Killing forked threads

When we create a new thread, we can keep track of its corresponding thread ID to kill it later manually.

In this recipe, we will be creating a command-line interface for forking new processes to download a huge file. A download will be initiated with the d command followed by a number. So, running d 1 will launch a thread to download item #1.

We will learn how to kill threads while they are still running. Our command to kill threads will look like k 1 in order to kill the downloaded item #1.

How to do it…

In a new file, which we call Main.hs, insert the following code:

1. Import the required packages as follows:

   import Control.Concurrent
   import qualified Data.Map as M

2. Let main call the helper download function:

   main = download (M.empty :: M.Map Int [ThreadId])

3. Define a function to take the user queries and appropriately respond as follows:

   download m = do
     input <- (getLine >>= return . words)
     respond m input >>= download

4. Respond to a download request:

   respond...