This website is out of date, please refer to the site on github.

Examining Hackage: extensible-effects

Posted on July 15, 2014
Tags: haskell

I had a few people tell me after my last post that they would enjoy a write up on reading extensible-effects so here goes.

I’m going to document my process of reading through and understanding how extensible-effects is implemented. Since this is a fairly large library (about 1k) of code, we’re not going over all of it. Rather we’re just reviewing the core modules and enough of the extra ones to get a sense for how everything is implemented.

If you’re curious or still have questions, the modules that we don’t cover should serve as a nice place for further exploration.

Which Modules

extensible-effects comes with quite a few modules, my find query reveals

$ find src -name "*.hs"

Whew! Well I’m going to take a leap and assume that extensible-effects is similar to the mtl in the sense that there are a few core modules, an then a bunch of “utility” modules. So there’s Control.Monad.Trans and then Control.Monad.State and a bunch of other implementations of MonadTrans.

If we assume extensible-effects is formatted like this, then we need to look at

  1. Data.OpenUnion1
  2. Control.Monad.Eff

And maybe a few other modules to get a feel for how to use these two. I’ve added Data.OpenUnion1 because it’s imported by Control.Monad.Eff so is presumably important.

Since Data.OpenUnion1 is at the top of our dependency DAG, we’ll start with it.


So we’re starting with Data.OpenUnion1. If the authors of this code have stuck to normal Haskell naming conventions, that’s an open union of type constructors, stuff with the kind * -> *.

Happily, this module has an export list so we can at least see what’s public.

    module Data.OpenUnion1( Union (..)
                          , SetMember
                          , Member
                          , (:>)
                          , inj
                          , prj
                          , prjForce
                          , decomp
                          , unsafeReUnion
                          ) where

So we’re looking at a data type Union, which we export everything for. Two type classes SetMember and Member, a type operator :>, and a handful of functions, most likely to work with Union.

So let’s figure out exactly what this union thing is

data Union r v = forall t. (Functor t, Typeable1 t) => Union (t v)

So Union r v is just a wrapper around some of functor applied to v. This seems a little odd, what’s this r thing? The docs hint that Member t r should always hold.

Member is a type class of two parameters with no members. In fact, greping the entire source reveals that the entire definition and instances for Member in this code base is

    infixr 1 :>
    data ((a :: * -> *) :> b)

    class Member t r
    instance Member t (t :> r)
    instance Member t r => Member t (t' :> r)

So this makes it a bit clearer, :> acts like a type level cons and Member just checks for membership!

Now Union makes a bit more sense, especially in light of the inj function

    inj :: (Functor t, Typeable1 t, Member t r) => t v -> Union r v
    inj = Union

So Union takes some t in r and hides it away in an existential applied to v. Now this is kinda like having a great nested bunch of Eithers with every t applied to v.

Dual to inj, we can define a projection from a Union to some t in r. This will need to return something wrapped in Maybe since we don’t know which member of r our Union is wrapping.

    prj :: (Typeable1 t, Member t r) => Union r v -> Maybe (t v)
    prj (Union v) = runId <$> gcast1 (Id v)

prj does some evil Typeable casts, but this is necessary since we’re throwing away all our type information with that existential. That Id runId pair is needed since gcast1 has the type

    -- In our case, `c ~ Id`
    gcast1 :: (Typeable t', Typeable t) => c (t a) -> Maybe (c (t' a))

They’re just defined as

    newtype Id a = Id { runId :: a }
      deriving Typeable

so just like Control.Monad.Identity.

Now let’s try to figure out what this SetMember thing is.

    class Member t r => SetMember set (t :: * -> *) r | r set -> t
    instance SetMember set t r => SetMember set t (t' :> r)

This is unhelpful, all we have is the recursive step with no base case! Resorting to grep reveals that our base case is defined in Control.Eff.Lift so we’ll temporarily put this class off until then.

Now the rest of the file is defining a few functions to operate over Unions.

First up is an unsafe “forced” version of prj.

    infixl 4 <?>

    (<?>) :: Maybe a -> a -> a
    Just a <?> _ = a
    _ <?> a = a

    prjForce :: (Typeable1 t, Member t r) => Union r v -> (t v -> a) -> a
    prjForce u f = f <$> prj u <?> error "prjForce with an invalid type"

prjForce is really exactly what it says on the label, it’s a version of prj that throws an exception if we’re in the wrong state of Union.

