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Unit testing IO in Haskell

by fractalsea on 10/15/15, 8:43 AM with 10 comments

• by gamegoblin on 10/15/15, 6:36 PM

  This is a good post in that it illustrates how it's possible to write useful Haskell that interacts with the real world without weaving IO through the entire codebase (as many beginners end up doing).

  I still find myself preferring to use a dependency injection style when writing Haskell. The code ends up looking similar, and more flexible, in my opinion (you can inject different dependencies based on runtime values).

  Here is an example using the method described in the article vs. dependency injection to print out the current time.

  Article:

      class MonadTime m where
          getTime :: m Integer
  
      class MonadPrint m a where
          doPrint :: a -> m ()
  
      instance MonadTime IO where
          getTime = do
              TOD s _ <- getClockTime
              return s
  
      instance Show a => MonadPrint IO a where
          doPrint = print
  
      printTime :: (Monad m, MonadPrint m Integer, MonadTime m) => m ()
      printTime = getTime >>= doPrint
  
      main :: IO ()
      main = printTime
  

  Dependency injected:

      data MonadPrint m a = MonadPrint { doPrint :: a -> m () }
  
      data MonadTime m = MonadTime { getTime :: m Integer }
  
      ioMonadTime :: IO Integer
      ioMonadTime = do
          TOD s _ <- getClockTime
          return s
  
      printTime :: Monad m => MonadPrint m Integer -> MonadTime m -> m ()
      printTime MonadPrint{..} MonadTime{..} = getTime >>= doPrint
  
      main :: IO ()
      main = printTime (MonadPrint print) (MonadTime ioMonadTime)
   
      

  Good read on the subject of being careful with overusing typeclasses:

  http://www.haskellforall.com/2012/05/scrap-your-type-classes...

• by LukeHoersten on 10/15/15, 6:02 PM

  Pusher is doing some awesome stuff with Haskell! They run a lot of the bitcoin exchange web socket APIs on their infrastructure. I'm sure they have other users too. Cool stuff.

• by bartq on 10/15/15, 3:05 PM

  finally someone did haskell examples in decent modern blog UI instead of old fashioned dark depressing terminal-like colors ;)