I’m talking more about availability of extension methods rather than typeclass instances per se. In Rust, typeclass instance is not available as a value (even trait objects used for having virtual methods are bound to specific struct or enum instances) but instead visibility of a typeclass adds extension methods to a type.
Example code in Rust https://www.ideone.com/SXefDQ
use struct_here::MyStruct;
//use trait_here::MyTrait; // rustc suggest that, IntelliJ inserts automatically
mod struct_here {
pub struct MyStruct;
impl MyStruct {}
}
mod trait_here {
use struct_here::MyStruct;
pub trait MyTrait {
fn print_hello(&self);
}
impl MyTrait for MyStruct {
fn print_hello(&self) {
println!("Hello, World!")
}
}
}
fn main() {
let instance = MyStruct {};
instance.print_hello(); // <- situation: I'm writing this line of code
}
Compiler error:
error: no method named `print_hello` found for type `struct_here::MyStruct` in the current scope
--> prog.rs:26:14
|
26 | instance.print_hello(); // <- situation: I'm writing this line of code
| ^^^^^^^^^^^
|
= help: items from traits can only be used if the trait is in scope; the following trait is implemented but not in scope, perhaps add a `use` for it:
= help: candidate #1: `use trait_here::MyTrait`
Analogous code in Scala https://www.ideone.com/kY7zRG
import TypeClasses1.struct_here.MyStruct
object TypeClasses1 {
object struct_here {
class MyStruct
}
object trait_here {
trait MyTrait[A] {
def printHello(): Unit
}
implicit val myTraitForMyStruct: MyTrait[MyStruct] =
new MyTrait[MyStruct] {
override def printHello(): Unit =
println("Hello, World!")
}
}
def main(args: Array[String]): Unit = {
val instance = new MyStruct
instance.printHello()
}
}
Compiler error:
Main.scala:22: error: value printHello is not a member of TypeClasses1.struct_here.MyStruct
instance.printHello()
I know that typeclasses work somewhat differently in Scala and Rust (and above Scala code can’t be fixed with extra import), but Rust’s approach doesn’t look inherently less capable. Typeclasses are ultimately used for extension methods and extra constants. Here’s how Rust typeclass can add a constant to a base type:
use struct_here::MyStruct;
use trait_here::MyTrait;
mod struct_here {
pub struct MyStruct;
impl MyStruct {}
}
mod trait_here {
use struct_here::MyStruct;
pub trait MyTrait {
const MY_CONSTANT: u8;
}
impl MyTrait for MyStruct {
const MY_CONSTANT: u8 = 8;
}
}
fn main() {
println!("{}", MyStruct::MY_CONSTANT); // MY_CONSTANT added to MyStruct
}
If I forget to add use trait_here::MyTrait; then rustc prints the following:
error[E0599]: no associated item named `MY_CONSTANT` found for type `struct_here::MyStruct` in the current scope
--> src/main.rs:23:30
|
5 | pub struct MyStruct;
| -------------------- associated item `MY_CONSTANT` not found for this
...
23 | println!("{}", MyStruct::MY_CONSTANT); // MY_CONSTANT added to MyStruct
| ----------^^^^^^^^^^^
| |
| associated item not found in `struct_here::MyStruct`
|
= help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope, perhaps add a `use` for it:
|
1 | use trait_here::MyTrait;
|
Overall, in Rust I do not even need to know the names of typeclasses. Rust compiler will find them for me and suggest them. Same for IntelliJ - it will suggest methods or constants from typeclasses and import those typeclasses (traits) immediately.
Things change when I use a generic type - then we need to have generic type constraints, i.e. explicitly type typeclass names, e.g. (snippet from my project):
fn mul_wide<A: FixI32, B: FixI32, R: FixI64>(a: &A, b: &B) -> R { ... }
but even then rustc can help when I omit something, e.g. when I change A: FixI32 to just A. Suggestions aren’t always working, but it’s better than no suggestions as in Scala. In this case rustc suggested me to change A to A: FixedPoint where FixedPoint is a supertrait of FixI32. Suggestion failed, but was somewhat close to truth. I can search for subtraits of FixedPoint - there are only 4 of them (FixI32, FixI64, FixU32 and FixU64) so choice is easy.