Next is a way of unsafely rejiggering the type level list that Union is indexed over.

    unsafeReUnion :: Union r w -> Union t w
    unsafeReUnion (Union v) = Union v

We need this for our last function, decom. This function partially unfolds our Union into an Either

    decomp :: Typeable1 t => Union (t :> r) v -> Either (Union r v) (t v)
    decomp u = Right <$> prj u <?> Left (unsafeReUnion u)

This provides a way to actually do some sort of induction on r by breaking out each type piece by piece with some absurd case for when we don’t have a :> b.

That about wraps up this little Union library, let’s move on to see how this is actually used.


Now let’s talk about the core of extensible-effects, Control.Eff. As always we’ll start by taking a look at the export list

    module Control.Eff(
                        Eff (..)
                      , VE (..)
                      , Member
                      , SetMember
                      , Union
                      , (:>)
                      , inj
                      , prj
                      , prjForce
                      , decomp
                      , send
                      , admin
                      , run
                      , interpose
                      , handleRelay
                      , unsafeReUnion
                      ) where

So right away we can see that we’re exporting stuff Data.Union1 as well as several new things, including the infamous Eff.

The first definition we come across in this module is VE. VE is either a simple value or a Union applied to a VE!

    data VE r w = Val w | E !(Union r (VE r w))

Right away we notice that “pure value or X” pattern we see with free monads and other abstractions over effects.

We also include a quick function to try to extract a pure value form Vals

    fromVal :: VE r w -> w
    fromVal (Val w) = w
    fromVal _ = error "extensible-effects: fromVal was called on a non-terminal effect."

Now we’ve finally reached the definition of Eff!

    newtype Eff r a = Eff { runEff :: forall w. (a -> VE r w) -> VE r w }

So Eff bears a striking resemblance to Cont. There are two critical differences though, first is that we specialize our return type to something constructed with VE r. The second crucial difference is that by universally quantifying over w we sacrifice a lot of the power of Cont, including callCC!

Next in Control.Eff is the instances for Eff

    instance Functor (Eff r) where
        fmap f m = Eff $ \k -> runEff m (k . f)
        {-# INLINE fmap #-}

    instance Applicative (Eff r) where
        pure = return
        (<*>) = ap

    instance Monad (Eff r) where
        return x = Eff $ \k -> k x
        {-# INLINE return #-}

        m >>= f = Eff $ \k -> runEff m (\v -> runEff (f v) k)
        {-# INLINE (>>=) #-}

Notice that these are all really identical to Conts instances. Functor adds a function to the head of the continuation. Monad dereferences m and feeds the result into f. Exactly as with Cont.

Next we can look at our primitive function for handling effects

    send :: (forall w. (a -> VE r w) -> Union r (VE r w)) -> Eff r a
    send f = Eff (E . f)

I must admit, this tripped me up for a while. Here’s how I read it, “provide a function, which when given a continuation for the rest of the program expecting an a, produces a side effecting VE r w and we’ll map that into Eff”.

Remember how Union holds functors? Well each of our effects must act like as a functor and wrap itself in that union. By being open, we get the “extensible” in extensible-effects.

Next we look at how to remove effects once they’ve been added to our set of effects. In mtl-land, this is similar to the collection of runFooT functions that are used to gradually strip a layer of transformers away.

The first step towards this is to transform the CPS-ed effectful computation Eff, into a more manageable form, VE

    admin :: Eff r w -> VE r w
    admin (Eff m) = m Val

This is a setup step so that we can traverse the “tree” of effects that our Eff monad built up for us.

Next, we know that we can take an Eff with no effects and unwrap it into a pure value. This is the “base case” for running an effectful computation.

    run :: Eff () w -> w
    run = fromVal . admin

Concerned readers may notice that we’re using a partial function, this is OK since the E case is “morally impossible” since there is no t so that Member t () holds.

Next is the function to remove just one effect from an Eff

    handleRelay :: Typeable1 t
                => Union (t :> r) v -- ^ Request
                -> (v -> Eff r a)   -- ^ Relay the request
                -> (t v -> Eff r a) -- ^ Handle the request of type t
                -> Eff r a
    handleRelay u loop h = either passOn h $ decomp u
      where passOn u' = send (<$> u') >>= loop

Next to send, this function gave me the most trouble. The trick was to realize that that decomp will leave us in two cases.

