On Bifunctor IO and Java's Checked Exceptions

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The Bifunctor IO data type is a hot topic in the Scala community. In this article however I’m expressing my dislike for it because it shares the same problems as Java’s Checked Exceptions.

What is IO? #

Normally the IO data type is expressed as:

sealed trait IO[+A] {

What this means is that IO is like a thunk, like a function with zero parameters, that upon execution will finally produce an A value, if successful. The type also signals possible side effects that might happen upon execution, but since it behaves like a function (that hasn’t been executed yet), when you are given an IO value you can consider it as being pure (and the function producing it has referential transparency). This means that IO can be used to describe pure computations.

Modern IO implementations for JVM are also capable of describing asynchronous processes, therefore you can also think of IO as being:

opaque type IO[+A] = () => Future[A]

If we had opaque types this would work well ;-)

Available implementations are:

Some cool presentations on this subject:

IO[+A] implements MonadError[IO, Throwable]. And if we’re talking of Cats-Effect or Monix, it also implements Sync among others.

This means that IO[+A] can terminate with error, it can terminate in a Throwable, actually reflecting the capabilities of the Java Runtime. This means that this code is legit:

import cats.effect.IO
import scala.util.Random

def genRandomPosInt(max: Int): IO[Int] =
  IO(Math.floorMod(Random.nextInt(), max))

The astute reader might notice that this isn’t a total function, as it could throw an ArithmeticException. An easy mistake to make.

What’s the Bifunctor IO? #

I’m going to call this data type BIO, to differentiate it from IO above:

sealed trait BIO[E, A] {

Such a type parameterizes the error type in E. This is more or less like usage of Either to express the error, but as you shall see below, they aren’t exactly equivalent:

opaque type BIO[E, A] = IO[Either[E, A]]

Or in case you’re throwing EitherT in the mix to make that less awkward:

import cats.data.EitherT

type BIO[E, A] = EitherT[IO, E, A]

Exposing the error type would allow one to be very explicit at compile time about the error:

def openFile(file: File): BIO[FileNotFoundException, BufferedReader] =
  // Made up API
  BIO.delayE {
    try Right(new BufferedReader(new FileReader(file)))
    catch { case e: FileNotFoundException => Left(e) }

def genRandomInt: BIO[Nothing, Int] =

You can see in the first function that we are very explicit about FileNotFoundException being an error that could happen, instructing readers that they should probably do error recovery.

And in the second function we could use Nothing as the error type to signal that this operation can in fact produce no error (not really, but let’s go with it 😉).

Available implementations:

  • scalaz/ioeffect, the Scalaz 8 IO, available as a backport for Scalaz 7, by John A. De Goes and other Scalaz contributors
  • cats-bio by Luka Jacobowitz, inspired by Scalaz 8’s IO, he took Cats-Effect’s IO and changed Throwable to E as a proof of concept
  • Worthy to mention is also Unexceptional IO, Luka’s precursor to his BIO implementation, inspired by Haskell’s UIO I think

Some articles on this subject:

The premise of these articles is that:

  1. our type system should stop us from being able to write nonsensical error handling code and give us a way to show anyone reading the code that we’ve already handled errors
  2. the performance of EitherT is bad and usage more awkward

Naturally, I disagree with the first assertion and I don’t think the second assertion is a problem 😀

The Problems of Java’s Checked Exceptions #

While I think that the Bifunctor IO is a cool implementation, that’s pretty useful for certain people, or certain use cases, I believe that ultimately it’s not a good default implementation, as it shares the same problems as Java’s Checked Exceptions. Or in other words, it’s ignoring decades of experience with exceptions, since their introduction in LISP and then in C++, Java, C# and other mainstream languages.

The web is littered with articles on why checked exceptions were a bad idea and many of those reasons are also very relevant for an IO[E, A]. Here’s just two such interesting articles:

But let me explain in more detail …

1. Composition Destroys Specific Error Types #

Let’s go with a more serious example:

import java.io._

def openFile(file: File): BIO[FileNotFoundException, BufferedReader] =
  // Made up API
  BIO.delayE {
    try Right(new BufferedReader(new FileReader(file)))
    catch { case e: FileNotFoundException => Left(e) }

def readLine(in: BufferedReader): BIO[IOException, String] =
  BIO.delayE {
    try Right(in.readLine())
    catch { case e: IOException => Left(e) }
    finally in.close()

def convertToNumber(nr: String): BIO[NumberFormatException, Long] =
  BIO.delayE {
    try Right(nr.toLong)
    catch { case e: NumberFormatException => Left(e) }

What would be the type of a composition of multiple IO values like this?

for {
  buffer <- openFile(file)
  line <- readLine(buffer)
  num <- convertToNumber(line)
} yield num

That’s right, you’ll have a Throwable on your hands. And this is assuming that we’ve got a flatMap that widens the result to the most specific super-type, otherwise you’ll have to take care of conversions manually, at each step. Also note that our usage of Throwable is irrelevant for the problem at hand. You could come up with your own error type, but Throwable is actually more practical, because we can simply cast it.

