[Haskell-cafe] Yet Another Forkable Class

John ExFalso 0slemi0 at gmail.com
Thu Aug 22 00:42:12 CEST 2013

TLDR: New forkable monad/transformer suggestion
http://pastebin.com/QNUVL12v(hpaste is down)


There are a dozen packages on hackage defining a class for monads that can
be forked, however none of these are modular enough to be useful in my

In particular the following are not addressed:
1. Cases when the child thread's monad is different from the parent's
2. Monad transformers (this is somewhat addressed with

I will try to demonstrate both issues with an example.

1. WebSockets

WebSockets is a monad that cannot itself be forked. This is because at any
given time there should only be a single thread listening on a websocket.
However there is a reasonable monad that can be forked off, namely one that
can send to the websocket - one that has access to the Sink.

So first off a "Forkable" class should not look like this:

class (MonadIO m, MonadIO n) => Forkable m where
    fork :: m () -> m ThreadId

But rather like this:

class Forkable m n where
    fork :: n () -> m ThreadId

For our example the instance would be

instance (Protocol p) => Forkable (WebSockets p) (ReaderT (Sink p) IO) where
    fork (ReaderT f) = liftIO . forkIO . f =<< getSink

Another example would be a child that should not be able to throw errors as
opposed to the parent thread.

2. ReaderT

Continuing from the previous example to demonstrate the need to distinguish
forkable transformers.
Say we have some shared state S that both parent and child should have
access to:

type Parent p = ReaderT (TVar S) (WebSockets p)
type Child p = ReaderT (TVar S) (ReaderT (Sink p) IO)

The "forkability" of Child from Parent should be implied, however with
Forkable we have to write a separate instance.

So what I suggest is a second class:

class ForkableT t where
    forkT :: (Forkable m n) => t n () -> t m ThreadId

And then:

instance ForkableT (ReaderT r) where
    forkT (ReaderT f) = ReaderT $ fork . f

We can also introduce a default for Forkable that uses a ForkableT instance:

class (MonadIO m, MonadIO n) => Forkable m n where
    fork :: n () -> m ThreadId
    default fork :: ForkableT t => t n () -> t m ThreadId
    fork = forkT

instance (Forkable m n) => Forkable (ReaderT r m) (ReaderT r n)

This means Child is automatically Forkable from Parent, no need to write a
specific case for our specific monads (and if we newtype it we can use

Note how MonadTransControl already solves the specific problem of lifting a
forking operation into ReaderT. However consider ResourceT from
Control.Monad.Resource: it is basically a ReaderT, however in order to
safely deallocate resources when sharing reference counting is needed. This
means a simple lift would not suffice.

We can nevertheless provide a default ForkableT based on MonadTransControl:
class ForkableT t where
    forkT :: (Forkable m n) => t n () -> t m ThreadId
    default forkT :: (MonadTransControl t, Forkable m n) => t n () -> t m
    forkT t = liftWith $ \run -> fork $ run t >> return ()

Actually resourcet's reference counting resourceForkIO also nicely
demonstrates the first problem:
type Parent p = ResourceT (WebSockets p)
type Child p = ResourceT (ReaderT (Sink p) IO)

Note how we cannot use resourceForkIO without touching the underlying

What do you think? Is there already an established way of modular forking?
I wouldn't like to litter hackage with another unusable Forkable class:)
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