Is it that is dangerous?

I wrote the post for recoverable errors in Haskell and someone has written a nice response post proposing that this pattern is actually dangerous. While reading it I wrote up some of my own notes to share in reply.

I’d like to preface with the fact that I’m grateful for discussion and criticism of this approach. I’d like to use this approach, but if someone finds genuine issues with it, then I’ll naturally have to look elsewhere.

Loss of code re-use

The first point made, and I think this is a solid point, is that you can’t re-use handy monadic functions like traverse or forever with this syntax. That’s a downer.

However, I think it also follows naturally. If ExceptT were easily a clean MonadUnliftIO instance, then we would have a clean interpretation of how such functions would behave under all the circumstances that unliftio is applicable in, most importantly threading and when IO appears in negative position (withX). As it happens, we don’t.

This also applies to free monads, by the way. When you run the monad, you still have to decide on an interpretation, figure out how things will interact, and the idea of doing so does not bring me confidence.

Indeed, I may want to decide on a case-by-case basis whether traverse should optimistically run all actions (even concurrently; which I’m doing in my compiler), only at the end checking all results, or whether it should fail hard on the first failing action. Happily, the pooledMapConcurrentlyN function doesn’t have to know or care about how I do error handling.

In conclusion, I don’t see this as a complete obvious loss, and in some way it’s also a gain. I think there might be some interesting ideas to explore in this area.

Lifting hides results

The second point, in my reading, was that with this syntax, a programmer will get into the habit of lifting all actions that aren’t returning Either E A. The downside being that if you use a function like timeout, like this:

result <- timeout (someAction :: IO (Either E A))

Then you’ll hit a type error,

Couldn't match type: Maybe (Either ErrorType Int)
                 with: Either e0 b0
In a stmt of a qualified 'do' block:
    result <- timeout 1000 action1

So, being in a rush, they ignore the code and just add an fmap pure call to fix the error:

result <- fmap pure (timeout 1000 action1)

Now we’ve lost the error, it didn’t short-circuit where it should have! Actually, we haven’t lose the error. In fact, we haven’t finished writing the code at this point. Assuming we have compiler warnings on, we’ll be told that result is unused.

Main.hs:10:7: warning: [-Wunused-matches]
    Defined but not used: ‘result’
10 |       result <- fmap pure (timeout 1000 action1)
   |       ^^^^^^

Furthermore, when we decide to use it, we’ll see that its type is

Maybe (Either ErrorType Int)

The error is still here, we have to do something with it. You can’t use it without having to deal with the error in some way. I think if the compiler tells you something and you ignore it, that’s your responsibility. It can’t save you from yourself.

This is the first instance of a running theme in the post that I’m replying to, which is that code that doesn’t use results is dangerous.

The try function

The next case considered was try, which has this type:

try :: Exception e => IO a -> IO (Either e a)

And what would happen if you wrote:

result <- try (someAction :: IO (Either E A))

So that the type would be:

result <- try (someAction :: IO (Either E A)) :: IO (Either E (Either E A))

The author claims that you won’t get a type error if you do something with the result. Let’s study it:

  1. try will catch exceptions thrown by the GHC exception mechanism.
  2. It will catch only those exceptions that are an instance of Exception.
  3. It will be reported back to the user in the Left case of the either.

In this example:

But I believe that this example is contrived. The whole point of the system is to avoid using the base exception system for our business logic. Why, then, would we implement an instance of Exception and then willingly throw our failure type? Finally, the code doesn’t lose any errors. In fact, the double Either would be enough to indicate to a programmer that something is off here.

Names are not type safety

As an aside, an easy to avoid confusion here is to simply use a type dedicated to this. Let’s call it Failure:

data Failure err ok = Failed err | Ok ok

Now, it’s not possible to be confused about Either as some random data and Failure which only ever is produced by a failure. Many have argued that Either itself is a form of boolean blindness. I used Either in my post as an aid to demonstrate a point; there’s no compelling reason otherwise to use it.

The author, citing Alexis King, would have us believe that names are not type safety. I disagree; all types are type safety. The attempt to put type techniques into “safe” and “unsafe” categories only serves to eliminate nuance.

Additionally, the author believes that syntax is not type safety. I think syntax is the main driving force behind type safety, or so I feel, having written the type generation stage of various compilers.

Finally (pun intended)

The last point the author brings up seems to be more of a criticism of any IO action returning Either, than the special syntax I proposed in my post.

In base there are two functions which are notable in one crucial aspect, their discarding of values:

finally :: MonadUnliftIO m => m a -> m b -> m a
bracket :: MonadUnliftIO m => m a -> (a -> m b) -> (a -> m c) -> m c

finally discards b, and bracket discards b.

The author writes that for,

someAction1 `finally` someAction2

would necessarily discard the result of someAction2. I think this is a good observation. Your result will be discarded, never to be seen again.

This does bring up a wider point, however: the type of bracket and friends is a problem. Why don’t they use ()? These functions can swallow results all the time, regardless of whether you’re using or writing normal Haskell. In Scala, Kotlin and Ceylon, bracket uses void or unit.

I’m glad the author brought this up, because we don’t currently have any tooling to find and catch these mistakes. Essentially, bracket/finally are risky functions to use due to this.1 This is what prompted the title of my reply – is it that’s dangerous, or is it bracket/finally?

Is it a criticism of I’m doubtful at the moment.


I don’t think there’s any reason to call this technique “dangerous”. I think that the examples in the post were slightly contrived (such as the dubious suggestion that you would implement Exception for your result type, or not using the result of an action that GHC would warn you about), and in the last point it seemed to be arguing against base rather than my proposed technique, to which I can only agree.

I’m unsure about the tone in the penultimate paragraph2, but overall I liked the criticism, it got me thinking. I do think there’s a wider discussion to be had about “use”, and perhaps my slow absorption of Rust thinking has me wishing there was less of a complacent attitude to throwing away results in Haskell’s base libraries. It’ll raise my alarms more readily when reviewing/auditing code.

It’ll still be a while before I’m even able to take advantage of QualifiedDo, but I still look forward to pushing the boundaries of this idea.

  1. Well, they’re risky anyway. Consider the case of an exception being raised in the handler or releaser. It’s complicated.↩︎

  2. This language seems tonally ambiguous, “shallow unsafe trick into a single weak spoiled framework”, but I’ll assume good faith.↩︎