I also blog frequently on the Yesod Web Framework blog, as well as the FP Complete blog.

Breaking changes, dependency trees

January 9, 2018

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My previous blog post discussed a possible upcoming breaking change to the conduit library: dropping finalizers. This is one of a number of other breaking changes I have planned. Another one is switching over from MonadBaseControl to MonadUnliftIO, for reasons I've discussed at length before and spoken about too.

Beyond this change, I have a number of others planned out as well, some more solidly than others. I've started a document describing some of these, and I wanted to bring up one point in this design space for some user feedback: conduit dependency trees.


The situation today is that we have a dependency graph that looks something like the following:

  • resourcet is at the base of the hierarchy, and defines some non-conduit-specific types and functions used throughout the conduit ecosystem. It currently depends on a number of packages, like monad-control, but that number will naturally drop as we move over to MonadUnliftIO exclusively.
  • conduit is designed to provide basic conduit functionality with fewer dependencies. It does depend on resourcet, and packages like monad-control. But it does not depend on bytestring, text, or vector, even though these are almost always wanted with conduit. It provides the Data.Conduit.List set of combinators, which are not the best ones out there.
  • conduit-extra adds lots of dependencies, including things like attoparsec, and provides a nicer set of helpers around bytestring and text.
  • And finally, at the top of the tree (or our tree for today), we've got conduit-combinators, which provides the combinators I actually recommend people use in the Data.Conduit.Combinator module. This has lots of dependencies, since it inherits from conduit-extra and also adds in some extra things like mwc-random.


  • You can use resourcet without touching the conduit ecosystem at all
  • You can use conduit without pulling in lots of resources
  • Data.Conduit.Combinators is fully loaded


  • The current dependency footprint even at the base is higher than I'd like, though that's getting fixed soon regardless.
  • The conduit package is not super useful on its own due to lack of bytestring, text, and vector support.
  • To get the functionality you want in either conduit-extra or conduit-combinators, you end up with a much larger dependency footprint.

Plans for the next version

I have a number of different ideas in mind. I'll start off with the most conservative plan, and mention some variants below.

  • As already mentioned, resourcet drops a bunch of dependencies. Nothing too interesting there.
  • conduit adds a dependency on bytestring, text, and vector as basic libraries everyone should be using anyway. We move over Data.Conduit.Combinators and provide most of its functionality in conduit itself, and start recommending against Data.Conduit.List, Data.Conduit.Binary, and Data.Conduit.Text.
  • conduit-extra basically remains as-is
  • conduit-combinators retains the extra functionality not present in the new conduit


  • The conduit package now provides most of the functionality you'll want on a day-to-day basis
  • The dependency footprint for the Data.Conduit.Combinators module is much reduced
  • We can finally get away from the not-well-named functions in Data.Conduit.List

There aren't necessarily downsides to this approach, as I think it's simply better than what we have today already. But I want to list out the alternatives, which will make clear some things that could be possibly better still.

  • What do we do with the mono-traversable package? It's currently a dependency of conduit-combinators, and the simplest path forward for the above is to make conduit depend on mono-traversable. However, this is a slightly heavier dependency footprint, requiring adding in unordered-containers and vector-algorithms. Alternatives: Strip down mono-traversable to have less deps Redefine parts of mono-traversable needed for conduit in conduit itself Going crazy: really move mono-traversable into conduit and swap the dependency tree around My inclination: minimize mono-traversable's dependencies a bit more (like dropping the split package, and maybe vector-algorithms) and make it a dependency of conduit.
  • Do we really need conduit-combinators as well as conduit-extra? It's just adding a few extra pieces of functionality over conduit-extra, and perhaps those should be folded into conduit-extra itself.
  • Some people may not like the heavier dep footprint of conduit now. Should we split off a conduit-core package providing the core data types, functions, and operators, and have conduit depend on that?
  • It feels almost silly to have the ResourceT data type live in a separate package. If we have conduit-core, that could be a logical place to put it, since it won't have any extra dependencies versus the resourcet package itself, and then we can turn resourcet into a backwards compatibility layer. Or it may be logical to place ResourceT in the unliftio-core package, since both concepts help with resource cleanup in monad transformers. The former is necessary for continuation-based monads, while the latter (MonadUnliftIO) works for simpler monads.

If people have feedback, I'm happy to hear about it. I've spent an unfortunate amount of time bouncing around between these different options, so hopefully writing it all down and hearing some outside opinions can help move this forward.

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