntax which can be interpreted like any other, and are thus simpler to reason about.


In order to maximize efficiency, fused-effects applies fusion laws, avoiding the construction of intermediate representations of effectful computations between effect handlers. In fact, this is applied as far as the initial construction as well: there is no representation of the computation as a free monad parameterized by some syntax type. As such, fused-effects avoids the overhead associated with constructing and evaluating any underlying free or freer monad.

Instead, computations are performed in a monad named Eff, parameterized by the carrier type for the syntax. This carrier is specific to the effect handler selected, but since it isn’t described until the handler is applied, the separation between specification and interpretation is maintained. Computations are written against an abstract effectful signature, and only specialized to some concrete carrier when their effects are interpreted.

Carriers needn’t be Functors (let alone Monads), allowing a great deal of freedom in the interpretation of effects. And since the interpretation is written as a typeclass instance which ghc is eager to inline, performance is excellent: approximately on par with mtl.

Finally, since the fusion of carrier algebras occurs as a result of the selection of the carriers, it doesn’t depend on complex RULES pragmas, making it very easy to reason about and tune.


Using built-in effects

Like other effect systems, effects are performed in a Monad extended with operations relating to the effect. In fused-effects, this is done by means of a Member constraint to require the effect’s presence in a signature, and a Carrier constraint to relate the signature to the Monad. For example, to use a State effect managing a String, one would write:

action :: (Member (State String) sig, Carrier sig m) => m ()

(Additional constraints may be necessary depending on the precise operations required, e.g. to make the Monad methods available.)

Multiple effects can be required simply by adding their corresponding Member constraints to the context. For example, to add a Reader effect managing an Int, we would write:

action :: (Member (State String) sig, Member (Reader Int) sig, Carrier sig m) => m ()

Different effects make different operations available; see the documentation for individual effects for more information about their operations. Note that we generally don’t program against an explicit list of effect components: we take the typeclass-oriented approach, adding new constraints to sig as new capabilities become necessary. If you want to name and share some predefined list of effects, it’s best to use the -XConstraintKinds extension to GHC, capturing the elements of sig as a type synonym of kind Constraint:

type Shared sig = ( Member (State String) sig
                  , Member (Reader Int)   sig
                  , Member (Writer Graph) sig

myFunction :: (Shared sig, Carrier sig m) => Int -> m ()

Running effects

Effects are run with effect handlers, specified as functions (generally starting with run…) invoking some specific Carrier instance. For example, we can run a State computation using runState:

example1 :: (Carrier sig m, Effect sig) => [a] -> m (Int, ())
example1 list = runState 0 $ do
  i <- get
  put (i + length list)

runState returns a tuple of both the computed value (the ()) and the final state (the Int), visible in the result of the returned computation.

Since this function returns a value in some carrier m, effect handlers can be chained to run multiple effects. Here, we get the list to compute the length of from a Reader effect:

example2 :: (Carrier sig m, Effect sig, Monad m) => m (Int, ())
example2 = runReader "hello" . runState 0 $ do
  list <- ask
  put (length (list :: String))

(Note that the type annotation on list is necessary to disambiguate the requested value, since otherwise all the typechecker knows is that it’s an arbitrary Foldable. For more information, see the comparison to mtl.)

When all effects have been handled, a computation’s final value can be extracted with run:

example3 :: (Int, ())
example3 = run . runReader "hello" . runState 0 $ do
  list <- ask
  put (length (list :: String))

run is itself actually an effect handler for the Void effect, which has no operations and thus can only represent a final result value.

Alternatively, arbitrary Monads can be embedded into effectful computations using the Lift effect. In this case, the underlying Monadic computation can be extracted using runM. Here, we use the MonadIO instance for Eff to lift putStrLn into the middle of our computation:

example4 :: IO (Int, ())
example4 = runM . runReader "hello" . runState 0 $ do
  list <- ask
  liftIO (putStrLn list)
  put (length list)

(Note that we no longer need to give a type annotation for list, since putStrLn constrains the type for us.)

