Haskell "exceptions"

Question

I've got a set of users, groups, and a mapping between users and groups. I have various functions that manipulate these sets, however one should not be able to add a user<->group mapping for a user that does not exist, nor remove a group which still has users as members, etc.

So basically I want these functions to throw "exceptions" that must be explicitly dealt with by the caller.

I first thought of returning something like this:

data Return r e = Success r | Exception e

And if the caller fails to pattern match against the Exception, they'll hopefully get a compiler warning, or at the very least have an obvious runtime error when there is a problem.

Is this the best approach, and is there a pre-packaged solution that does this? Note I need to throw and catch "exceptions" in pure code, not the IO Monad.

Answer 1

Yes, this is a good approach, and it's in the standard library: Return r e is the same as Either e r. You can even write code like you would using exceptions in IO, too (i.e. without having to explicitly handle the errors at each step with pattern matching): the Monad instance for Either propagates the errors, just like the Maybe monad does (but with the additional e value in the case of an error). For example:

data MyError
    = Oops String
    | VeryBadError Int Int

mightFail :: T -> Either MyError Int
mightFail a = ...

foo :: T -> T -> Int -> Either MyError Int
foo a b c = do
    x <- mightFail a
    y <- mightFail b
    if x == y
        then throwError (VeryBadError x y)
        else return (x + y + c)

If mightFail a or mightFail b returns Left someError, then foo a b c will, too; the errors are automatically propagated. (Here, throwError is just a nice way of writing Left, using the functions from Control.Monad.Error; there's also catchError to catch these exceptions.)

Answer 2

The Return r e type that you are describing is exactly the standard type

data Either a b = Left a | Right b

You might want to use the so called "error monad" (a more suitable name is "exception monad") of the mtl package. (Alternatively, there's ExceptionT in the monadLib package if you don't want to use mtl.) This allows you to do error handling in pure code by invoking throwError and catchError. Here you can find an example that shows how to use it.