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extensible-effects, monad transformers and interleaving effects


In their paper “Extensible Effects”, Kiselyov, Sabry and Swords (KSS) introduce an effect system for Haskell that is an alternative to the classical monad transformers approach. There is a Haskell library of the same name implementing the ideas of the paper. Dan Doel noted in a recent article that this is one of many papers which investigate effects and their associated handlers, in large part aiming to move from what is seen to be the “static” approach of monad transformers, to the supposedly more “dynamic” approach of algebraic effects.

Dan’s article is a defence of the classical monad transformers approach, and he argues convincingly “that the staticness [of monad transformers] can actually be a useful property, that certain ways of harnessing it can make the confusing constructions less so, and … that algebraic effects perhaps aren’t so different from what we are already using”.

KSS give two examples where they claim their approach is superior to monad transformers. The first is that it is awkward to combine exceptions with non-determinism. However, Dan implements a simple alternative to catchError and ensures that the exception handler removes from the set of effects. In this way he efficiently disposes with the supposed awkwardess. The second objection of KSS is that a certain interleaving of a read effect with coroutines is impossible to achieve with monad transformers. In this article I will demonstrate that contrary to KSS’s assertion, this interleaving is possible with monad transformers, and moreover straightforward.

Non-determinism and exceptions

catchError from the mtl runs a monad transformer stack that contains an “exception raising” layer and handles any exceptions with a handler that can potentially rethrow them. The definition has the following signature

catchError :: MonadError e m => m a -> (e -> m a) -> m a

The monad m is the same in the argument and return value, thus any exceptions that are rethrown are rethrown at exactly the same level of the stack. As KSS note, this does result in a particular inflexibility when mixing effects.

However, Dan realised that handlers should remove from the set of effects. That is, the monad of the return value should be “smaller” than the monad of the argument. He defines localCatch with the signature

localCatch :: Monad m => ErrorT e m a -> (e -> m a) -> m a

which handles exceptions at the outermost level and gives the user the chance to rethrow them at a lower level. Using this formulation yields the results that KSS are seeking with no further difficulties.

Read effects and coroutines

KSS provide the following type CoT which is a co-routine transformer

type CoT a m = ContT (Y m a) m
data Y m a = Done | Y a (() -> m (Y m a))

and define a function which mixes read effects with coroutines. The idea is to use local to modify the local state of a read operation. local has the signature

MonadReader r m => (r -> r) -> m a -> m a

and the function is

th3 :: MonadReader Int m => CoT Int m ()
th3 = ay >> ay >> local (+10) (ay >> ay)
    where ay = ask >>= yield

They demonstrate that this has unexpected behaviour. They also consider the opposite order of effects

th4 :: Monad m => ReaderT Int (CoT Int m) ()
th4 = ay >> ay >> local (+10) (ay >> ay)
    where ay = ask >>= lift . yield

but this does not work either. However, what I learned from Dan’s article is that handlers should remove from the set of effects, so instead of local we can try implementing localLocal (awkwardly named to be consistent with Dan’s terminology!) which removes the read effect.

localLocal :: MonadReader a m => (a -> r) -> ReaderT r m b -> m b
localLocal f m = ask >>= runReaderT m . f

Using localLocal highlights the problem. The ay outside the localLocal needs to have a different type than the ay inside the localLocal.

th3_explicit :: Monad m => CoT Int (ReaderT Int m) ()
th3_explicit = ay1 >> ay1 >> localLocal (+10) (ay2 >> ay2)
    where ay1 :: Monad m => CoT Int (ReaderT Int m) ()
          ay1 = lift ask >>= yield
          ay2 :: Monad m => ReaderT Int (CoT Int m) ()
          ay2 = ask >>= lift . yield

This implementation gives exactly the results that KSS are looking for. Furthermore it’s not hard to remove the duplication by defining a monad transformer class MonadCo which abstracts over continuation monads and provides a yieldG operation (a transformer-polymorphic version of yield). This leads to a neat and general implementation

th3_MTL :: (MonadCo Int m, MonadReader Int m) => m ()
th3_MTL = ay >> ay >> localLocal (+10) (ay >> ay)
    where ay :: (MonadCo r m, MonadReader r m) => m ()
          ay = ask >>= yieldG


With the right primitives, combining complicated effects using monad transformers can be straightforward. What I’ve presened here is not evidence that monad transformers are the best way to go, but it does show that more thought is needed before we declare them obsolete!

There are some further comparisons between extensible-effects and mtl.