Required compiler extensions

To use effects, you’ll need a relatively-uncontroversial set of extensions: -XFlexibleContexts, -XFlexibleInstances, and -XMultiParamTypeClasses.

When defining your own effects, you’ll need -XTypeOperators to declare a Carrier instance over (:+:), and -XUndecidableInstances to satisfy the coverage condition for this instance. -XLambdaCase provides a measure of syntactic convenience when handling an effect type with handleSum. You may need -XKindSignatures if GHC cannot correctly infer the type of your handler; see the documentation on common errors for more information about this case.

The following invocation, taken from the teletype example, should suffice for any use or construction of effects:

{-# LANGUAGE DeriveFunctor, FlexibleContexts, FlexibleInstances, GeneralizedNewtypeDeriving,
    KindSignatures, LambdaCase, MultiParamTypeClasses, TypeOperators, UndecidableInstances #-}

Defining new effects

Effects are a powerful mechanism for abstraction, and so defining new effects is a valuable tool for system architecture. Effects are modelled as (higher-order) functors, with an explicit continuation denoting the remainder of the computation after the effect.

It’s often helpful to start by specifying the types of the desired operations. For our example, we’re going to define a Teletype effect, with read and write operations, which read a string from some input and write a string to some output, respectively:

data Teletype (m :: * -> *) k
read :: (Member Teletype sig, Carrier sig m) => m String
write :: (Member Teletype sig, Carrier sig m) => String -> m ()

Effect types must have two type parameters: m, denoting any computations which the effect embeds, and k, denoting the remainder of the computation after the effect. Note that since Teletype doesn’t use m, the compiler will infer it as being of kind * by default. The explicit kind annotation on m corrects that.

Next, we can flesh out the definition of the Teletype effect by providing constructors for each primitive operation:

data Teletype (m :: * -> *) k
  = Read (String -> k)
  | Write String k
  deriving (Functor)

The Read operation returns a String, and hence its continuation is represented as a function taking a String. Thus, to continue the computation, a handler will have to provide a String. But since the effect type doesn’t say anything about where that String should come from, handlers are free to read from stdin, use a constant value, etc.

On the other hand, the Write operation returns (). Since a function () -> k is equivalent to a (non-strict) k, we can omit the function parameter.

In addition to a Functor instance (derived here using -XDeriveFunctor), we need two other instances: HFunctor and Effect. HFunctor, named for “higher-order functor,” has one non-default operation, hmap, which applies a function to any embedded computations inside an effect. Since Teletype is first-order (i.e. it doesn’t have any embedded computations), the definition of hmap can be given using coerce:

instance HFunctor Teletype where
  hmap _ = coerce

Effect plays a similar role to the combination of Functor (which operates on continuations) and HFunctor (which operates on embedded computations). It’s used by Carrier instances to service any requests for their effect occurring inside other computations—whether embedded or in the continuations. Since these may require some state to be maintained, handle takes an initial state parameter (encoded as some arbitrary functor filled with ()), and its function is phrased as a distributive law, mapping state functors containing unhandled computations to handled computations producing the state functor alongside any results.

Since Teletype’s operations don’t have any embedded computations, the Effect instance only has to operate on the continuations, by wrapping the computations in the state and applying the handler:

instance Effect Teletype where
  handle state handler (Read    k) = Read (handler . (<$ state) . k)
  handle state handler (Write s k) = Write s (handler (k <$ state))

Now that we have our effect datatype, we can give definitions for read and write:

read :: (Member Teletype sig, Carrier sig m) => m String
read = send (Read ret)

write :: (Member Teletype sig, Carrier sig m) => String -> m ()
write s = send (Write s (ret ()))

This gives us enough to write computations using the Teletype effect. The next section discusses how to run Teletype computations.

