Weaken constraint from Monoid to Semigroup

[leifmetcalf]

This breaks a bunch of stuff:

• startUpdateNetwork and startUpdateNetworkWithValue now return
  'Maybe a' instead of 'a' if no change has happened since the last
  sample.

• 'Update' no longer has an Applicative instance. An Applicative
  instance isn't possible (nicely) with only a Semigroup constraint.

I've also removed IOMonoid since, as of base 4.9.0.0, IO has an
identical Monoid instance.

[maoe]

Can we switch back to Travis or GitHub Actions? We don't use GitLab for this project.

[pacak]

Why is this change needed? Why do we need enummaps?

[leifmetcalf]

Can we switch back to Travis or GitHub Actions? We don't use GitLab for this project.

I don't have the permissions to setup Github actions (or Gitlab CI for that matter). I can add a travis config.

[maoe]

I don't have the permissions to setup Github actions (or Gitlab CI for that matter).

I think it's enabled by default unless explicitly disabled. If you use Travis please use haskell-ci to generate a config. I'm fine either of GitHub Actions or Travis.

[leifmetcalf]

I think it's enabled by default unless explicitly disabled. If you use Travis please use haskell-ci to generate a config. I'm fine either of GitHub Actions or Travis.

I've added an actions workflow. You can see a run here: https://github.com/leifmetcalf/euphoria/actions/runs/144463831

[maoe]

I'm still not sure why the Semigroup constraint is better. Could you elaborate a bit?

[leifmetcalf]

I'm still not sure why the Semigroup constraint is better. Could you elaborate a bit?

You cannot implement eventToUpdate :: Event a -> Update a with a Monoid constraint (what would sampling eventToUpdate mempty return?). Instead, we currently implement eventToUpdate :: Event a -> Update (Just a), and we handle the handle the Nothing case (i.e. the case where no event has occurred since the last sample) with code like this:

g :: Maybe Text -> IO ()
g Nothing = pure ()
g (Just s) = putStrLn s

We can avoid this by changing the constraint in Update from Monoid to Semigroup. Instead of handling the no-event case in the FRP code, we handle it in the sampling code:

startUpdateNetwork :: Update a -> IO (IO (Maybe a) {- sample -}, IO () {- step -})

[leifmetcalf]

CI: https://github.com/leifmetcalf/euphoria/actions/runs/144493676

[leifmetcalf]

@maoe Why would we want an Applicative instance for update?