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Rust: “Explain GATs Like I'm 5 Years Old”

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Re: Rust: “Explain GATs Like I'm 5 Years Old”

#41

Post by gpderetta »

bsaul wrote: post #18
josephcsible wrote: post #14

Earlier quoted context omitted.

Rust getting more of Haskell's type system features is a good thing. The stronger your type system is, the more mistakes you can discover as compiler errors instead of runtime bugs.

If a typesystem has any impact on the popularity of a language, then i assume rust doesn’t want to copy too much of haskell and needs to know where to stop..

C++ sorta has the equivalent of this feature in the form of nested templates. C++ is kind of popular and rust wants to be a direct replacement.

Next step: template template parameters!

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#42

Post by jackmott42 »

the__alchemist wrote: post #38
kibwen wrote: post #35

Earlier quoted context omitted.

You mention that you use Rust. In that case, you're almost certainly familiar with generics, and you're likely familiar with associated types. If you've ever found yourself struggling to write an associated type that needed a generic parameter, this feature is for you. And if you haven't ever needed to do such a thing, that's totally fine, and you can happily ignore it and content yourself with benefiting from librar…

I appreciate the explanation. I've been more of a firmware or application developer in Rust than library developer, so will keep this in the hip pocket.

Yeah basically if the day comes when you need these, you will just naturally use them and it will work and you will never think anything of it.

Whereas I stumbled upon a use for these ~4 years ago and was just mad that what I naturally wanted to do didn't work =)

But yeah in certain programming domains, probably most, it just doesn't come up.

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#43

Post by kibwen »

nrc wrote: post #40
kibwen wrote: post #30

Earlier quoted context omitted.

GATs are an essential part of Rust's long-term async/await story. It's not language-wankery, this is driven by actual real-world use cases. Rust's own stdlib would have benefited from this in many places had this feature existed years ago.

No, they're not. GATs are used internally in the compiler to implement async methods, but there is absolutely no need for them to be in the language for that (just like generators are used to implement await but are not part of the (stable) language). GATs may or may not be useful, but IMO the actual real-world use cases are pretty weak.

Yes, I've read the stabilization thread and I'm aware of your stance. :P Whether or not it's exposed, the mechanism still needs to exist in the compiler, and enough library authors are clamoring to use it that I think the argument in favor of exposing them is stronger than you're giving it credit for.

You mention generators, but a lot of people are clamoring for those as well, and I fully expect them to be exposed to end-users someday.

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#45

Post by wisnoskij »

I feel like they are making languages too complicated. Most projects should just be garbage collected, most don't need to deal with pointers or anything advanced. We need variables, functions, conditionals, loops, exception handling, struts, and a library with a bunch of standard utilities (including simple I/O and string managment). Everything else should be optional and kept out of sight unless you are looking for it IMHO.

I am probably just old and set in my ways, but I have been learning rust over the last few days, and it is really syntax dense. Their is no reason it needs to be this complex. For example I think "String" and "&static' str" are different types, and no one explains what "&static'" is but I dont think it is changeable. It is not like ' or static means anything and can change, it is just basically random syntax you have to memorize. OK, I am sure it means something like a reference to a static character or something, if you dont even think it is a good idea to inform people what it means, dont implement the syntax like that.

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#46

Post by aarobot »

nicoburns wrote: post #34
the__alchemist wrote: post #28

I'm filing this one along with Monads and Haskell as programming concepts that I'll never understand. Rust is my favorite language, but that explanation, lauded directly below it as This is personally the easiest to understand example I've seen of GATs. , was incomprehensible.

I think the easiest way to understand it is this: Suppose you have a concrete type like Vec Regular generics enable you to make the contained type generic, so you have: Vec where T can be i32, u32, String, etc. GATs allow you to make the container generic. So you can have: T where T might be Vec, Option, Box, etc.

Awesome example.

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#47

Post by wisnoskij »

unsafecast wrote: post #4
jstx1 wrote: post #2

As a beginner (having finished the book and not much more) the example in the top comment is very difficult to parse and hold in my head. At a glance I have no idea what we're doing and why.

Basically, before this you couldn't have generic types in traits, like this: trait X { type Y ; } You couldn't express "a thing that contains another thing" in one of those type expressions before, and also use the contained type. It's needed for Mappable, because it's an abstraction over the concept of modifying a value inside a container. IMO the implementation that they have is a bit obtuse, but I don't know if yo…

I really wonder about the usefulness of implementing a language feature to reduce the need to copy paste in an editor.

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#48

Post by mcronce »

jackmott42 wrote: post #42
the__alchemist wrote: post #38

Earlier quoted context omitted.

I appreciate the explanation. I've been more of a firmware or application developer in Rust than library developer, so will keep this in the hip pocket.

Yeah basically if the day comes when you need these, you will just naturally use them and it will work and you will never think anything of it. Whereas I stumbled upon a use for these ~4 years ago and was just mad that what I naturally wanted to do didn't work =) But yeah in certain programming domains, probably most, it just doesn't come up.

Yeah - it's almost better to think of GATs as less of a feature and more of a weird restriction being removed. That's obviously super reductive knowing the amount of work that went into it, but from an end developer standpoint, it's an explanation that helped make it make sense to me.

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#49

Post by fallat »

nicoburns wrote: post #34
the__alchemist wrote: post #28

I'm filing this one along with Monads and Haskell as programming concepts that I'll never understand. Rust is my favorite language, but that explanation, lauded directly below it as This is personally the easiest to understand example I've seen of GATs. , was incomprehensible.

I think the easiest way to understand it is this: Suppose you have a concrete type like Vec Regular generics enable you to make the contained type generic, so you have: Vec where T can be i32, u32, String, etc. GATs allow you to make the container generic. So you can have: T where T might be Vec, Option, Box, etc.

THIS is literally the best explanation. My god people have forgotten how to explain anything anymore. Thank you.

Re: Rust: “Explain GATs Like I'm 5 Years Old”

#50

Post by philzook »

nicoburns wrote: post #34
the__alchemist wrote: post #28

I'm filing this one along with Monads and Haskell as programming concepts that I'll never understand. Rust is my favorite language, but that explanation, lauded directly below it as This is personally the easiest to understand example I've seen of GATs. , was incomprehensible.

I think the easiest way to understand it is this: Suppose you have a concrete type like Vec Regular generics enable you to make the contained type generic, so you have: Vec where T can be i32, u32, String, etc. GATs allow you to make the container generic. So you can have: T where T might be Vec, Option, Box, etc.

What you're describing sounds to me like higher kinded types, which are of some relationship to GATs (GATs enable a better encoding of higher kinded types as I understand it), but are not GATs. https://docs.rs/higher/latest/higher/
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