8 ways to report errors in Haskell revisited

by Edward Z. Yang

In 2007, Eric Kidd wrote a quite popular article named 8 ways to report errors in Haskell. However, it has been four years since the original publication of the article. Does this affect the veracity of the original article? Some names have changed, and some of the original advice given may have been a bit... dodgy. We’ll take a look at each of the recommendations from the original article, and also propose a new way of conceptualizing all of Haskell’s error reporting mechanisms.

I recommend reading this article side-to-side with the old article.

1. Use error

No change. My personal recommendation is that you should only use error in cases which imply programmer error; that is, you have some invariant that only a programmer (not an end-user) could have violated. And don’t forget, you should probably see if you can enforce this invariant in the type system, rather than at runtime. It is also good style to include the name of the function which the error is associated with, so you say “myDiv: division by zero” rather than just “Division by zero.”

Another important thing to note is that error e is actually an abbreviation for throw (ErrorCall e), so you can explicitly pattern match against this class of errors using:

import qualified Control.Exception as E

example1 :: Float -> Float -> IO ()
example1 x y =
  E.catch (putStrLn (show (myDiv1 x y)))
          (\(ErrorCall e) -> putStrLn e)

However, testing for string equality of error messages is bad juju, so if you do need to distinguish specific error invocations, you may need something better.

2. Use Maybe a

No change. Maybe is a convenient, universal mechanism for reporting failure when there is only one possible failure mode and it is something that a user probably will want to handle in pure code. You can easily convert a returned Maybe into an error using fromMaybe (error "bang") m. Maybe gives no indication what the error was, so it’s a good idea for a function like head or tail but not so much for doSomeComplicatedWidgetThing.

3. Use Either String a

I can’t really recommend using this method in any circumstance. If you don’t need to distinguish errors, you should have used Maybe. If you don’t need to handle errors while you’re in pure code, use exceptions. If you need to distinguish errors in pure code, for the love of god, don’t use strings, make an enumerable type!

However, in base 4.3 or later (GHC 7), this monad instance comes for free in Control.Monad.Instances; you no longer have to do the ugly Control.Monad.Error import. But there are some costs to having changed this: see below.

4. Use Monad and fail to generalize 1-3

If you at all a theoretician, you reject fail as an abomination that should not belong in Monad, and refuse to use it.

If you’re a bit more practical than that, it’s tougher to say. I’ve already made the case that catching string exceptions in pure code isn’t a particularly good idea, and if you’re in the Maybe monad fail simply swallows your nicely written exception. If you’re running base 4.3, Either will not treat fail specially either:

-- Prior to base-4.3
Prelude Control.Monad.Error> fail "foo" :: Either String a
Loading package mtl- ... linking ... done.
Left "foo"

-- After base-4.3
Prelude Control.Monad.Instances> fail "foo" :: Either String a
*** Exception: foo

So you have this weird generalization that doesn’t actually do what you want most of the time. It just might (and even so, only barely) come in handy if you have a custom error handling application monad, but that’s it.

It’s worth noting Data.Map does not use this mechanism anymore.

5. Use MonadError and a custom error type

MonadError has become a lot more reasonable in the new world order, and if you are building your own application monad it’s a pretty reasonable choice, either as a transformer in the stack or an instance to implement.

Contrary to the old advice, you can use MonadError on top of IO: you just transform the IO monad and lift all of your IO actions. I’m not really sure why you’d want to, though, since IO has it’s own nice, efficient and extensible error throwing and catching mechanisms (see below.)

I’ll also note that canonicalizing errors that the libraries you are interoperating is a good thing: it makes you think about what information you care about and how you want to present it to the user. You can always create a MyParsecError constructor which takes the parsec error verbatim, but for a really good user experience you should be considering each case individually.

6. Use throw in the IO monad

It’s not called throwDyn and catchDyn anymore (unless you import Control.OldException), just throw and catch. You don’t even need a Typeable instance; just a trivial Exception instance. I highly recommend this method for unchecked exception handling in IO: despite the mutation of these libraries over time, the designers of Haskell and GHC’s maintainers have put a lot of thought into how this exceptions should work, and they have broad applicability, from normal synchronous exception handling to asynchronous exception handling, which is very nifty. There are a load of bracketing, masking and other functions which you simply cannot do if you’re passing Eithers around.

Make sure you do use throwIO and not throw if you are in the IO monad, since the former guarantees ordering; the latter, not necessarily.

7. Use ioError and catch

No reason to use this, it’s around for hysterical raisins.

8. Go nuts with monad transformers

This is, for all intents and purposes, the same as 5; just in one case you roll your own, and in this case you compose it with transformers. The same caveats apply. Eric does give good advice here shooing you away from using this with IO.

Here are some new mechanisms which have sprung up since the original article was published.

9. Checked exceptions

Pepe Iborra wrote a nifty checked exceptions library which allows you to explicitly say what Control.Exception style exceptions a piece of code may throw. I’ve never used it before, but it’s gratifying to know that Haskell’s type system can be (ab)used in this way. Check it out if you don’t like the fact that it’s hard to tell if you caught all the exceptions you care about.

10. Failure

The Failure typeclass is a really simple library that attempts to solve the interoperability problem by making it easy to wrap and unwrap third-party errors. I’ve used it a little, but not enough to have any authoritative opinion on the matter. It’s also worth taking a look at the Haskellwiki page.


There are two domains of error handling that you need to consider: pure errors and IO errors. For IO errors, there is a very clear winner: the mechanisms specified in Control.Exception. Use it if the error is obviously due to an imperfection in the outside universe. For pure errors, a bit more taste is necessary. Maybe should be used if there is one and only one failure case (and maybe it isn’t even that big of a deal), error may be used if it encodes an impossible condition, string errors may be OK in small applications that don’t need to react to errors, custom error types in those that do. For interoperability problems, you can easily accomodate them with your custom error type, or you can try using some of the frameworks that other people are building: maybe one will some day gain critical mass.

It should be clear that there is a great deal of choice for Haskell error reporting. However, I don’t think this choice is unjustified: each tool has situations which are appropriate for its use, and one joy of working in a high level language is that error conversion is, no really, not that hard.