fusedeffects
A fast, flexible, fused effect system.
https://github.com/fusedeffects/fusedeffects
LTS Haskell 20.9:  1.1.2.1 
Stackage Nightly 20230130:  1.1.2.1 
Latest on Hackage:  1.1.2.1 
fusedeffects1.1.2.1@sha256:b3766ca1869a7082ddcced5a95ce60196c05b51fbd3d97aa79acb98de3940079,5261
Module documentation for 1.1.2.1
 Control
 Control.Algebra
 Control.Carrier
 Control.Carrier.Accum
 Control.Carrier.Choose
 Control.Carrier.Cull
 Control.Carrier.Cut
 Control.Carrier.Empty
 Control.Carrier.Error
 Control.Carrier.Fail
 Control.Carrier.Fresh
 Control.Carrier.Interpret
 Control.Carrier.Lift
 Control.Carrier.NonDet
 Control.Carrier.Reader
 Control.Carrier.State
 Control.Carrier.Throw
 Control.Carrier.Trace
 Control.Carrier.Writer
 Control.Effect
 Control.Effect.Accum
 Control.Effect.Catch
 Control.Effect.Choose
 Control.Effect.Cull
 Control.Effect.Cut
 Control.Effect.Empty
 Control.Effect.Error
 Control.Effect.Fail
 Control.Effect.Fresh
 Control.Effect.Labelled
 Control.Effect.Lift
 Control.Effect.NonDet
 Control.Effect.Reader
 Control.Effect.State
 Control.Effect.Sum
 Control.Effect.Throw
 Control.Effect.Trace
 Control.Effect.Writer
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.
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 ‘Deadpendency’, a software dependency project health tracking tool
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.
Changes
v1.1.2.1
 Adds support for
ghc
9.4 andbase
4.17.
v1.1.2.0

Adds
MonadUnliftIO
instances forReaderC
,LiftC
, andInterpretC
. (#420) 
Adds
Accum
(#391) (by @turion) Adds an
Accum
effect  Adds a churchencoded
Control.Carrier.Accum.Church
carrier  Adds a strict
Control.Carrier.Accum.Strict
carrier  Adds an impure
Control.Carrier.Accum.IORef
carrier (#430)  Defines
Algebra
instances for the three mentioned carriers, and forControl.Monad.Trans.Accum
fromtransformers
 Adds an

Defines
Algebra
,Alternative
,Applicative
,Foldable
,Functor
,Monad
,MonadFail
,MonadFix
,MonadIO
,MonadPlus
,MonadTrans
,MonadUnliftIO
,MonadZip
, andTraversable
instances forControl.Effect.Choose.Choosing
. (#419) 
Adds an
IORef
based carrier inControl.Carrier.State.IORef
. (#422)
v1.1.1.3
 Adds support for
inspectiontesting
0.5.
v1.1.1.2
 Adds support for
ghc
9.2.1 andbase
4.16.
v1.1.1.1
 Adds support for
ghc
9.0 &base
4.15.
v1.1.1
 Defines
MonadFix
instances forLabelled
andUnderLabel
. (#402)
v1.1

Adds a churchencoded
State
carrier inControl.Carrier.State.Church
. (#363) 
Adds a churchencoded
Error
carrier inControl.Carrier.Error.Church
. (#203) 
Adds a churchencoded
Empty
carrier inControl.Carrier.Empty.Church
. (#203) 
Adds a churchencoded
Writer
carrier inControl.Carrier.Writer.Church
. (#369) 
Adds a churchencoded
Fresh
carrier inControl.Carrier.Fresh.Church
. (#373) 
Defines
Algebra
instances forControl.Monad.Trans.Maybe.MaybeT
,Control.Monad.Trans.RWS.CPS
, andControl.Monad.Trans.Writer.CPS
. (#366) 
Adds
evalEmpty
andexecEmpty
handlers for theEmpty
carriers as conveniences for usingempty
to signal early returns. (#371)
Backwardsincompatible changes