So assuming a flatMap that doesn’t automatically widen the error type of the result, what you’ll have to deal with is actually worse:

for {
  buffer <- openFile(file).leftWiden[Throwable]
  line <- readLine(buffer).leftWiden[Throwable]
  num <- convertToNumber(line).leftWiden[Throwable]
} yield num

Not sure how people feel about this, but to me this isn’t an improvement over the status quo, far from it, this is just noise polluting the code. And before you say anything in its defence, make sure the argument doesn’t also apply to Java and everything you dislike about it 😉

2. You Don’t Recover From Errors Often #

Imagine a piece of code like this:

for {
  r1 <- op1
  r2 <- op2
  r3 <- op3
} yield r1 + r2 + r3

So we are executing 3 operations in sequence and each of them can fail, we don’t know which or how.

Does it matter? Most of the time, you don’t care. Most of the time it is irrelevant. Most of the time you can’t even recover until later.

Due to this uncertainty about which operations trigger errors and which don’t, the premise of a Bifunctor IO is that we’re forced to do attempt (error recovery) everywhere, but that is not a correct premise. The way exceptions work and why they were introduced in LISP and later in C++, is that you only catch exceptions at the point were you can actually do something about it, otherwise it’s fine to live in blissful ignorance.

Empirical evidence suggests that most checked exceptions in Java are either ignored or re-thrown, forcing people to write catch blocks that are meaningless and even error prone.

You can even find some studies on handling of checked exceptions in Java projects, although I’m unsure about how good they are. For example there’s Analysis of Exception Handling Patterns in Java Projects, which states that:

“Results of this study indicate that most programmers ignore checked exceptions and leave them unnoticed. Additionally, it is observed that classes higher in the exception class hierarchy are more frequently used as compared to specific exception subclasses.”

Consider that in case of a web server the recovery might be something as simple as showing the user an HTTP 500 status. HTTP 500 statuses are a problem, but only if they happen and when they start to show up, you can then go back and fix what needs to be fixed.

Also remember the FileNotFoundException we mentioned above? Well, in most cases there’s not much you can do about it. It’s not like you’ve got much choice in the knowledge that the file is missing, most of the time the important bit being that an error, any error, happened.

To quote Anders Hejlsberg, the original designer of C#:

“It is funny how people think that the important thing about exceptions is handling them. That is not the important thing about exceptions. In a well-written application there’s a ratio of ten to one, in my opinion, of try/finally to try/catch. Or in C#, using statements, which are like try/finally.”

In other words the most important part of exceptions are the finalizers, recovery being less frequent.

3. The Error Type is an Encapsulation Leak #

Lets say that we have this function:

def foo(param: A): BIO[FileNotFoundException, B]

By saying that it can end with a FileNotFoundException, we are instructing all callers, at all call sites, to handle this error as part of the exposed API.

It’s pretty obvious that FileNotFoundException can happen due to trying to open a file on disk that is missing. It’s a very specific error, isn’t it, the kind of error we’re supposed to like if we’re fans of EitherT or of the Bifunctor IO.

Well, what happens if we change foo to make an HTTP request instead, or maybe we turn it into something that reads a memory location. Now all of a sudden FileNotFoundException is no longer a possibility.

def foo(param: A): BIO[Unit, B]

This then bubbles down to all call sites, effectively breaking backwards compatibility, so all that depend on your foo will have to upgrade and recompile. And as the author of foo you’ll be faced with two choices:

  1. break compatibility
  2. keep lying to your users that foo can end with a FileNotFoundException and thus leave them with unreachable code - which is something that some Java libraries are known to have done

NOTE: there are cases in which you want to break binary compatibility in case the error type changes. That is precisely the use case for which the Bifunctor IO or EitherT are recommended.

4. It Pushes Complexity to the User #

On utility I deeply understand the need to parameterize all things. But the question is, what else could we parameterize and why aren’t we doing it?

  • we could have a type parameter that says whether the operation is blocking-IO bound, or CPU bound and in this way we could avoid running an IO that’s CPU-bound on a thread-pool meant for blocking I/O or vice-versa
  • we could add a type parameter for the execution model — is it synchronous or asynchronous?
  • we could describe the side effect with a type parameter — i.e. is it doing PostgreSQL queries, or ElasticSearch inserts and in this way the type becomes more transparent and you could come up with rules for what’s safe to execute in parallel or what not
  • add your own pet peeve …

I’m fairly sure that people have attempted these. I’m fairly sure that there might even be libraries around that are useful in certain specific instances. But they are not mainstream.

We aren’t doing it because adding type parameters to the types we are using leads to the death of the compiler, not to mention our own understanding of the types involved, plus usage becomes that much harder, because by introducing type parameters, values with different type arguments no longer compose without explicit conversion / widening, pushing a lot of complexity to the user.

