Unit testing IO in Haskell (revisited)

Posted on January 23, 2016

A couple of months back I posted an article on the Pusher blog about a technique we had tried for unit testing IO in Haskell. Essentially it involved switching out IO for typeclasses, and then making IO an instance of those typeclasses, as well as mocks.

This generated a lot of interesting discussion in the comments, on reddit and Hacker News. After reading Gregory Collins’ comment we decided to try out his approach and so far it has worked out much better for us. Here is an example of how this would work for mocking a socket:

data Socket =
  Socket {
    send :: B.ByteString -> IO (),
    receive :: Int -> IO B.ByteString,
    close :: IO ()

-- Create a real socket by partially applying IO actions to
-- a real socket
fromNetworkSocket :: NS.Socket -> Socket
fromNetworkSocket socket =
    (NS.sendAll socket)
    (flip NS.recv socket)
    (NS.close socket)

-- Create a mock by defining up front what it will respond
-- with, and a callback that will be called with data
-- written to it (could also use an MVar instead)
  :: B.ByteString
  -> (B.ByteString -> IO ())
  -> IO Socket
mkMockSocket input outputCB = do
  leftOver <- newMVar input
  return $
      (recv' leftOver)
      (error "Did not expect socket to be closed")
  recv' leftOver i =
      (\input -> swap $ B.splitAt i input)

We really liked this technique because it avoids creating lots of typeclesses, and having to do a lot of newtype gymnastics when creating mocks. We gradually used it throughout our codebase and it turned out to be a great way of defining the interfaces of all modules that perform IO actions.

Hopefully the example provided you with a gist of how this would work. As always, let me know if you have any questions/suggestions/corrections.