Changes
alg
’s signature, giving it an initial state, and a distributive law which must be applied to each computation in the signature. This change allowsAlgebra
instances to be derived usingGeneralizedNewtypeDeriving
andDerivingVia
, while also obviating the need forhmap
,handleCoercible
, or thethread
method ofEffect
. This furthermore increases the expressiveness of effects, allowing effects with higherorder positions yielding concrete types, e.g.m ()
, to be run anywhere in the stack, not just above anyEffect
requiring algebras. (#359, #361) 
Changes the signatures of
runInterpret
andrunInterpretState
analogously; also reorders the parameters torunInterpretState
to take the signature before the state parameter. (#359) 
Removes
Algebra
’s superclass constraint requiring aHFunctor
instance for the signature. (#359) 
Removes
handleCoercible
. Algebras which formerly used it when handling the tail of the signature may now composecoerce
onto the homomorphism passed toalg
. (#359) 
Removes
HFunctor
. Effects are no longer required to haveHFunctor
instances, and so the class is redundant. (#359) 
Removes
Effect
. The new signature foralg
(see above) obviates the need for threading handlers through effects, replacing that by threading them through algebras instead. (#361) 
Redefines
thread
as a wrapper aroundalg
, composing context functors and distributive laws together. (Note that its type has also changed to take the context last and to decompose the handler for the two carriers.) (#361) 
Renames
Control.Effect.Interpret.Handler
toInterpreter
. (#361) 
Reorders the parameters to the higherorder function passed to
Control.Effect.Lift.liftWith
for consistency withalg
and to reflect its purpose of lifting Kleisli arrows in some underlying monad into the context modulo the context’s state. (#361) 
Redefines all effects as GADTs. Since we no longer require
Functor
,HFunctor
, orEffect
instances, we no longer need to use continuations to allow distinct result types per constructor.Algebra
instances for these effects can be ported forwards by removing the continuations. Userdefined effects are not impacted, but we recommend migrating to GADT definitions of them for convenience and ease of comprehension going forwards. (#365) 
Removes
Control.Carrier.State.Lazy.runStateC
, which was supposed to have been removed in 1.0.
v1.0.2.2
 Adds support for
ghc
8.10 &base
4.14. (#376)
v1.0.2.0

Adds a
state
operation for theState
effect. (#353) 
Adds a function reassociating sums leftwards to
Control.Effect.Sum
. (#354) 
Inlines
inj
. (#354) 
Adds labelled effects in
Control.Effect.Labelled
. Labelled effects allow flexible disambiguation and dependency for parametric effects, enabling better type inference, restricted usage, and associated type parameters. (#354) 
Adds labelled interface for
Reader
andState
effects inControl.Effect.Reader.Labelled
andControl.Effect.State.Labelled
. The functions in this interface are identical to their parent effect save that they accept a label parameter as an explicit type argument, suitable for use with an explicit type application; this can clean up code that would otherwise need an invocation ofrunUnderLabel
to associate a labelled operation with its label. (#354) 
Adds a
sendIO
operation for theLift IO
effect. (#360) 
Inlines the
Reader
operations. (#347)
v1.0.0.1
 Adds passthrough
Algebra
instances forAp
andAlt
, allowing the invocation of effects inside these structures without extraneous constructor applications.
v1.0.0.0

Adds an
Empty
effect, modelling nondeterminism without choice (#196). 
Adds an
EmptyC
carrier forEmpty
. (#196) 
Adds a
Choose
effect, modelling nondeterminism without failure (#198). 
Adds a
Throw
effect, modelling failure with a value. (#247) 
Adds a
Catch
effect which can be used withThrow
(or other kinds of failure) to model recoverable failure. (#247) 
Adds a
oneOf
function toControl.Effect.NonDet
to provide an idiom for the common case of nondeterministically selecting from a container. (#201) 
Adds a
foldMapA
function toControl.Effect.NonDet
mapping containers into nondeterministic computations using a supplied function. (#204) 
Defines a new
Has
constraint synonym, conveniently combiningCarrier
andMember
constraints and used for all effect constructors. (#217) 
Allows effects to be defined and handled as sums of other effects, while still using the constructors for the component effects. This has been used to redefine
NonDet
as a sum ofEmpty
andChoose
, andError
as a sum ofThrow
andCatch
. (#199, #219, #247) 
Defines
Carrier
instances for a number of types inbase
, includingEither
,Maybe
,[]
, andIO
. (#206) 
Defines
Carrier
instances for a number of types intransformers
. (#226) 
Defines an
evalFresh
handler forControl.Carrier.Strict.FreshC
, taking the initial value. (#267)
Backwardsincompatible changes

