Client server example doesn't build with ghc-8.4.3 about distributed-closure HOT 5 CLOSED

Unfortunate BC breaking in GHC 8.4. Do you know what is causing it? I tried enabling TypeSynonymInstances but even then it's giving the same error.

It would be very convenient to be able to continue writing the instances inside the withStatic block as-is.

from distributed-closure.

Ryan Scott told me that the problem is because of https://ghc.haskell.org/trac/ghc/ticket/13267

from distributed-closure.

Thankfully, the workaround is fairly simple: just don't generate instance Serializable SerializableInt! (It wasn't doing anything before anyways, since Serializable is a type synonym, not a class.)

from distributed-closure.

OK in that case, I propose the following change. Instead of

type Serializable a = (Binary a, Typeable a)

have

class (Binary a, Typeable a) => Serializable a
instance (Binary a, Typeable a) => Serializable a

If we do that, then we don't have a constraint synonym anymore, thus working around the fact that GHC doesn't support defining instances for those anymore. @mpickering @RyanGlScott might either of you have any thoughts about the downsides of such an alternative definition?

from distributed-closure.

That would also work fine. As far as downsides go:

1. It's yet another class that you have to generate code for to accommodate. (With the type synonym version, you don't have to generate any extra code in order to use it.)
2. It requires defining a flexible, undecidable catch-all instance to use, which you may or may not find appealing depending on your tastes.

tl;dr You can't go wrong with either version, so pick which one suits you better.

from distributed-closure.