[noelwelsh]

What Zig is doing is called "capability passing". I don't know if the Zig team is aware of this field of work, or have independently arrived upon it, but that's what is achieved by passing IO, memory allocators, and other stuff around.

The core idea is that you create a "capability" for any action you want to track, such as using IO, allocating memory, or in your example making cross core or cross server calls. Now to perform these actions code has to have access to the capability. It's a very simple, but powerful, model.

The Effekt language formalizes this, and adds safety properties: https://effekt-lang.org/ Scala 3 also has this.

The papers are pretty readable, if you ignore the middle bits which go into the formal models.

[epolanski]

That looks like a convoluted way to describe dependency injection to me, what am I missing.

[noelwelsh]

You're right, they are related. One difference is simply lineage. Capabilities come out of the erights / security world (e.g. [0]), while dependency injection comes from the XP / agile / Martin Fowler world. More interesting is that to do capabilities correctly you need some type system extensions, namely capture checking. Essentially, this means you delimit where a program can hold a reference to a value derived from a capability. So if you have a function that allocates memory, you can say "there are no references to any allocated memory outside this function call" and hence no use-after-free bugs. It gives a form of resource management that is simpler than Rust's lifetimes. See [1] and [2]. (Technically it's a modal type system versus Rust's substructural type system.) To my mind it's an obvious thing for Zig to add.

Shameless plug. If you're interested in more on this, for a programmer's rather than academic perspective, this is going into my book [3]. I'm writing the chapter of capability passing right now.

Back to writing!

[0]: https://blog.acolyer.org/2016/02/16/capability-myths-demolis...

[1]: https://effekt-lang.org/tour/captures

[2]: https://docs.scala-lang.org/scala3/reference/experimental/cc...

[3]: https://functionalprogrammingstrategies.com/

[epolanski]

So it's essentially low level dependency injection where it's not just about the interface but the system and it's resources too, right?

I will take a look at your content, these topics interest me a lot.

[noelwelsh]

That's a reasonable way of looking at it, but capabilities are not restricted to low-level system properties. Here's a terminal UI system built around three main capabilities:

- layout (adding components to the component tree) - event (handling user input events) - react (reacting to changes in reactive values)

https://github.com/creativescala/terminus/tree/main/ui/share...

This is the case study I'm using for the book chapter.

[Veserv]

Capability passing and dependency injection are just convoluted ways to describe passing parameters instead of using globals.

Capability passing is just extending that to the level of imports. Your files do not import the global I/O library and then use io.print(). They are passed a I/O library as the parameter {io} which defines a print() function and then call io.print().