This is why EitherT is cool, even with all of its problems. It’s cool because it can be bolted on, when you need it, adding that complexity only when necessary.

The Bifunctor IO[E, A] looks cool, but what happens downstream to the types using it? Monix’s Iterant for example is Iterant[F[_], A]. Should it be Iterant[F[_], E, A]? Or maybe Iterant[F[Throwable, _], A]? Or Iterant[F[_, _], E, A]?

If I parameterize the error in Iterant, how could it keep on working with the current IO that doesn’t have a E parameter? And if Iterant works with IO[Throwable, _], then what’s the point of IO[E, A] anyway?

Note that having multiple type parameters is a problem in Haskell too. Martin Odersky already expressed his dislike for type classes of multiple type parameters, such as MonadError and it’s pretty telling that type classes with multiple type parameters are not part of standard Haskell.

5. The Bifunctor IO Doesn’t Reflect the Runtime #

I gave this piece of code above and I’m fairly sure that you missed the bug in it:

def readLine(in: BufferedReader): BIO[IOException, String] =
  BIO.delayE {
    try Right(in.readLine())
    catch { case e: IOException => Left(e) }
    finally in.close()

The bug is that in.close() can throw exceptions as well. Actually on top of the JVM even pure, total functions can throw InterruptedException for example.

So what happens next?

Well the Bifunctor IO cannot represent just any Throwable. By making E generic, it means that handling of Throwable is out. So at this point there are about 3 possibilities:

  1. crash the process, which would be the default, naive implementation
  2. your thread crashes without making a sound, logging to a stderr that gets redirected to /dev/null
  3. use something like a custom Java Thread.UncaughtExceptionHandler, or Scalaz’s specific “fiber” error reporter to report such errors somewhere

Also the astute reader should notice that by replacing the MonadError handling and recovery by a simple reporter there’s no way to do back-pressured retries. The nature of bugs is that many bugs are non-deterministic. Maybe you’re doing an HTTP request and you’re expecting a number in return, but it gives you an unexpected response - maybe it has a maximum limit of concurrent connections or something.

When making requests to web services, wouldn’t it be better to give them some slack? Wouldn’t it be better to do retries with exponential backoff a couple of times before crashing? Or maybe use utilities such as TaskCircuitBreaker? Of course it is. And in the environments I worked on, such instances are very frequent and the processes have to be really resilient to failure and resiliency is built-in only when having the assumption that everything can fail for unknown reasons.

In the grand scheme of things, the reason for why this is a huge problem is because IO should reflect the runtime, because IO effectively replaces Java’s call-stack. But the Bifunctor IO no longer does.

In the words of Daniel Spiewak, who initiated the Cats-Effect project:

“ The JVM runtime is typed to a first order. Which happens to be exactly what the type parameter of IO reflects. I'm not talking about code in general, just IO. IO is the runtime, the runtime is IO. ”


“ The whole purpose of IO as an abstraction is to control the runtime. If you pretend that the runtime has a property which it does not, then that control is weakened and can be corrupted (in this case, by uncontrolled crashes). ”


“ IO needs to reflect and describe the capabilities of the runtime, for good or for bad. All it takes is an "innocent" throw to turn it all into a lie, and you can't prevent that. ”


I agree with that and it shows which developers worked a lot in dynamic environments, this great divide being between those that think types can prove correctness in all cases and those that don’t.

If you’re in the former camp, I think Hillel Wayne is eager to prove you wrong 😉

IO Cannot Be an Alias of the Bifunctor IO #

You might be temped to say that:

type IO[A] = BIO[Throwable, A]

This is not true and it gave birth to, what I like to call, the great “No True Functor” debate and fallacy 😜

But details about it would take another article to explain.

So it’s enough to say that cats.effect.IO and monix.eval.Task has got you covered in all cases, whereas a Bifunctor IO needs to pretend that developers on top of the JVM can work only with total functions, on top of an environment that actively proves you wrong, thus applying the “let it crash” philosophy on top of a runtime that makes this really expensive to do so (i.e. the JVM is not Erlang).

This is another great divide in mentality, although I can see the merits of the arguments on the other side. In such cases it’s relevant by what kind of problems you got burned or not in the past I guess.

Final Words #

I am not saying that the Bifunctor IO[E, A] is not useful.

I’m pretty sure it will prove useful for some use-cases, the same kind of use-cases for which EitherT is useful, except with a less orthogonal design. Well you gain some performance in that process, although when you’re using EitherT it’s debatable whether it matters for those particular use cases.

What I am saying is that:

  1. let’s not ignore the two decades of experience we had with Java’s checked exceptions, preceded by another two decades of experience with exceptions in other languages
  2. EitherT is useful because it can be bolted on when the need arises, or otherwise it can be totally ignored by people like myself, so let’s not throw the baby with the bath water

I do think that IO[E, A] will be a great addition to the ecosystem, as an option over the current status quo. Scala is a great environment.

That’s all.

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