recursion-schemes VS unliftio

I've got another article planned where I talk through exactly why and how this function works, but it's worth noting that I didn't come up with it myself -- I simply ported it from the Haskell library that first introduced this technique.

Reasonably proud of my part 2 solution, although would like to try using a recursion scheme rather than unstructured recursion:

Yeah, that project is pretty much at the bottom of my list, unfortunately. My top projects these days are mgmt, klister, recursion-schemes, and hint... And that's already too much!

In a programming context, recursion schemes can be used to write recursive (or corecursive) functions, by automating/abstracting away the common boilerplate part of actually doing the recursion. They take the form of polymorphic higher-order functions, which can be imported from a library like this classic one.

Maybe I didn't link the best post. It is unfortunately the only one I know that uses Rust. If you are able to read Haskell, the documentation for the recursion-schemes package might be a better resource?

Writing something as a type of fold over an infinite sequence is nicer than using recursion directly in my opinion. See: https://hackage.haskell.org/package/recursion-schemes

Consider recursion schemes! It let's you separate the logic of how your recursion is structured on your data, and the logic of what you're doing on each recursion stage. So e.g. you can write the core logic of a recursive linked list summation as just fun x accum -> x + accum, and then you just find the appropriate recursion scheme to pipe the list values into x and handle recursing to build accum (a catamorphism in this case)

It's a pretty complicated bug, documented in details at https://github.com/recursion-schemes/recursion-schemes/issues/50

It’s also trivial and easy in Haskell — you just need an instance of `Foldable` or `Traversable` on your collection, and then you can fold or traverse it in a configurable way. Or for recursive structures, use https://hackage.haskell.org/package/recursion-schemes. Or even just pass a traversal function as an argument for maximum flexibility.

https://hackage.haskell.org/package/recursion-schemes is the "normal" library for this type of generalized folding. It even contains Base instances for Maybe and Either.

FWIW, your suggestion is very similar to a proposed instance for MonadUnliftIO (ExceptT e), except that effectfuls use of the type system means that it doesn't suffer from one of the proposed downsides (dubious interaction with catchAny).

Possibly interesting thread here: https://github.com/fpco/unliftio/issues/68

Any way quoting u/snoyberg : https://github.com/fpco/unliftio/issues/17#issuecomment-363655106

That's not true. It's just writing that instance is a bit tricky: https://github.com/fpco/unliftio/issues/68

However, ExceptT cannot be an instance of MonadUnliftIO – because it necessarily requires multiple exit points. See this discussion which should give you an idea of how hairy and unpredictable this can be.