[Haskell-cafe] Yet Another Forkable Class

[oleg at okmij.org]

Perhaps effect libraries (there are several to choose from) could be a
better answer to Fork effects than monad transformers. One lesson from
the recent research in effects is that we should start thinking what
effect we want to achieve rather than which monad transformer to
use. Using ReaderT or StateT or something else is an implementation
detail. Once we know what effect to achieve we can write a handler, or
interpreter, to implement the desired operation on the World, obeying
the desired equations. And we are done.

For example, with ExtEff library with which I'm more familiar, the
Fork effect would take as an argument a computation that cannot throw
any requests. That means that the parent has to provide interpreters
for all child effects. It becomes trivially to implement:

> Another example would be a child that should not be able to throw errors as
> opposed to the parent thread.
It is possible to specify which errors will be allowed for the child
thread (the ones that the parent will be willing to reflect and
interpret). The rest of errors will be statically prohibited then.

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

This is a good illustration of too much implementation detail. Why do we
need to know of (Sink p) as a Reader layer? Would it be clearer to
define an Effect of sending to the socket? Computation's type will
make it patent the computation is sending to the socket.
The parent thread, before forking, has to provide a handler for that
effect (and the handler will probably need a socket). 

Defining a new class for each effect is possible but not needed at
all. With monad transformers, a class per effect is meant to hide the
ordering of transformer layers in a monad transformer stack. Effect
libraries abstract over the implementation details out of the
box. Crutches -- extra classes -- are unnecessary. We can start by
writing handlers on a case-by-case basis. Generalization, if any,
we'll be easier to see. From my experience, generalizing from concrete
cases is easier than trying to write a (too) general code at the
outset. Way too often, as I read and saw, code that is meant to be
reusable ends up hardly usable.