On behalf of Java developers everywhere - thank you Project Lombok!
this post was submitted on 15 Nov 2024
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Implementing Equality in Haskell:
deriving (Eq, Ord)
After learning how easy it was to implement functional programming in Rust (it's almost like the language requires it sometimes), I decided to go back and learn the one I had heard about the most.
It opened my mind. Rust takes so many cues from Haskell, I don't even know where to begin. Strong typing, immutable primitives, derived types, Sum types. Iterating and iterables, closures, and pattern matching are big in Haskell.
I'm not saying Rust uses these because Graydon Hoare wanted a more C-like Haskell, but it is clear it took a lot of elements from the functional paradigm, and the implementations the designers were familiar with had descended through Haskell at some point.
Also, deriving is not the same as implementing. One is letting the compiler make an educated guess about what you want to compare, the other is telling it specifically what you want to compare. You're making, coincidentally, a bad comparison.
The first iteration of the Rust compiler was written in OCaml...
Don't need the Ord instance for equality, just Eq is sufficient. Ord is for inequalities.
The point of the post is that most mainstream languages don't provide a way to automatically derive point-wise equality by value, even though it's pervasively used everywhere. They instead need IDEs to generate the boilerplate rather than the compiler handling it.
The problem with rust, I always find is that when you're from the previous coding generation like myself. Where I grew up on 8 bit machines with basic and assembly language that you could actually use moving into OO languages.. I find that with rust, I'm always trying to shove a round block in a square hole.
When I look at other projects done originally in rust, I think they're using a different design paradigm.
Not to say, what I make doesn't work and isn't still fast and mostly efficient (mostly...). But one example is, because I'm used to working with references and shoving them in different storage. Everything ends up surrounded by Rc\ or Rc\> and accessed with blah.as_ptr().borrow().x etc.
Nothing wrong with that, but the code (to me at least) feels messy in comparison to say C# which is where I do most of my day job work these days. But since I see often that things are done very different in rust projects I see online, I feel like to really get on with the language I need a design paradigm shift somewhere.
I do still persist with rust because I think it's way more portable than other languages. By that I mean it will make executable files for linux and windows with the same code that really only needs the standard libraries installed on the machine. So when I think of writing a project I want to work on multi platforms, I'm generally looking at rust first these days.
I just realised this is programmerhumor. Sorry, not a very funny comment. Unless you're a rust developer and laughing at my plight of trying to make rust work for me.
I find these videos give a very visual explanation and help to put you into the right mindset: http://intorust.com/
(You can skip the first two videos.)
Sort of when it clicked for me, was when I realized that your code needs to be a tree of function calls.
I mean, that's what all code is anyways, with a main-function at the top calling other functions which call other functions. But OOP adds a layer to that, i.e. objects, and encourages to do all function calls between objects. You don't want to do that in Rust. You kind of have to write simpler code for it to fall into place.
To make it a bit more concrete:
You will have functions which hold ownership over some data, typically because they instantiated a struct. These sit at the root of a sub-tree, where you pass access to this data down into further functions by borrowing it to them.
You don't typically want to pass ownership all over the place, nor do you typically want to borrow (or pass references) to functions which are not part of this sub-tree.
Of course, there's situations where this isn't easily possible, e.g. when having two independent threads talking to each other, and then you do need Rc or Arc, but yeah, the vast majority of programming problems can be solved with trees of function calls.
Do you have some public code you could link to that you’re having this issue with? There isn’t a one-size-fits-all solution for Rc/RefCell, I think.
The current thing I'm working on (processor for iptv m3u files) isn't public yet, it's still in the very early stages. Some of the "learning to fly" rust projects I've done so far are here though:
https://git.nerfed.net/r00ty/bingo_rust (it's a multi-threaded bingo game simulator, that I made because of the stand-up maths video on the subject).
https://git.nerfed.net/r00ty/spectrum_screen (this is a port of part of a general CPU emulation project I did in C#, it emulates the ZX spectrum screen, you can load in the 6912 byte screens and it will show it in a 2x scaled window).
I think both of these are rather using Arc\> because they both operate in a threaded environment. Bingo is wholly multi-threaded and the spectrum screen is meant to be used by a CPU emulator running in another thread. So not quite the same thing. But you can probably see a lot of jamming the wrong shape in the wrong hole in both of those.
The current project isn't multi-threaded. So it has a lot of the Rc/Rc\ action instead.
EDIT: Just to give the reason for Rc\ in the current project. I'm reading in a M3U file and I'm going to be referencing it against an Excel file. So in the structure for the m3u file, I have two BtreeMaps, one for order by channel number and one by name. Each containing references to the same Channel object.
Likewise the same channel objects are stored in the structure for the Excel file that is read in (searched for in the m3u file structure).
BTreeMaps used because in different scenarios the contents will be output in either name order or channel order. So just better to put them in, in that order in the first place.
The bingo one actually uses crossbeam channels instead of mutexes, so that's nice. I haven't looked too closely at it though.
I don't think you can do too much about the Spectrum one if you want to keep the two threads, but here's what I would change related to thread synchronization. Lemmy doesn't seem to allow me to attach patch files for whatever reason so have an archive instead... https://dblsaiko.net/pub/tmp/patches.tar.bz2 (I wrote a few notes in the commit messages)
Just to give the reason for Rc in the current project. I’m reading in a M3U file and I’m going to be referencing it against an Excel file. So in the structure for the m3u file, I have two BtreeMaps, one for order by channel number and one by name. Each containing references to the same Channel object.
