In my current project, we’re working on a large-ish code base that is written in Scala and uses cats effect as an effect system in large parts of the code base. If you’re not familiar with what an effect system is, I think the most important detail is that it’s a tool that gives you certain superpowers if you promise to be honest about it when your code does things that can be considered “effectful”, such as interacting with the network or reading files. We use the IO monad from cats effect, which is a wonderful way of writing async, concurrent code that looks a lot like synchronous code.

This is the first project where I’ve been using an effect system in Scala, and it’s been a fascinating experience. Something that I think is kind of challenging in our case is that we’ve inherited a code base where a lot of code is dishonest about being effectful – that’s to say, it simply existed before the rest of the code started using an effect system. Many of the libraries that we use do not integrate with our IO monad and use exceptions to handle errors, so there’s actually quite a lot of this code. Previously I’ve only worked with IO in Haskell, where using it isn’t optional so this kind of challenge has been new to me and I don’t really have a good idea about how to handle it.

On the whole, I feel very happy about working with cats effects, but I do think that there’s an issue with this kind of ecosystem where it’s hard to gradually adopt it.

Despite our happiness with cats effects, we’ve ended up having to look at and consider ZIO for some of the things we do. Namely, we’ve been instructed to offer GraphQL APIs and after testing a few options, we decided that caliban looks like the best option for us. It integrates really nicely with ZIO, and specifically ZQuery, which is a very cool library that makes it easier to batch and cache GraphQL queries efficiently. There’s a cool paper you can read about Haxl, which inspired ZQuery, although I have to warn that the paper isn’t light reading.

A theory we had early on, is that it would be simpler for developers to work with only one effect system in the code base, and so if we needed to have ZIO anyway, we should consider porting the rest of the code base away from cats effects. This is something we decided to timebox and make an attempt at, and I thought it might be interesting to share some of the things we’ve learned in the process.

Devising a method to refactor

There are a lot of common signatures between IO and ZIO, and we had strong reasons to believe that in many places, just changing the type would be enough. The first thing we did was to define an IO alias for ZIO, so that we could update the effect type we used across the whole code base by only editing import statements:

type IO[+A] = zio.Task[A]

Replacing all imports cats.effect.IO with our IO alias was simple to do with simple search and replace, and some manual touch up for the most complex imports.

After doing this, there was a fairly limited number of patterns we had to update, and some of them, we could fix by adding extension methods to our IO alias. Here’s a non-exhaustive list of things we needed to change:

  • IO.raiseError became
  • IO.pure became ZIO.succeed
  • IO.attempt became ZIO.either
  • IO.apply | IO.delay became ZIO.attemptBlocking
  • IOApp became ZIOAppDefault
  • We had to import import zio.interop.catz.* all over the place for EitherT and things like that to work.

On the whole, this stuff wasn’t so bad and took me the better part of a day to fix in the whole code base (which is maybe 150k lines?) by simply hitting compile and fixing the next error, one after another. It was boring, but not very hard.

Once that was done, I very quickly had a code base that compiled, and almost all the tests immediately passed. Most of the few test failures were easy to fix and were simple things like wrong translation of IO methods to ZIO methods.

The actual hard part

There was one test failure that bothered me. In particular, the test was doing an actual call to a http4s service, and it was expecting to have some error handling done, but it simply wasn’t happening. It looked like our error handling was simply being ignored by ZIO, and I had a hard time figuring out why.

This honestly took me a few hours to figure out and I only realized what was wrong completely by accident. At some point I randomly remembered reading documentation about cats effects 2 that mentioned something about expecting only pure functions to be provided to map and flatMap. I decided to look up this section and see why my brain thought it might be relevant.

When using map or flatMap it is not recommended to pass a side effectful function, as mapping functions should also be pure. […] Note that as far as the actual behavior of IO is concerned, something like IO.pure(x).map(f) is equivalent with IO(f(x)) and IO.pure(x).flatMap(f) is equivalent with IO.defer(f(x)).

But you should not rely on this behavior, because it is NOT described by the laws required by the Sync type class and those laws are the only guarantees of behavior that you get. For example the above equivalence might be broken in the future in regards to error handling. So this behavior is currently there for safety reasons, but you should regard it as an implementation detail that could change in the future.

I quickly realized that due to our code base having a lot of dishonest code that pretended to be pure, but wasn’t, we had a lot of places where we were passing side effectful functions to map and flatMap. cats effects is saying it can handle that (and that we should feel bad about doing it!), but how about ZIO?

An interesting difference between ZIO and IO is that ZIO has an error channel in the type, and we’ll often see a signature like ZIO[Env, Throwable, A]. ZIO can also represent an “infallible” value, that is, something that can’t possibly fail: ZIO[Env, Nothing, A]. A value like that has to always contain an A, and could never contain an exception. If ZIO.flatMap were to allow functions that could throw exceptions, it would have to always return some variant like ZIO[Env, Throwable, A], which means you couldn’t compose ZIO[Env, Nothing, A] at all. So how does ZIO handle this?

So it turns out that this is what’s called a defect in ZIO. This is for unexpected and unrecoverable errors, like a throwing an Exception in map or flatMap. You can move such defects to the error channel of the ZIO monad by using .catchSomeDefect, like this:

// some dishonest code that says it can't fail
val infallible: ZIO[Any, Nothing, String] = ???
val fallible: ZIO[Any, Throwable, String] = infallible.catchSomeDefect {
  case e: JsonParseException =>

Doing this operation at the top level of our http4s service made the test pass and for a moment, I was happy to see that we were able to refactor the entire code base in only a few days. But then I started having doubts.

There’s a lot of error handling code in a 150k line code base. There’s an impossibly large amount of calls to .map and .flatMap. How could we possibly verify that we’ve installed all the necessary .catchSomeDefect calls in the right places? How could we possibly hope to verify that we’ve done this correctly? I started to feel a bit of despair and realized I that this decision was too big to make without consulting others.

Opting for the safer route

After discussing this with the teams that share this code base, we decided that it was hard to accept this risk. The task of refactoring to use ZIO was something we had speculatively decided to be a good idea, but it was unclear to us what we would risk by not doing it. It seemed pretty clear that we would have to take a risk that was hard to quantify, in order to proceed with the refactoring, and for very uncertain benefits, it seemed best to let this one go. This is one of the cases where I think timeboxing is very valuable, in the end, this task did not cost us much time, and we learned a lot from it. It’s hard to say for sure, but I suspect that if we had sunk more cost into this, it would’ve been harder to stop.

At this point we have both effect systems in use in our code base, and so far it seems to be working out fine. It also turns out that we’re taking the GraphQL part of the code out of the main code base and developing it in a separate repository, so we don’t have to worry about mixing the two effect systems in the same code base, so it may have all worked out for the best.

Some lessons learned

  • Time boxing is very valuable, and it is fun to work against the clock. ⏰
  • Doing a lot of work in a short time, then throwing it away is fine! 🗑️
  • Conducting experiments is sometimes the best way to make sure that decisions are well-founded. ✅
  • But often, there’s no right or wrong choice anyway, just different degrees of risk. 🎲
  • There’s a significant and big difference in the way that exceptions in .map and .flatMap are handled between IO and ZIO. 🤔
  • It seems like gradually adopting an effect system is very hard, fraught with perils and probably not worth it in most cases. 🤔