  1. Some effect producing a v, Union r v
  2. A t producing a v, t v

If we have a t v, then we’re all set since we know exactly how to map that to a Eff r a with h.

Otherwise we need to take this effect, add it back into our computation. send (<$> u') takes the rest of the computation, that continuation and feeds it the v that we know our effects produce. This gives us the type Eff r v, where that outer Eff r contains our most recent effect as well as everything else. Now to convert this to a Eff r a we need to transform that v to an a. The only way to do that is to use the supplied loop function so we just bind to that.

Last but not least is a function to modify an effect somewhere in our effectful computation. A grep reveals will see this later with things like local from Control.Eff.Reader for example.

To do this we want something like handleRelay but without removing t from r. We also need to generalize the type so that t can be anywhere in our. Otherwise we’ll have to prematurally solidify our stack of effects to use something like modify.

    interpose :: (Typeable1 t, Functor t, Member t r)
              => Union r v
              -> (v -> Eff r a)
              -> (t v -> Eff r a)
              -> Eff r a
    interpose u loop h = maybe (send (<$> u) >>= loop) h $ prj u

Now this is almost identical to handleRelay except instead of using decomp which will split off t and only works when r ~ t :> r', we use prj! This gives us a Maybe and since the type of u doesn’t need to change we just recycle that for the send (<$> u) >>= loop sequence.

That wraps up the core of extensible-effects, and I must admit that when writing this I was still quite confused as to actually use Eff to implement new effects. Reading a few examples really helped clear things up for me.


The State monad has always been the sort of classic monad example so I suppose we’ll start here.

    module Control.Eff.State.Lazy( State (..)
                                 , get
                                 , put
                                 , modify
                                 , runState
                                 , evalState
                                 , execState
                                 ) where

So we’re not reusing the State from Control.Monad.State but providing our own. It looks like

    data State s w = State (s -> s) (s -> w)

So what is this supposed to do? Well that s -> w looks a continuation of sorts, it takes the state s, and produces the resulting value. The s -> s looks like something that modify should use.

Indeed this is the case

    modify :: (Typeable s, Member (State s) r) => (s -> s) -> Eff r ()
    modify f = send $ \k -> inj $ State f $ \_ -> k ()

    put :: (Typeable e, Member (State e) r) => e -> Eff r ()
    put = modify . const

we grab the continuation from send and add a State effect on top which uses our modification function s. The continuation that State takes ignores the value it’s passed, the current state, and instead feeds the program computation the () it’s expecting.

get is defined in a similar manner, but instead of modifying the state, we use State’s continuation to feed the program the current state.

    get :: (Typeable e, Member (State e) r) => Eff r e
    get = send (inj . State id)

So we grab the continuation, feed it to a State id which won’t modify the state, and then inject that into our open union of effects.

Now that we have the API for working with states, let’s look at how to remove that effect.

    runState :: Typeable s
             => s                     -- ^ Initial state
             -> Eff (State s :> r) w  -- ^ Effect incorporating State
             -> Eff r (s, w)          -- ^ Effect containing final state and a return value
    runState s0 = loop s0 . admin where
     loop s (Val x) = return (s, x)
     loop s (E u)   = handleRelay u (loop s) $
                           \(State t k) -> let s' = t s
                                           in loop s' (k s')

runState first preps our effect to be pattern matched on with admin. We then start loop with the initial state.

loop has two components, if we have run into a value, then we don’t interpret any effects, just stick the state and value together and return them.

If we do have an effect, we use handleRelay to split out the State s from our effects. To handle the case where we get a VE w, we just loop with the current state. However, if we get a State t k, we update the state with t and pass the continuation k.

From runState evalState and execState.

    evalState :: Typeable s => s -> Eff (State s :> r) w -> Eff r w
    evalState s = fmap snd . runState s

    execState :: Typeable s => s -> Eff (State s :> r) w -> Eff r s
    execState s = fmap fst . runState s

That wraps up the interface for Control.Eff.State. The nice bit is this makes it a lot clearer how to use send, handleRelay and a few other functions from the core.


Now we’re on to Reader. The interesting thing here is that local highlights how to use interpose properly.