So basically it's channels indexed by channel number and name? That one is actually one of the easy cases. Store indices instead:
struct Channels {
data: Vec,
by_number: BTreeMap,
by_name: BTreeMap,
}
// untested but I think it should compile
fn get_channel_by_name(ch: &Channels, name: &str) -> Option<&Channel> {
Some(&self.data[*ch.by_name.get(name)?])
}
Go is really good for std library, windows and Linux from same code and static binaries BTW.
Fun story from before Rust was getting popular (years ago). So, I did a performance comparison to determine what language we should write our rules engine in. I compared Go, Rust, Node, and some others not worth mentioning.
At the time, I had experience with all but Rust.
Even knowing nothing, and working from scratch, the Rust POC was significantly faster. Just way, way, better.
That being said, I still chose Go due to productivity based on the language knowledge of the team to ease the transition (Go was closer to what they knew already), and while it was good for them to learn Go, I look back on it and realize Rust would have been a great opportunity to invest in their careers and have them learn it instead.
A hindsight is 20/20 experience for me.
I'm much more impressed by the fact that a type can implement PartialEq and not Eq. Now that's nice design!
In Go "==" operator works for everything by default, I like it more:
type A struct {
Name string
Quality int
}
func main() {
var x A
var y A
fmt.Printf("%v", x == y)
}
(if all you want is to compare all corresponding fields which you usually want)
Yeah, I came to Rust from Scala and Kotlin, where equality is default-implemented (for case class and data class respectively, which is basically all we ever used), so this meme surprised me a bit.
I do actually like that you can decide a type cannot be compared, because sometimes it really just doesn't make sense. How would you compare two HTTP clients, for example? But yeah, it certainly is a choice one can disagree with.
Haskell: deriving Eq
Ahh, the comment I was looking for
I would have also accepted: "Haskell did it first."
OCaml has ppx_deriving. PureScript has derive instance.
Is that because it's that simple, or just that the boilerplate is pre-written in the standard library (or whatever it's called in rust)?
Yes, it is that simple. In Rust if you have a structure Person and you want to allow testing equality between instances, you just add that bit of code before the struct definition as follows:
#[derive(PartialEq, Eq)]
struct Person {
name: String,
age: u32,
}
In Rust, PartialEq and Eq are traits, which are similar to interfaces in Java. Manually implementing the PartialEq trait in this example would be writing code that returns something like a.name == b.name && a.age == b.age. This is pretty simple but with large data structures it can be a lot of boilerplate.
There also exist other traits such as Clone to allow creating a copy of an instance, Debug for getting a string representation of an object, and PartialOrd and Ord for providing an ordering. Each of these traits can be automatically implemented for a struct by adding #[derive(PartialEq, Eq, Clone, Debug, PartialOrd, Ord)] before it.
Derive macros are a godsend. There's macros to automatically implement serialization as well. Basically a Trait that can automatically be implemented when derived
i've only read about rust, but is there a way to influence those automatic implementations?
equality for example could be that somethings literally point to the same thing in memory, or it could be that two structs have only values that are equal to each other
Not for the built-in Eq derive macro. But you can write your own derive macros that do allow you to take options, yeah.
But then you realise that the types of 10 constituent fields don't implement Eq, PartialEq...
I mean, if your IDE does it for you, is it really that much better that it's shorter?
not the IDE, its the compiler. this is also not some AI shit, in many cases (not all) the compiler can actually figure out how to do this, because it's not hard, it would just be a lot of boilerplate if written manually.
Why did you even bring up AI? IDEs have been able to generate equality functions for decades without AI.
It's kinda neat to have this defined directly in the language so that compilers can implement it, but creating equality function is so low effort that this doesn't really seem like a big deal.
Like, you define the members in a class, then you tell your IDE to generate getters, constructor, equals, hashcode, etc all in like 5 seconds.
I like it, it's nice when the language itself defines reasonable defaults for things, but realistically you're saving yourself a few seconds of effort.
Yeah but if the class changes you need to update everything, you got all that boilerplate taking up space for no real reason, etc...
The Rust way's just a lot cleaner imo.
It's like 5 seconds to regenerate it. Boilerplate doesn't matter, just collapse it. The only real issue is remembering to update it, if you make a change.
Like I said, I prefer for rust does it, it's just not a big deal
Isn't it obvious? More code to skim, scroll over and maintain if something changes. If you add a struct field, your manual EQ implementation still compiles and seems to work but is wrong and will lead to bugs. Yes, solving this for 99,999% of cases with an attribute is just far superior and does make a difference (while keeping it easy to manually implement it if needed). Hash and Ord and some other traits can be implemented in a similar fashion btw..
I said it was better, just not much better.
The maintenance costs of equals is nearly zero. Scrolling over boilerplate seems like a real stretch, like saying a novel with a picture every chapter is harder to read.
I like that you can't accidentally forget to update it, which is kinda nice but is rarely a concern.
And it's a bit more readable, which is nice.
It's better, but folks are talking like it's Super Jesus and I think it's more like finding a dollar in the parking lot.