Haskell is a marvellous language, but there are some things I don't like about it. My least favorite: Haskell has no fewer than 8 different APIs for reporting errors.
To make a bad situation worse, the choice of API varies between popular libraries. To give a particularly unfortunate example, Network.URI.parseURI and Network.HTTP.simpleHTTP report errors in entirely different ways, turning a "download this URL" program into a page of code, nearly half of which is devoted to dealing with various kinds of errors. (The rest is boilerplate that could be refactored into a nice wrapper.)
Let's begin with a toy function, the simplest possible program that could actually fail:
myDiv x y = x / y
As every algebra student knows, we can't divide by zero. Using this function as our example, let's take a look at all the different ways we can implement error-reporting in Haskell.
The most popular way to report errors in Haskell is
which works as follows:
myDiv1 :: Float -> Float -> Float myDiv1 x 0 = error "Division by zero" myDiv1 x y = x / y
(This is similar to the error-reporting that's built into integer division, actually.)
We can catch the error using
import qualified Control.Exception as E example1 :: Float -> Float -> IO () example1 x y = E.catch (putStrLn (show (myDiv1 x y))) (\err -> putStrLn (show err))
There are two limitations here: Our error is a free-form string, and we can
only catch errors from within the
IO monad. So this only
works for smaller, informal programs.
What if we don't have access to the
IO monad? Well, we can
always use Haskell's
Maybe type to represent a computation
that might fail:
myDiv2 :: Float -> Float -> Maybe Float myDiv2 x 0 = Nothing myDiv2 x y = Just (x / y) example2 x y = case myDiv2 x y of Nothing -> putStrLn "Division by zero" Just q -> putStrLn (show q)
And thanks to the magic of monads, we can actually string together calls to
myDiv2 quite nicely:
divSum2 :: Float -> Float -> Float -> Maybe Float divSum2 x y z = do xdy <- myDiv2 x y xdz <- myDiv2 x z return (xdy + xdz)
This approach to error-reporting is used by Network.URI.parseURI, which is included with most Haskell compilers.
Either String a
But what if we want to have different error messages for different errors?
In that case, we can use
Either to represent computations
which might return either an error message or a value:
myDiv3 :: Float -> Float -> Either String Float myDiv3 x 0 = Left "Divison by zero" myDiv3 x y = Right (x / y) example3 x y = case myDiv3 x y of Left msg -> putStrLn msg Right q -> putStrLn (show q)
Once again, we can treat
Either String as a monad, allowing us
to combine these computations with a minimum of fuss:
divSum3 :: Float -> Float -> Float -> Either String Float divSum3 x y z = do xdy <- myDiv3 x y xdz <- myDiv3 x z return (xdy + xdz)
This approach is used by many small programs that need to recover from multiple kinds of non-IO errors, but I don't think it appears in the standard Haskell libraries.
fail to generalize 1--3
But what if we don't care what
monad our caller is using? In that case, we can rewrite our code to work
in any monad
m, and use
fail to report the error.
myDiv4 :: (Monad m) => Float -> Float -> m Float myDiv4 x 0 = fail "Divison by zero" myDiv4 x y = return (x / y)
This will do the right thing if our caller is expecting
example4a x y = case myDiv4 x y of Nothing -> putStrLn "Division by zero" Just q -> putStrLn (show q) example4b x y = case myDiv4 x y of Left msg -> putStrLn msg Right q -> putStrLn (show q)
You can even use it with the
example4c x y = E.catch (do q <- myDiv4 x y putStrLn (show q)) (\err -> putStrLn (show err))
This style of error-reporting is used widely in the standard libraries, because it's so flexible. You can find several examples in Data.Map.
If you're writing new Haskell libraries for public consumption, and all your errors are strings, please consider using this error-reporting method.
MonadError and a custom error type
What if we want to keep track of specific types of errors? In that case,
we could use the error
Error type class:
import Control.Monad.Error data CustomError = DivByZero | OutOfCheese | MiscError String instance Show CustomError where show DivByZero = "Division by zero" show OutOfCheese = "Out of cheese" show (MiscError str) = str instance Error CustomError where noMsg = MiscError "Unknown error" strMsg str = MiscError str
This works like the
fail example, but instead of using error
messages, we use error values:
myDiv5 :: (MonadError CustomError m) => Float -> Float -> m Float myDiv5 x 0 = throwError DivByZero myDiv5 x y = return (x / y) example5 :: Float -> Float -> Either CustomError String example5 x y = catchError (do q <- myDiv5 x y return (show q)) (\err -> return (show err))
Note that this approach will work in almost any monad except the IO monad. This approach will also fail if we start mixing libraries, because each library will define its own set of errors, and we'll need to write code which converts them all to our preferred error type.
This approach is used by many popular libraries, including parsec. An
unusual variant of this approach is used by Network.HTTP, which
returns values of type
IO (Either ConnError a), but doesn't
ConnError an instance of
throwDyn in the IO monad
We can also use our custom error type in the
IO monad, thanks
import Data.Typeable data CustomError = DivByZero | OutOfCheese | MiscError String deriving (Typeable) myDiv6 :: Float -> Float -> IO Float myDiv6 x 0 = E.throwDyn DivByZero myDiv6 x y = return (x / y) example6 x y = E.catchDyn (do q <- myDiv6 x y putStrLn (show q)) handler where handler :: CustomError -> IO () handler err = putStrLn (show err)
This relies on the fact that
Exception is extensible, thanks
DynException constructor. If you're working in the IO
monad, this approach is almost ideal for production code: You get support
for custom exception types, it's easy to make libraries compatible, and
it's compatible will all the other
IO-based examples we've
Note that this very flexible approach could generalized to
IO monads by making
Exception an instance of
Error, and writing appropriate versions of
MonadError. This would actually be very convenient for people
who have to work with many libraries at once. But I'll refrain from
actually providing code, because there's too many error-reporting
This is a close cousin to the
throwDyn example above. It also
myDiv7 :: Float -> Float -> IO Float myDiv7 x 0 = ioError (userError "Division by zero") myDiv7 x y = return (x / y) example7 :: Float -> Float -> IO () example7 x y = catch (do q <- myDiv7 x y putStrLn (show q)) (\err -> putStrLn (show err))
This one is pretty rare, as far as I can tell.
8. Go nuts with monad transformers
Several of the error-reporting approaches we've seen are based on
IO monads. Most of these can can be generalized to the
equivalent monad transformers. For example,
type ErrIO = ErrorT CustomError IO myDiv8 :: Float -> Float -> ErrIO Float myDiv8 x 0 = throwError DivByZero myDiv8 x y = return (x / y) example8 x y = do result <- runErrorT (myDiv8 x y) case result of Left err -> putStrLn (show err) Right q -> putStrLn (show q)
In the IO monad, this is usually a bad idea (though, again, I've seen it).
Instead, consider using
throwDyn. But if you're working with
a base monad other than
IO, this can occasionally be useful.
A plea for consistency
Several of these error-reporting approaches offer interesting insights into Haskell. And most of them have legitimate uses.
But I'd be just as happy if we could standardize on two or three of the above whenever possible!
Update: Don Stewart has started a thread about Haskell error-handling conventions on the Haskell library list.