fused-effects

by fused-effects

fused-effects / fused-effects

A fast, flexible, fused effect system for Haskell

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A fast, flexible, fused effect system for Haskell

Build Status hackage

Overview

fused-effects
is an effect system for Haskell that values expressivity, efficiency, and rigor. It provides an encoding of algebraic, higher-order 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

fused-effects
itself.

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.

In

fused-effects
, 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.

Higher-order effects

Unlike some 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, 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,

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 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

fused-effects
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 

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 

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 

(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 

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 

(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

fused-effects
. 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 3-clause license.

Development

Development of

fused-effects
is typically done using
cabal v2-build
:
cabal v2-build # build the library
cabal v2-test  # build and run the examples and tests

The package is available on hackage, and can be used by adding it to a component’s

build-depends
field in your
.cabal
file.

Testing

fused-effects
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

fused-effects
adheres to the Package Versioning Policy standard.

Benchmarks

To run the provided benchmark suite, use

cabal v2-bench
. You may wish to provide the
-O2
compiler option to view performance under aggressive optimizations.
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).

Contributed packages

Though we aim to keep the

fused-effects
core minimal, we encourage the development of external
fused-effects
-compatible libraries. If you’ve written one that you’d like to be mentioned here, get in touch!

Projects using
fused-effects

Comparison to other effect libraries

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.

In addition,

mtl
and
fused-effects
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
fused-effects
with minimal code changes.
fused-effects
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 third-party 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

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.

Finally, thanks to the fusion and inlining of carriers,

fused-effects
is only marginally slower than equivalent
mtl
code (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
newtype
s.

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
.

Comparison to
polysemy

Like

polysemy
,

fused-effects
is a batteries-included effect system capable of scoped, reinterpretable algebraic effects.

As of GHC 8.8,

fused-effects
outperforms
polysemy
, though new effects take more code to define in
fused-effects
than
polysemy
(though the
Control.Carrier.Interpret
module provides a low-friction API for rapid prototyping of new effects). Like
freer-simple
and unlike
fused-effects
, 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

fused-effects
, 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
monad-control
package to lift effects between contexts rather than implementing an
Algebra
-style class itself.

Acknowledgements

The authors of fused-effects would like to thank:

  • Tom Schrijvers, Nicholas Wu, and all their collaborators for the research that led to
    fused-effects
    ;
  • Alexis King for thoughtful discussions about and suggestions regarding our methodology;
  • the authors of other effect libraries, including
    eff
    ,
    polysemy
    , and
    capabilities
    , for their exploration of the space.

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