Defining effect handlers

Effects only specify actions, they don’t actually perform them. That task is left up to effect handlers, typically defined as functions calling interpret to apply a given Carrier instance.

Following from the above section, we can define a carrier for the Teletype effect which runs the calls in an underlying MonadIO instance:

newtype TeletypeIOC m a = TeletypeIOC { runTeletypeIOC :: m a }

instance (Carrier sig m, MonadIO m) => Carrier (Teletype :+: sig) (TeletypeIOC m) where
  ret = TeletypeIOC . ret

  eff = TeletypeIOC . handleSum (eff . handleCoercible) (\ t -> case t of
    Read    k -> liftIO getLine      >>= runTeletypeIOC . k
    Write s k -> liftIO (putStrLn s) >>  runTeletypeIOC   k)

Here, ret is responsible for wrapping pure values in the carrier, and eff is responsible for handling an effectful computations. Since the Carrier instance handles a sum (:+:) of Teletype and the remaining signature, eff has two parts: a handler for Teletype (alg), and a handler for teletype effects that might be embedded in other effects in the signature.

In this case, since the Teletype carrier is just a thin wrapper around the underlying computation, we can use handleCoercible to handle any embedded TeletypeIOC carriers by simply mapping coerce over them.

That leaves alg, which handles Teletype effects with one case per constructor. Since we’re assuming the existence of a MonadIO instance for the underlying computation, we can use liftIO to inject the getLine and putStrLn actions into it, and then proceed with the continuations, unwrapping them in the process.

Users could use interpret directly to run the effect, but it’s more convenient to provide effect handler functions applying interpret and then unwrapping the carrier:

runTeletypeIO :: (MonadIO m, Carrier sig m) => Eff (TeletypeIOC m) a -> m a
runTeletypeIO = runTeletypeIOC . interpret

In general, carriers don’t have to be Functors, let alone Monads. However, sometimes—especially in cases where the carrier is a thin wrapper like this—they can be more convenient to write using (derived) Monad instances. In this case, by using -XGeneralizedNewtypeDeriving, we can derive Functor, Applicative, Monad, and MonadIO instances for TeletypeIOC:

newtype TeletypeIOC m a = TeletypeIOC { runTeletypeIOC :: m a }
  deriving (Applicative, Functor, Monad, MonadIO)

This allows us to use liftIO directly on the carrier itself, instead of only in the underlying m; likewise with >>=, >>, and pure:

instance (MonadIO m, Carrier sig m) => Carrier (Teletype :+: sig) (TeletypeIOC m) where
  ret = pure
  eff = handleSum (TeletypeIOC . eff . handleCoercible) (\ t -> case t of
    Read    k -> liftIO getLine      >>= k
    Write s k -> liftIO (putStrLn s) >>  k)

Project overview

This project builds a Haskell package named fused-effects. The library’s sources are in src, with doctests (property tests written in documentation comments) attached to most functions. Unit tests are in test, and library usage examples are in examples. Further documentation can be found in docs.

This project adheres to the Contributor Covenant code of conduct. By participating, you are expected to uphold this code.

Finally, this project is licensed under the BSD 3-clause license.


Development of fused-effects is typically done using cabal new-build:

cabal new-build # build the library
cabal new-test  # build and run the examples, unit tests, and doctests

The package is available on hackage, and can be used by adding it to a component’s build-depends field in your .cabal file.


Though fused-effects is suitable for production work, it is currently in a pre-release state. Though we will attempt to comply with the Haskell Package Versioning Policy standard, we make no concrete guarantees of API stability between versions < Once v1.0.0.0 lands, all changes will abide by the PVP MAJOR.MAJOR.MINOR.PATCH standard.


fused-effects has been benchmarked against a number of other effect systems. See also @patrickt’s benchmarks.

