Solution to https://github.com/effectfully-ou/haskell-challenges/tree/master/h5-prerun-action

Prerun.hs

import Control.Exception (finally)
import Control.Concurrent.STM (atomically)
import Control.Concurrent.STM.TMVar (TMVar, newEmptyTMVarIO, takeTMVar, putTMVar, readTMVar)

prerun :: (IO a -> IO (IO b)) -> IO (IO a -> IO b)
prerun f = do
  mailbox <- newEmptyTMVarIO
  -- mailbox for actions (IO a)
  mb <- f (join (atomically (readTMVar mailbox)))
  -- run the action inside the mailbox up to n times
  pure $
    \ma -> do
      atomically (putTMVar mailbox ma)
      -- fill mailbox with a specific action
      -- this will block if other threads attempt to run f
      mb `finally` atomically (takeTMVar mailbox)
      -- empty the mailbox so other threads can run f