Extensible effects (, )
Implement effectful computations in a modular way!
The main monad of this package is Eff
from Control.Eff
.
Eff r a
is parameterized by the effectlist r
and the monadicresult type
a
similar to other monads.
It is the intention that all other monadic computations can be replaced by the
use of Eff
.
In case you know monad transformers or mtl
:
This library provides only one monad that includes all your effects instead of
layering different transformers.
It is not necessary to lift the computations through a monad stack.
Also, it is not required to lift every Monad*
typeclass (like MonadError
)
though all transformers.
Quickstart
To experiment with this library, it is suggested to write some lines within
ghci
.
Recommended Procedure:
 get
extensibleeffects
by doing one of the following:
 add
extensibleeffects
as a dependency to a existing cabal or stack project
git clone https://github.com/suhailshergill/extensibleeffects.git
 start
stack ghci
or cabal repl
inside the project
 import
Control.Eff
and Control.Eff.QuickStart
 start with the examples provided in the documentation of the
Control.Eff.QuickStart
module
Tour through Extensible Effects
This section explains the basic concepts of this library.
The Effect List
import Control.Eff
The effect list r
in the type Eff r a
is a central concept in this library.
It is a typelevel list containing effect types.
If r
is the empty list, then the computation Eff r
(or Eff '[]
) does not
contain any effects to be handled and therefore is a pure computation.
In this case, the result value can be retrieved by run :: Eff '[] a > a
For programming within the Eff r
monad, it is almost never necessary to list
all effects that can appear.
It suffices to state what types of effects are at least required.
This is done via the Member t r
typeclass. It describes that the type t
occurs inside the list r
.
If you really want, you can still list all Effects and their order in which
they are used (e.g. Eff '[Reader r, State s] a
).
Handling Effects
Functions containing something like Eff (x ': r) a > Eff r a
handle effects.
The transition from the longer list of effects (x ': r)
to just r
is a typelevel indicator that the effect x
has been handled.
Depending on the effect, some additional input might be required or some
different output than just a
is produced.
The handler functions typically are called run*
, eval*
or exec*
.
Most common Effects
The most common effects used are Writer
, Reader
, Exception
and State
.
Writer
, Reader
and State
all provide lazy and strict variants. Each has
its own module that exposes a common interface. Importing one or the other
controls whether the effect is strict or lazy in its inputs and outputs. It's
recommended that you use the lazy variants by default unless you know you need
strictness.
In this section, only the core functions associated with an effect are
presented.
Have a look at the modules for additional details.
The Exception Effect
import Control.Eff.Exception
The exception effect adds the possibility to exit a computation preemptively
with an exception.
Note that the exceptions from this library are handled by the programmer and
have nothing to do with exceptions thrown inside the Haskell runtime.
throwError :: Member (Exc e) r => e > Eff r a
runError :: Eff (Exc e ': r) a > Eff r (Either e a)
An exception can be thrown using the throwError
function.
Its return type is Eff r a
with an arbitrary type a
.
When handling the effect, the resulttype changes to Either e a
instead of
just a
.
This indicates how the effect is handled: The returned value is either the
thrown exception or the value returned from a successful computation.
The State Effect
import Control.Eff.State.{Lazy  Strict}
The state effect provides readable and writable state during a computation.
get :: Member (State s) r => Eff r s
put :: Member (State s) r => s > Eff r ()
modify :: Member (State s) r => (s > s) > Eff r ()
runState :: s > Eff (State s ': r) a > Eff r (a, s)
The get
function fetches the current state and makes it available within
subsequent computation. The put
function sets the state to a given value.
modify
updates the state using a mapping function by combining get
and
put
.
The stateeffect is handled using the runState
function.
It takes the initial state as an argument and returns the final state and
effectresult.
The Reader Effect
import Control.Eff.Reader.{Strict  Lazy}
The reader effect provides an environment that can be read.
Sometimes it is considered as readonly state.
ask :: Member (Reader e) r => e > Eff r e
runReader :: e > Eff (Reader e ': r) a > Eff r a
ask
can be used to retrieve the environment provided to runReader
from
within a computation which has the Reader
effect.
The Writer Effect
import Control.Eff.Writer.{Strict  Lazy}
The writer effect allows one to collect messages during a computation.
