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README
A fast, flexible, fused effect system for Haskell
Overview
fusedeffects
is an effect system for Haskell that values expressivity, efficiency, and rigor. It provides an encoding of algebraic, higherorder effects, includes a library of the most common effects, and generates efficient code by fusing effect handlers through computations. It is suitable for use in hobbyist, research, and industrial contexts.
Readers already familiar with effect systems may wish to start with the usage instead. For those interested, this talk at Strange Loop outlines the history of and motivation behind effect systems and fusedeffects
itself.
<! Setup, hidden from the rendered markdown.
{# LANGUAGE ConstraintKinds, FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, UndecidableInstances #}
module Main (module Main) where
import Control.Algebra
import Control.Carrier.Lift
import Control.Carrier.Reader
import Control.Carrier.State.Strict
import Control.Monad.IO.Class (liftIO)
main :: IO ()
main = pure ()
>
Algebraic effects
In fusedeffects
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.
In fusedeffects
, effect types provide syntax and carrier types provide semantics. Effect types are datatypes with one constructor for each action, invoked using the send
builtin. Carriers are monads, with an Algebra
instance specifying how an effect’s constructors should be interpreted. Carriers can handle more than one effect, and multiple carriers can be defined for the same effect, corresponding to different interpreters for the effect’s syntax.
Higherorder effects
Unlike some other effect systems, fusedeffects
offers higherorder (or scoped) effects in addition to firstorder algebraic effects. In a strictly firstorder algebraic effect system, operations like local
or catchError
, which specify some action limited to a given scope, must be implemented as interpreters, hardcoding their meaning in precisely the manner algebraic effects were designed to avoid. By specifying effects as higherorder functors, this limitation is removed, meaning that these operations admit 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.
Fusion
In order to maximize efficiency, fusedeffects
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, fusedeffects
avoids the overhead associated with constructing and evaluating any underlying free or freer monad.
Instead, computations are performed in a carrier type for the syntax, typically a monad wrapping further monads, via an instance of the Carrier
class. 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.
Since the interpretation of effects 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 easy to reason about and tune.
Usage
Package organization
The fusedeffects
package is organized into two module hierarchies:
 those under
Control.Effect
, which provide effects and functions that invoke these effects’ capabilities.  those under
Control.Carrier
, which provide carrier types capable of executing the effects described by a given effect type.
An additional module, Control.Algebra
, provides the Algebra
interface that carrier types implement to provide an interpretation of a given effect. You shouldn’t need to import it unless you’re defining your own effects.
Invoking effects
Each module under the Control.Effect
hierarchy provides a set of functions that invoke effects, each mapping to a constructor of the underlying effect type. These functions are similar to, but more powerful than, those provided by mtl
. In this example, we invoke the get
and put
functions provided by Control.Effect.State
, first extracting the state and then updating it with a new value:
action1 :: Has (State String) sig m => m ()
action1 = get >>= \ s > put ("hello, " ++ s)
The Has
constraint requires a given effect (here State
) to be present in a signature (sig
), and relates that signature to be present in a carrier type (m
). We generally, but not always, program against an abstract carrier type, usually called m
, as carrier types always implement the Monad
typeclass.
To add effects to a given computation, add more Has
constraints to the signature/carrier pair sig
and m
. For example, to add a Reader
effect managing an Int
, we would write:
action2 :: (Has (State String) sig m, Has (Reader Int) sig m) => m ()
action2 = do
i < ask
put (replicate i '!')
Running effects
Effects are run with effect handlers, specified as functions (generally starting with run…
) unpacking some specific monad with a Carrier
instance. For example, we can run a State
computation using runState
, imported from the Control.Carrier.State.Strict
carrier module:
example1 :: Algebra sig m => [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. The get
function is resolved with a visible type application, due to the fact that effects can contain more than one state type (in contrast with mtl
’s MonadState
, which limits the user to a single state type).
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 :: Algebra sig 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 Lift Identity
effect, whose only operation is to lift a result value into a computation.
Alternatively, arbitrary Monad
s can be embedded into effectful computations using the Lift
effect. In this case, the underlying Monad
ic computation can be extracted using runM
. Here, we use the MonadIO
instance for the LiftC
carrier 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
When defining your own effects, you may need XKindSignatures
if GHC cannot correctly infer the type of your constructor; see the documentation on common errors for more information about this case.
When defining carriers, you’ll need XTypeOperators
to declare a Carrier
instance over (:+:
), XFlexibleInstances
to loosen the conditions on the instance, XMultiParamTypeClasses
since Carrier
takes two parameters, and XUndecidableInstances
to satisfy the coverage condition for this instance.
The following invocation, taken from the teletype example, should suffice for most use or construction of effects and carriers:
{# LANGUAGE FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, TypeOperators, UndecidableInstances #}
Defining new effects
The process of defining new effects is outlined in docs/defining_effects.md
, using the classic Teletype
effect as an example.
Project overview
This project builds a Haskell package named fusedeffects
. The library’s sources are in src
. 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 3clause license.