Related work

fused-effects is an encoding of higher-order algebraic effects following the recipes in Effect Handlers in Scope (Nicolas Wu, Tom Schrijvers, Ralf Hinze), Monad Transformers and Modular Algebraic Effects: What Binds Them Together (Tom Schrijvers, Maciej Piróg, Nicolas Wu, Mauro Jaskelioff), and Fusion for Free—Efficient Algebraic Effect Handlers (Nicolas Wu, Tom Schrijvers).

Comparison to mtl

Like mtl, fused-effects provides a library of monadic effects which can be given different interpretations. In mtl this is done by defining new instances of the typeclasses encoding the actions of the effect, e.g. MonadState. In fused-effects, this is done by defining new instances of the Carrier typeclass for the effect.

Also like mtl, fused-effects allows scoped operations like local and catchError to be given different interpretations. As with first-order operations, mtl achieves this with a final tagless encoding via methods, whereas fused-effects achieves this with an initial algebra encoding via Carrier instances.

Unlike mtl, effects are automatically available regardless of where they occur in the signature; in mtl this requires instances for all valid orderings of the transformers (O(n²) of them, in general).

Also unlike mtl, there can be more than one State or Reader effect in a signature. This is a tradeoff: mtl is able to provide excellent type inference for effectful operations like get, since the functional dependencies can resolve the state type from the monad type. On the other hand, this behaviour can be recovered in fused-effects using newtype wrappers with phantom type parameters and helper functions, e.g.:

newtype Wrapper s m a = Wrapper { runWrapper :: Eff m a }
  deriving (Applicative, Functor, Monad)

instance Carrier sig m => Carrier sig (Wrapper s m) where …

getState :: (Carrier sig m, Member (State s) m) => Wrapper m s
getState = get

Indeed, Wrapper can now be made an instance of MonadState:

instance (Carrier sig m, Member (State s) m) => MTL.MonadState s (Wrapper s m) where
  get = get
  put = put

Thus, the approaches aren’t mutually exclusive; consumers are free to decide which approach makes the most sense for their situation.

Unlike fused-effects, mtl provides a ContT monad transformer; however, it’s worth noting that many behaviours possible with delimited continuations (e.g. resumable exceptions) are directly encodable as effects. Further, fused-effects provides a relatively large palette of these, including resumable exceptions, tracing, resource management, and others, as well as tools to define your own.

Finally, thanks to the fusion and inlining of carriers, fused-effects is approximately as fast as mtl (see benchmarks).

Comparison to freer-simple

Like freer-simple, fused-effects uses an initial encoding of library- and user-defined effects as syntax which can then be given different interpretations. In freer-simple, this is done with a family of interpreter functions (which cover a variety of needs, and which can be extended for more bespoke needs), whereas in fused-effects this is done with Carrier instances for newtypes.

(Technically, it is possible to define handlers like freer-simple’s interpret using fused-effects, but passing handlers in as higher-order functions defeats the fusion and inlining of Carrier instances which makes fused-effects so efficient.)

Unlike fused-effects, in freer-simple, scoped operations like catchError and local are implemented as interpreters, and can therefore not be given new interpretations.

Unlike freer-simple, fused-effects has relatively little attention paid to compiler error messaging, which can make common (compile-time) errors somewhat more confusing to diagnose. Similarly, freer-simple’s family of interpreter functions can make the job of defining new effect handlers somewhat easier than in fused-effects. Further, freer-simple provides many of the same effects as fused-effects, plus a coroutine effect, but minus resource management and random generation.

Finally, fused-effects has been benchmarked as faster than freer-simple.


  • Loosens the bounds on QuickCheck to accommodate 0.12.

  • Adds support for ghc 8.6.2, courtesy of @jkachmar.
  • Adds a Cut effect which adds committed choice to nondeterminism.
  • Adds a Cull effect which adds pruning to nondeterminism.
  • Adds an example of using NonDet, Cut, and a character parser effect to define parsers.
  • Fixes the table of contents links in the README.

  • Adds a runNonDetOnce handler which terminates immediately upon finding a solution.

Initial release.

Depends on 4 packages:
Used by 1 package:
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