It is sometimes referred to as writeonly state, which gets retrieved at the
end of the computation.
tell :: Member (Writer w) r => w > Eff r ()
runWriter :: (w > b > b) > b > Eff (Writer w ': r) a > Eff r (a, b)
runListWriter :: Eff (Writer w ': r) a > Eff r (a, [w])
Running a writer can be done in several ways.
The most general function is runWriter
which folds over all written values.
However, if you only want to collect the values written, the runListWriter
function does that.
Note that compared to mtl, the value written has no Monoid constraint on it and
can be collected in any way.
Using multiple Effects
The main benefit of this library is that multiple effects can be included
with ease.
If you need state and want to be able exit the computation with an exception,
the type of your effectful computation would be the one of myComp
below.
Then, both the state and exception effectfunctions can be used.
To handle the effects, both the runState
and runError
functions have to be
provided.
myComp :: (Member (Exc e) r, Member (State s) r) => Eff r a
run1 :: (Either e a, s)
run1 = run . runState initalState . runError $ myComp
run2 :: Either e (a, s)
run2 = run . runError . runState initalState $ myComp
However, the order of the handlers does matter for the final result.
run1
and run2
show different executions of the same effectful computation.
In run1
, the returned state s
is the last state seen before an eventual
exception gets thrown (similar to the semantics in typical imperative
languages), while in run2
the final state is returned only if the whole
computation succeeded  transaction style.
Tips and tricks
There are several constructs that make it easier to work with the effects.
If only a part of the result is necessary for further computation, have a
look at the eval*
and exec*
functions which exist for some effects.
The exec*
functions discard the result of the computation (the a
type).
The eval*
functions discard the final result of the effect.
Instead of writing
(Member (Exc e) r, Member (State s) r) => ...
it is
possible to use the type operator <::
and write
[ Exc e, State s ] <:: r => ...
, which has the same meaning.
It might be convenient to include the necessary language extensions and disable
classconstraint warnings in your project's .cabal
file (or package.yaml
if
you're using stack
).
Explanation is a work in progress.
Other Effects
Work in progress.
Integration with IO
IO
or any other monad can be used as a base type for the Lift
effect.
There may be at most one instance of the Lift
effect in the effects list, and it
must be handled last. Control.Eff.Lift
exports the runLift
handler and
lift
function which provide the ability to run arbitrary monadic actions.
Also, there are convenient type aliases that allow for shorter type constraints.
f :: IO ()
f = runLift $ do printHello
printWorld
 These two functions' types are equivalent.
printHello :: SetMember Lift (Lift IO) r => Eff r ()
printHello = lift (putStr "Hello")
printWorld :: Lifted IO r => Eff r ()
printWorld = lift (putStrLn " world!")
Note that, since Lift
is a terminal effect, you do not need to use run
to
extract pure values. Instead, runLift
returns a value wrapped in whatever
monad you chose to use.
Additionally, the Lift
effect provides MonadBase
, MonadBaseControl
, and
MonadIO
instances that may be useful, especially with packages like
liftedbase,
liftedasync, and other
code that uses those typeclasses.
Work in progress.
Writing your own Effects and Handlers
Work in progress.
Other packages
Some other packages may implement various effects. Here is a rather incomplete
list:
Background
extensibleeffects
is based on the work of
Extensible Effects: An Alternative to Monad Transformers.
The paper and
the followup freer paper
contain details. Additional explanation behind the approach can be found on Oleg's website.
Limitations
AmbiguityFlexibility tradeoff
The extensibility of Eff
comes at the cost of some ambiguity. A useful
pattern to mitigate this ambiguity is to specialize calls to effect handlers
using
type application
or type annotation. Examples of this pattern can be seen in
Example/Test.hs.
Note, however, that the extensibility can also be traded back, but that detracts
from some of the advantages. For details see section 4.1 in the
paper.
Some examples where the cost of extensibility is apparent:

Common functions can't be grouped using typeclasses, e.g. the ask
and
getState
functions can't be grouped in the case of:
class Get t a where
ask :: Member (t a) r => Eff r a
ask
is inherently ambiguous, since the type signature only provides
a constraint on t
, and nothing more. To specify fully, a parameter
involving the type t
would need to be added, which would defeat the
point of having the grouping in the first place.

Code requires a greater number of type annotations. For details see
#31.