Development
Development of fusedeffects
is typically done using cabal v2build
:
cabal v2build # build the library
cabal v2test # build and run the examples and tests
The package is available on hackage, and can be used by adding it to a component’s builddepends
field in your .cabal
file.
Testing
fusedeffects
comes with a rigorous test suite. Each law or property stated in the Haddock documentation is checked using generative tests powered by the hedgehog
library.
Versioning
fusedeffects
adheres to the Package Versioning Policy standard.
Benchmarks
To run the provided benchmark suite, use cabal v2bench
. You may wish to provide the O2
compiler option to view performance under aggressive optimizations. fusedeffects
has been benchmarked against a number of other effect systems. See also @patrickt’s benchmarks.
Related work
fusedeffects
is an encoding of higherorder 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).
Contributed packages
Though we aim to keep the fusedeffects
core minimal, we encourage the development of external fusedeffects
compatible libraries. If you’ve written one that you’d like to be mentioned here, get in touch!
fusedeffectslens
provides combinators to use thelens
library fluently inside effectful computations.fusedeffectsexceptions
provides handlers for exceptions thrown in theIO
monad.fusedeffectsresumable
provides resumable exceptions, which can also serve as a limited form of coroutines.fusedeffectsmwcrandom
provides a performant, highquality source of random data, as well as values from common numerical distributions.fusedeffectsreadline
provides a REPL effect that interfaces withhaskeline
for its UI.fusedeffectsparser
provides parsercombinator style effects similar to parsing libraries such astrifecta
.fusedeffectsoptics
provides combinators for theoptics
ecosystem.
Projects using fusedeffects
semantic
, a program analysis toolkitnowhaskell
, a client library for AWS Lambda
Comparison to other effect libraries
Comparison to mtl
Like mtl
, fusedeffects
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 fusedeffects
, this is done by defining new instances of the Carrier
typeclass for the effect.
Also like mtl
, fusedeffects
allows scoped operations like local
and catchError
to be given different interpretations. As with firstorder operations, mtl
achieves this with a final tagless encoding via methods, whereas fusedeffects
achieves this with an initial algebra encoding via Carrier
instances.
In addition, mtl
and fusedeffects
are similar in that they provide instances for the monad types defined in the transformers
package (Control.Monad.Reader
, Control.Monad.Writer
, etc). This means that applications using mtl
can migrate many existing transformers
based monad stacks to fusedeffects
with minimal code changes. fusedeffects
provides its own hierarchy of carrier monads (under the Control.Carrier
namespace) that provide a more fluent interface for new code, though it may be useful to use transformers
types when working with thirdparty libraries.
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.
Unlike fusedeffects
, 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.
Finally, thanks to the fusion and inlining of carriers, fusedeffects
is only marginally slower than equivalent mtl
code (see benchmarks).
Comparison to freersimple
Like freersimple
, fusedeffects
uses an initial encoding of library and userdefined effects as syntax which can then be given different interpretations. In freersimple
, 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 fusedeffects
this is done with Carrier
instances for newtype
s.
Unlike fusedeffects
, in freersimple
, scoped operations like catchError
and local
are implemented as interpreters, and can therefore not be given new interpretations.
Unlike freersimple
, fusedeffects
has relatively little attention paid to compiler error messaging, which can make common (compiletime) errors somewhat more confusing to diagnose. Similarly, freersimple
’s family of interpreter functions can make the job of defining new effect handlers somewhat easier than in fusedeffects
. Further, freersimple
provides many of the same effects as fusedeffects
, plus a coroutine effect, but minus resource management and random generation.
Finally, fusedeffects
has been benchmarked as faster than freersimple
.
Comparison to polysemy
Like polysemy
, fusedeffects
is a batteriesincluded effect system capable of scoped, reinterpretable algebraic effects.
As of GHC 8.8, fusedeffects
outperforms polysemy
, though new effects take more code to define in fusedeffects
than polysemy
(though the Control.Carrier.Interpret
module provides a lowfriction API for rapid prototyping of new effects). Like freersimple
and unlike fusedeffects
, polysemy provides custom type errors if a given effect invocation is ambigous or invalid in the current context.
Comparison to eff
eff
is similar in many ways to fusedeffects
, but is slightly more performant due to its representation of effects as typeclasses. This approach lets GHC generate better code in exchange for sacrificing the flexibility associated with effects represented as data types. eff
also uses the monadcontrol
package to lift effects between contexts rather than implementing an Algebra
style class itself.
Acknowledgements
The authors of fusedeffects would like to thank:
 Tom Schrijvers, Nicholas Wu, and all their collaborators for the research that led to
fusedeffects
;  Alexis King for thoughtful discussions about and suggestions regarding our methodology;
 the authors of other effect libraries, including
eff
,polysemy
, andcapabilities
, for their exploration of the space.
*Note that all licence references and agreements mentioned in the fusedeffects README section above
are relevant to that project's source code only.