Renames the
Carrier
class toAlgebra
and itseff
method toalg
, and moved the responsibilities ofControl.Carrier
toControl.Algebra
. This makes the library more consistent with the literature and encourages a style of naming that focuses on morphisms rather than objects. (#285, #294) 
Fixes unlawful behaviour in the
Applicative
instance forErrorC
, which had different behaviour between<*>
andap
in the presence of a divergent rhs. In order to accomplish this,ErrorC
has been defined as a wrapper aroundControl.Monad.Trans.Except.ExceptT
. (#228) 
Improves the performance of
runInterpret
using reflection, changing its signature slightly (#193, h/t @ocharles). 
Removes
Control.Effect.Random
(and the dependencies onrandom
&MonadRandom
) in favour of a newfusedeffectsrandom
package (#200). 
Removes
fmap'
andhandlePure
, both deprecated in 0.5.0.0 (#205). 
Redefines
NonDetC
as a Churchencoded binary tree instead of a Churchencoded list (#197). 
Removes the
OnceC
carrier forCull
effects, replacing it with the composition ofCullC
on some otherAlternative
carrier, e.g.NonDetC
(#204). 
Moves all the carriers into their own modules in the
Control.Carrier
namespace. Several have also been renamed, e.g. the variousTrace
carriers are all namedTraceC
within their separate modules, and should be imported qualified if disambiguation is required. This simplifies naming schemes, and ensures that the choice of e.g. strict or lazy carrier is always made consciously and expliclty, instead of defaulting to whichever is exported by the effect module (#204). 
Removes the reexport of
Member
from all carrier modules, reexportingHas
in its place.Has
constraints should generally be used instead, and specialist cases can importControl.Effect.Sum
forMember
. (#217) 
Redesigns & renames the handlers for churchencoded nondeterminism carriers to standardize naming and usage patterns. (#207)
 The primary handlers (
runChoose
,runNonDet
,runCut
,runCull
) take multiple continuations.  Handlers which return an
Alternative
are suffixed withA
, e.g.runNonDetA
.  Handlers which return a
Monoid
are suffixed withM
, e.g.runNonDetM
.  Handlers which return a
Semigroup
are suffixed withS
, e.g.runChooseS
.
 The primary handlers (

Removes
InterposeC
&runInterpose
due to their inefficiency. They can be replaced with use ofInterpretC
/runInterpret
for the desired effect. (#223) 
Removes
prj
fromMember
, as it was only used inInterposeC
(see above), and was generally inadvisable due to its lack of modularity. (#223) 
Removes the
Resource
effect and carrier. Both have been relocated tofusedeffectsexceptions
. (#268) 
Redefines
Fail
as a synonym forThrow String
. (#247) 
Removes
Resumable
and its carriers. Both have been relocated tofusedeffectsresumable
; they can also be usefully and flexibly replaced by arbitrary effects,Lift
, andInterpretC
. (#269) 
Changes
Control.Carrier.Fresh.Strict.runFresh
to take and return the initial & final values, respectively, allowing for safer operation. (#267) 
Removes
resetFresh
, as it was unsafe. Greater safety and control over the generation of fresh values can be obtained by use ofrunFresh
. (#267) 
Removes
PureC
;Data.Functor.Identity.Identity
should be used instead. Note thatrun
is still provided as a convenient synonym forrunIdentity
. (#307) 
Removes the
Pure
effect. It’s unlikely that this will require changes, asPure
had no operations, butLift Identity
should be used instead. (#307) 
Redefines the
Lift
effect, allowing inner contexts to run actions in outer contexts, e.g. to interoperate withControl.Exception
. (#306) 
Removes
MonadUnliftIO
instances as they’ve been subsumed by the new definition ofLift
. Additionally, theReaderT
&IdentityT
types defined intransformers
may be useful. (#306)
v0.5.0.1
 Adds support for ghc 8.8.1.
v0.5.0.0

Derives
Generic1
instances for all nonexistentiallyquantified effect datatypes. 
Derives
Foldable
&Traversable
instances for:+:
. 
Defines
MonadFix
instances for all of the carriers. 
Reexports
run
,:+:
, andMember
fromControl.Effect.Carrier
, reducing the number of imports needed when defining new effects. 
Reexports
Carrier
,Member
, andrun
from the various effect modules, reducing the number of imports needed when using existing effects.
Backwardsincompatible changes

