An easy to use, fast extensible effects library with seamless integration with
the existing Haskell ecosystem.
Easy to use API (comparable with usage of the MonadUnliftIO class).
Correct semantics in presence of runtime exceptions (no more discarded state
Seamless integration with the existing ecosystem (
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
Support for statically (implementation determined at compile time) and
dynamically (implementation determined at run time) dispatched effects.
Do we really need yet another library for handling effects? There's
eff and probably a few more.
It needs to be noted that of all of them only the work-in-progress
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
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
issues related to its use of
delimited continuations underneath.
It's true that its "effects as classes" approach is widely known and used often.
These are problematic enough that the ReaderT design
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
it uses is essentially a
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.
Eff monad being a reader allows for seamless interoperability with
ubiquitous classes such as
MonadUnliftIO and solves
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.
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):
It needs to be noted however that such
NonDet effect handler in existing
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.
effectful is an extensible effects library that aims to be the replacement
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
and their derivatives) by providing equivalent effects with improved
semantics, performance, usability and making it easy to reuse them for your
It doesn't try to make monad transformers obsolete, so you're free to
use it with
ListT etc. when necessary.
The library is split among several packages:
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
package provides an optional GHC plugin for improving disambiguation of
for more information).
provides utilities for generating bits of effect-related boilerplate via
re-exports public modules of
effectful-core and additionally provides most
features of the
package divided into appropriate effects.
For the examples see the Introduction sections of
To all contributors of existing effect libraries - thank you for putting the
time and effort to explore the space. In particular, conversations in issue
repositories were invaluable in helping me discover and understand challenges in
Resources that inspired the rise of this library and had a lot of impact on its