effectful

An easy to use, fast extensible effects library with seamless integration with
the existing Haskell ecosystem.

Main features:

Very fast
(benchmarks).
Easy to use API (comparable with usage of the MonadUnliftIO class).
Correct semantics in presence of runtime exceptions (no more discarded state
updates).
Seamless integration with the existing ecosystem (exceptions,
monad-control, unliftio-core, resourcet etc.).
Support for thread local and shared state (e.g. StateT provides a thread
local state, while MVar holds a shared state, both approaches have their
merits).
Support for statically (implementation determined at compile time) and
dynamically (implementation determined at run time) dispatched effects.

Motivation

Do we really need yet another library for handling effects? There's
freer-simple,
fused-effects,
polysemy,
eff and probably a few more.

It needs to be noted that of all of them only the work-in-progress eff library
is a promising proposition because of reasonable performance characteristics
(see the talk Effects for Less
for more information) and potential for good interoperability with the existing
ecosystem.

The second point is arguably the most important, because it allows focusing on
things that matter instead of reinventing all kinds of wheels, hence being a
necessary condition for broader adoption of the library.

Unfortunately, the development of eff has stalled due to a
few
subtle
issues related to its use of
delimited continuations underneath.

What about `mtl`?

It's true that its "effects as classes" approach is widely known and used often.

However:

mtl style effects are
slow.
The majority of popular monad transformers (except ReaderT) used for effect
implementations are rife with subtle
issues.

These are problematic enough that the ReaderT design
pattern was
invented. Its fundamentals are solid, but it's not an effect system.

A solution? Use the ReaderT pattern as a base and build around it to make an
extensible effects library! This is where effectful comes in. The Eff monad
it uses is essentially a ReaderT over IO on steroids, allowing us to extend
its environment with data types representing effects.

This concept is quite simple, so:

It's reasonably easy to understand what is going on under the hood.
The Eff monad being a reader allows for seamless interoperability with
ubiquitous classes such as MonadBaseControl and MonadUnliftIO and solves
issues
of monad transformers mentioned above.

What is more, the Eff monad is concrete, so GHC has many possibilities for
optimization, which results in a very fast code at a default optimization
level. There is no need to explicitly mark functions with INLINE pragmas or
enable additional optimization passes, it just works.

Any downsides?

As always, there's no free lunch. The Eff monad doesn't support effect
handlers that require the ability to suspend or capture the rest of the
computation and resume it later (potentially multiple times). This prevents
effectful from providing (in particular):

A NonDet effect handler that executes multiple
Alternative
branches and collects their results.
A Coroutine effect.

It needs to be noted however that such NonDet effect handler in existing
libraries is
broken
and none of the ones with support for higher order effects provide the
Coroutine effect, so arguably it's not a big loss.

If you need such capability in your application, there are well established
libraries such as conduit or
list-t that can be used with
effectful without any hassle.

Summary

effectful is an extensible effects library that aims to be the replacement
for:

The bare ReaderT pattern by being essentially its enriched version.
Monad transformer stacks typically encountered in the wild (i.e. consisting of
a dozen of newtype'd ExceptT, ReaderT, StateT and WriterT transformers
and their derivatives) by providing equivalent effects with improved
semantics, performance, usability and making it easy to reuse them for your
own effects.

It doesn't try to make monad transformers obsolete, so you're free to
use it with ConduitT, ContT, ListT etc. when necessary.

Package structure

The library is split among several packages:

The effectful-core
package contains the core of the library along with basic effects. It aims for
a small dependency footprint and provides building blocks for more advanced
effects.
The effectful-plugin
package provides an optional GHC plugin for improving disambiguation of
effects (see
here
for more information).
The effectful-th package
provides utilities for generating bits of effect-related boilerplate via
Template Haskell.
The effectful package
re-exports public modules of effectful-core and additionally provides most
features of the unliftio
package divided into appropriate effects.

Examples

For the examples see the Introduction sections of
Effectful.Dispatch.Dynamic
and
Effectful.Dispatch.Static
(when in doubt, start with dynamic dispatch).

Acknowledgements

To all contributors of existing effect libraries - thank you for putting the
time and effort to explore the space. In particular, conversations in issue
trackers of cleff, eff, freer-simple, fused-effects and polysemy
repositories were invaluable in helping me discover and understand challenges in
the space.

Resources

Resources that inspired the rise of this library and had a lot of impact on its
design.

Talks:

Effects for Less by Alexis King.
Monad Transformer State by Michael Snoyman.

Blog posts:

ReaderT design pattern by Michael Snoyman.
Exceptions Best Practices by Michael Snoyman.

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