[Haskell-cafe] Unifying an IO read into a TChan?

Adam adam at ahri.net
Fri Aug 9 15:28:08 UTC 2019

Hello Café!

It's my first post here so do tell me if I get something wrong :)

I'm in the process of writing an event database; it should adhere to ACID
principles and is specifically aimed at persisting then broadcasting events
in some event-driven system.

The code is up on GitHub and I expect to build a Hackage package soon, for
now I'm prototyping with Stack generating the .cabal file, but this will
change when things settle down.

My question is about a function I have:

readEvents :: Word64 -> Connection -> IO ([IndexedEvent], TChan
readEvents from conn = do
    (ec, chan) <- atomically $ (,)
        <$> (readTVar $ evCount conn)
        <*> (dupTChan $ readSubs conn)

    evsFromDisk <- readEventsRange from ec conn
    pure (evsFromDisk, chan)

For context please see the whole file:

1. What I'd like to understand is how I might unify the return type into a
single `TChan IndexedEvent` which would contain first the `evsFromDisk` and
then pull from the broadcast channel `chan`. The only way I can think to do
it is to fork a thread to write to a new broadcast channel - which might be
unexpected for a client of my API and seems quite heavyweight.

I have a connected couple of questions too, related to lazy IO as I'm still
learning Haskell!

2. does my `readEventsRange` function pull all events into memory or does
lazy IO magically avoid this by streaming the data through from the disk?

3. if I did fork a thread and write the `evsFromDisk` and the sit in a `
forever` reading off `chan` and onto my new broadcast channel, would this
result in all the written events being read into memory or does the
laziness spread so that only when the client does a `readTChan` will the
data flow through from the disk/`chan`?

Thanks for reading,
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