schedSelectClock should only use a Semigroup

[turion]

With GHC 8.4, Semigroups become more of a thing, so it's time to relax the type signature of schedSelectClock to requiring cl only to be a Semigroup.

[turion]

We support GHC >= 8, so I think no conditional imports are needed.

[SolviQorda]

So I had a go at this - and the following compiles. Have I understood what you were asking for in the issue?

schedSelectClocks
  :: (Monad m, Semigroup cl, Clock m cl)
  => Schedule m (SelectClock cl a) (SelectClock cl b)
schedSelectClocks = Schedule {..}
  where
    startSchedule subClock1 subClock2 = do
      (runningClock, initialTime) <- startClock
        $ mainClock subClock1 <> mainClock subClock2
      let
        runningSelectClocks = concatS $ proc _ -> do
          (time, tag) <- runningClock -< ()
          returnA                     -< catMaybes
            [ (time, ) . Left  <$> select subClock1 tag
            , (time, ) . Right <$> select subClock2 tag ]
      return (runningSelectClocks, initialTime)

[turion]

Yes, that's the idea! Feel free to apply this to all other schedules with a Monoid requirement and send a PR. (Use git grep Monoid)

[SolviQorda]

I've managed to get all but one of the Schedules onto Semigroups and compile.

My only problem is in Concurrently.hs, with concurrentlyWriter. I'm trying to find a way for Semigroups to work with Monad Transformers.

I'm happy to keep trying with concurrentlyWriter, or to send a PR with all the changes except concurrentlyWriter --> whatever is more useful!

[turion]

Ah, the Monoid in concurrentlyWriter is of a different nature! Have a look at The Monad instance for WriterT. It requires Monoid w, and that's the sole reason for requiring it in concurrentlyWriter.

[turion]

So please do send the PR with everything you have so far :)

[SolviQorda]

Ah I thought that might be the case - glad that I didn't spend hours trying to refactor something I couldn't 😂