BSD-3-Clause licensed by Athan Clark
Maintained by [email protected]
This version can be pinned in stack with:chan-,1474

Module documentation for

Depends on 3 packages(full list with versions):
Used by 1 package in lts-22.22(full list with versions):


This is just some extra Chan and TChan kit that might help the average user. It relies on spawning threads with async and either canceling (debouncing) or waiting (throttling) messages.

Unfortunately, the current design is slightly untyped in the sense that the channel which you supply is the output channel, and the returned channel is the one you would write to. I’m not sure how this should be fixed. There’s a series of “typed” alternatives to each underlying channel type, that allows users to restrict them to “read-only”, “write-only”, or “Writable a => …” etc.

An example might be the following:

import Control.Concurrent.Chan (readChan)
import Control.Concurrent.Chan.Scope (Scope (..))
import Control.Concurrent.Chan.Typed (ChanRW)
import Control.Concurrent.Chan.Extra (allowReading, throttleStatic, intersperseStatic)

-- For example, some websockets:

data SomeMessage
  = Ping
  | Foo
  -- | ...

throttleLayer :: ChanRW 'Read SomeMessage -> IO (ChanRW 'Write SomeMessage)
throttleLayer output = do
  (x,_) <- throttleStatic output 1000000 -- nanoseconds, = 1 second
  pure x

pingLayer :: ChanRW 'Read SomeMessage -> IO (ChanRW 'Write SomeMessage)
pingLayer output = do
  (x,_,_) <- intersperseStatic output (pure Ping) 1000000
  pure x

performWebsocket :: ChanRW 'Read SomeMessage -> IO ()
performWebsocket output = do
  sendable <- pingLayer . allowReading =<< throttleLayer output
  emittingThread <- async $ forever $ do
    -- the thread that actually facilitates the sending of messages queued in the chans
    msg <- readChanRW output
    send msg -- something like that - it'll include Ping messages for us,
             -- and throttle the outgoing messages at the same time.
  writeChanRW sendable Foo