As always, we start by looking at what exactly this module provides

    module Control.Eff.Reader.Lazy( Reader (..)
                                  , ask
                                  , local
                                  , reader
                                  , runReader
                                  ) where

The definition of Reader is refreshingly simple

    newtype Reader e v = Reader (e -> v)

Keen readers will note that this is just half of the State definition which makes sense; Reader is half of State.

ask is defined almost identically to get

    ask :: (Typeable e, Member (Reader e) r) => Eff r e
    ask = send (inj . Reader)

We just feed the continuation for the program into Reader. A simple wrapper over this gives our equivalent of reads

    reader :: (Typeable e, Member (Reader e) r) => (e -> a) -> Eff r a
    reader f = f <$> ask

Next up is local, which is the most interesting bit of this module.

    local :: (Typeable e, Member (Reader e) r)
          => (e -> e)
          -> Eff r a
          -> Eff r a
    local f m = do
      e <- f <$> ask
      let loop (Val x) = return x
          loop (E u) = interpose u loop (\(Reader k) -> loop (k e))
      loop (admin m)

So local starts by grabbing the view of the environment we’re interested in, e. From there we define our worker function which looks a lot like runState. The key difference is that instead of using handleRelay we use interpose to replace each Reader effect with the appropriate environment. Remember that interpose is not going to remove Reader from the set of effects, just update each Reader effect in the current computation.

Finally, we simply rejigger the computation with admin and feed it to loop.

In fact, this is very similar to how runReader works!

    runReader :: Typeable e => Eff (Reader e :> r) w -> e -> Eff r w
    runReader m e = loop (admin m)
        loop (Val x) = return x
        loop (E u) = handleRelay u loop (\(Reader k) -> loop (k e))


Now between Control.Eff.Reader and Control.Eff.State I felt I had a pretty good handle on most of what I’d read in extensible-effects. There was just one remaining loose end: SetMember. Don’t remember what that was? It was a class in Data.OpenUnion1 that was conspicuously absent of detail or use.

I finally found where it seemed to be used! In Control.Eff.Lift.

First let’s poke at the exports of his module

    module Control.Eff.Lift( Lift (..)
                           , lift
                           , runLift
                           ) where

This module is designed to lift an arbitrary monad into the world of effects. There’s a caveat though, since monads aren’t necessarily commutative, the order in which we run them in is very important. Imagine for example the difference between IO (m a) and m (IO a).

So to ensure that Eff can support lifted monads we have to do some evil things. First we must require that we never have to lifted monads and we always run the monad last. This is a little icky but it’s usefulness outweighs such ugliness.

To ensure condition 1, we need SetMember.

    instance SetMember Lift (Lift m) (Lift m :> ())

So we define a new instance of SetMember. Basically this says that any Lift is a SetMember ... r iff Lift m is the last item in r.

To ensure condition number two we define runLift with the more restrictive type

    runLift :: (Monad m, Typeable1 m) => Eff (Lift m :> ()) w -> m w

We can now look into exactly how Lift is defined.

    data Lift m v = forall a. Lift (m a) (a -> v)

So this Lift acts sort of like a “suspended bind”. We postpone actually binding the monad and simulate doing so with a continuation a -> v.

We can define our one operation with Lift, lift.

    lift :: (Typeable1 m, SetMember Lift (Lift m) r) => m a -> Eff r a
    lift m = send (inj . Lift m)

This works by suspending the rest of the program in a our faux binding to be unwrapped later in runLift.

    runLift :: (Monad m, Typeable1 m) => Eff (Lift m :> ()) w -> m w
    runLift m = loop (admin m) where
     loop (Val x) = return x
     loop (E u) = prjForce u $ \(Lift m' k) -> m' >>= loop . k

The one interesting difference between this function and the rest of the run functions we’ve seen is that here we use prjForce. The reason for this is that we know that r is just Lift m :> (). This drastically simplifies the process and means all we’re essentially doing is transforming each Lift into >>=.

That wraps up our tour of the module and with it, extensible-effects.

Wrap Up

This post turned out a lot longer than I’d expected, but I think it was worth it. We’ve gone through the coroutine/continuation based core of extensible-effects and walked through a few different examples of how to actually use them.

If you’re still having some trouble putting the pieces together, the rest of extensible effects is a great collection of useful examples of building effects.

I hope you had as much fun as I did with this one!

Thanks to Erik Rantapaa a much longer post than I led him to believe

comments powered by Disqus