Elegant haskell case/error handling in sequential monads

Question

Because I oversimplified in my other question before, I would like to give a more clear example here.

How can I handle situations where I have to check for certian conditions in a sequential way without nesting multiple cases? With "sequential way" I mean getting a value (e.g. from stdin), checking this value for a certain condition and depending on the outcome getting another value and so on.

Example:

sequen :: IO String
sequen = do
  a <- getLine
  case a of
    "hi" -> do
      putStrLn "hello!"
      b <- getLine
      case b of
        "how are you?" -> do
          putStrLn "fine, thanks"
          return "nice conversation"
        _ -> return "error 2"
    _ -> return "error 1"

I know that there are better ways to write such a chat bot, it should just demonstrate the sequential nature of the problem. As you can see, with every nested case, the code also gets indented deeper.

Is there a way to better structure such code? I'm thinking of handling the "errors" on one place and describing the "success-path" without the error handling distributed all over it.

Answer 1

Of course. This is precisely what EitherT was made for. You can get it from Control.Monad.Trans.Either in the eitherT package.

import Control.Monad.Trans.Class
import Control.Monad.Trans.Either

main = do
    e <- runEitherT $ do
        a <- lift getLine
        case a of
            "hi" -> lift $ putStrLn "hello!"
            _    -> left 1
        b <- lift getLine
        case b of
            "how are you?" -> lift $ putStrLn "fine, thanks!"
            _              -> left 2
        return "nice conversation"
    case e of
        Left  n   -> putStrLn $ "Error - Code: " ++ show n
        Right str -> putStrLn $ "Success - String: " ++ str

EitherT aborts the current code block whenever it encounters a left statement, and people typically use this to indicate error conditions.

The inner block's type is EitherT Int IO String. When you runEitherT it, you get IO (Either Int String). The Left type corresponds to the case where it failed with a left and the Right value means it successfully reached the end of the block.

Answer 2

I wrote a series of posts a while back going over my own learnings of the Either & EitherT types. You can read it here: http://watchchrislearn.com/blog/2013/12/01/working-entirely-in-eithert/

I use the errors package to get a bunch of nice helpers around using EitherT (left and right functions for instance to return lifted versions of Left and Right).

By extracting your potential failure conditions into their own helpers, you can make the mainline of your code read totally sequentially, with no case statements checking results.

From that post, you can see how the runEitherT section is a sequential chunk of work, it just happens to have the failure mechanics of EitherT. Obviously this code is fairly contrived to show how MaybeT plays inside of EitherT as well. In real code it'd just be the story you were wanting to tell, with a single Left/Right at the end.

import Control.Error
import Control.Monad.Trans

-- A type for my example functions to pass or fail on.
data Flag = Pass | Error

main :: IO ()
main = do
  putStrLn "Starting to do work:"

  result <- runEitherT $ do
      lift $ putStrLn "Give me the first input please:"
      initialText <- lift getLine
      x <- eitherFailure Error initialText

      lift $ putStrLn "Give me the second input please:"
      secondText <- lift getLine
      y <- eitherFailure Pass (secondText ++ x)

      noteT ("Failed the Maybe: " ++ y) $ maybeFailure Pass y

  case result of
    Left  val -> putStrLn $ "Work Result: Failed\n " ++ val
    Right val -> putStrLn $ "Work Result: Passed\n " ++ val

  putStrLn "Ok, finished. Have a nice day"

eitherFailure :: Monad m => Flag -> String -> EitherT String m String
eitherFailure Pass  val = right $ "-> Passed " ++ val
eitherFailure Error val = left  $ "-> Failed " ++ val

maybeFailure :: Monad m => Flag -> String -> MaybeT m String
maybeFailure Pass  val = just $ "-> Passed maybe " ++ val
maybeFailure Error _   = nothing

Answer 3

Since you are necessarily in the IO monad, you are better off using the IO monad's error handling capabilities instead of stacking an error monad on top of IO. It avoids all of the heavy lifting:

import Control.Monad ( unless )
import Control.Exception ( catch )
import Prelude hiding ( catch )
import System.IO.Error ( ioeGetErrorString )

main' = do
  a <- getLine
  unless (a == "hi") $ fail "error 1"
  putStrLn "hello!"
  b <- getLine
  unless (b == "how are you?") $ fail "error 2"
  putStrLn "fine, thanks"
  return "nice conversation"

main = catch main' $ return . ioeGetErrorString

In this case, your errors are simply Strings, which are thrown by IO's fail, as a userError. If you want to throw some other type, you will need to use throwIO instead of fail.

Answer 4

At some point the EitherT package was deprecated (though transformers-either offers a similar API). Fortunately there's an alternative to EitherT that doesn't even require installing a separate package.

The standard Haskell installation comes with the Control.Monad.Trans.Except module (from the transformers package, which is bundled with GHC), which behaves almost identically to EitherT. The resulting code is almost identical to the code in Gabriella Gonzalez's answer, but using runExceptT instead of runEitherT and throwE instead of left.

import Control.Monad.Trans.Class
import Control.Monad.Trans.Except

main = do
    e <- runExceptT $ do
        a <- lift getLine
        case a of
            "hi" -> lift $ putStrLn "hello!"
            _    -> throwE 1
        b <- lift getLine
        case b of
            "how are you?" -> lift $ putStrLn "fine, thanks!"
            _              -> throwE 2
        return "nice conversation"
    case e of
        Left  n   -> putStrLn $ "Error - Code: " ++ show n
        Right str -> putStrLn $ "Success - String: " ++ str

(Note that the aforementioned transformers-either package is in fact a wrapper for ExceptT designed for providing compatibility with code that still uses EitherT.)