Replaces
runResource
with an equivalent function that usesMonadUnliftIO
to select the correct unlifting function (a lawithResource
, which is removed in favor ofrunResource
). 
Changes the signature of
eff
fromsig m (m a) > m a
tosig m a > m a
, requiring effects to holdm k
in their continuation positions instead of merelyk
. This was done in order to improve interoperability with other presentations of higherorder syntax, e.g.bound
; syntax used withbound
can now be givenHFunctor
andCarrier
instances.To upgrade effects used with previous versions, change any continuations from
k
tom k
. If no existential type variables appear in the effect, you can deriveGeneric1
, and thenceHFunctor
&Effect
instances. Otherwise, implement the required instances by hand. Since continuation positions now occur inm
,hmap
definitions will have to apply the higherorder function to these as well. 
Adds
Functor
constraints tohmap
andMonad
constraints tohandle
, allowing a greater variety of instances to be defined (e.g. for recursivelynested syntax). 
Replaces the default definitions of
hmap
andhandle
with derivations based onGeneric1
instead ofCoercible
. Therefore, firstorder effects wishing to derive these instances will requireGeneric1
instances, presumably derived usingXDeriveGeneric
. 
Moves
send
fromControl.Effect.Sum
toControl.Effect.Carrier
. Likewise removes the reexport ofsend
fromControl.Effect
. 
Deprecates
fmap'
in favour offmap
. 
Deprecates
handlePure
in favour ofhmap
.
v0.4.0.0
Backwardsincompatible changes
 Removes APIs deprecated in 0.3.0.0, including
Eff
,interpret
,ret
, and thehandle*
family of helper functions.
Other changes
 Adds the ability to derive default instances of
HFunctor
andEffect
for firstorder effects, using theXDeriveAnyClass
extension.  Adds a generic
Interpose
effect that enables arbitrary “eavesdropping” on other effects.
0.3.1.0
 Improved speed of
Reader
,State
,Writer
, andPure
effects by defining and inlining auxiliaryApplicative
methods.  Adds
runInterpret
&runInterpretState
handlers inControl.Effect.Interpret
as a convenient way to experiment with effect handlers without defining a new carrier type andCarrier
instance. Such handlers are somewhat less efficient than customCarrier
s, but allow for a smooth upgrade path when more efficiency is required.  Added
unliftiocore
as a dependency so as to provide a blessed API for unliftstyle effects and a solution to the cubiccaller problem.
0.3.0.0
Backwardsincompatible changes
 Adds
Monad
as a superclass ofCarrier
, obviating the need for a lot of constraints, andMonad
instances for all carrier types. This is a backwardsincompatible change, as any carriers users have defined now requireMonad
instances. Note that in many cases carriers can be composed out of existing carriers and monad transformers, and thus these instances can often be derived usingXGeneralizedNewtypeDeriving
. We also recommend compiling withWredundantconstraints
as many of these can now be removed.  Replaces
AltC
with a new carrier,NonDetC
, based on Ralf Hinze’s work in Deriving Backtracking Monad Transformers. This is a backwardsincompatible change.AltC
was equivalent to theListT
monad transformer, and had the same wellknown limitation to commutative monads. Therefore, the elimination ofEff
required a more durable approach.  Removes
Branch
. This is a backwardsincompatible change, but was necessitated by the difficulty of implementing correctApplicative
&Monad
instances for carriers which used it. Carriers which were employingBranch
internally should be reimplemented usingNonDetC
or a similar approach; seeCutC
andCullC
for examples.  Renames
Control.Effect.Void
,Void
, andVoidC
toControl.Effect.Pure
,Pure
, andPureC
respectively. This is a backwardsincompatible change for code mentioningVoidC
; it should be updated to referencePureC
instead.
Deprecations
Eff
andinterpret
, in favour of computing directly in the carriers. This enables the compiler to perform significant optimizations; see the benchmarks for details. Handlers can simply remove theEff
wrapping the carrier type & any use ofinterpret
. As above, we also recommend compiling withWredundantconstraints
as many of these can now be removed.ret
, in favor ofpure
orreturn
.handleEither
,handleReader
,handleState
,handleSum
, andhandleTraversable
in favour of composing carrier types directly. Carriers can be composed from other carriers andeff
defined withhandleCoercible
; and other definitions can usehandlePure
&handle
directly.
All deprecated APIs will be removed in the next release.
Other changes
 Adds a lazy
State
carrier inControl.Effect.State.Lazy
 Rewrites
CutC
using an approach related toNonDetC
, with the addition of a continuation to distinguishempty
fromcutfail
.  Rewrites
CullC
usingListC
andReaderC
.  Moves
OnceC
fromControl.Effect.NonDet
toControl.Effect.Cull
to avoid cyclic dependencies.  Adds a
runCutAll
handler forCut
effects, returning a collection of all results.
0.2.0.2
 Loosens the bounds on QuickCheck to accommodate 2.x.
0.2.0.1
 Fixes the benchmarks, and builds them in CI to avoid regressing them again.
0.2.0.0
 Adds
listen
,listens
, andcensor
operations toWriter
.  Provides explicit type parameters to
run
style functions inState
,Reader
,Writer
, andError
. This is a backwardsincompatible change for clients using these functions in combination with visible type applications.  Adds benchmarks of
WriterC
/VoidC
wrapped withEff
against their unwrapped counterparts.  Adds
Functor
,Applicative
, andMonad
instances forWriterC
.  Adds
Functor
,Applicative
, andMonad
instances forVoidC
.  Fixes a space leak with
WriterC
.  Removes the
Functor
constraint onasks
andgets
.  Adds
bracketOnError
,finally
, andonException
toResource
.  Adds
sendM
toLift
.
0.1.2.1
 Loosens the bounds on QuickCheck to accommodate 0.12.
0.1.2.0
 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.
0.1.1.0
 Adds a
runNonDetOnce
handler which terminates immediately upon finding a solution.
0.1.0.0
Initial release.