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Rust traits only don't support higher order types because they are missing from the whole language, i.e. it's not a philosophical difference between traits and

by dbaupp 12y ago

Rust traits only don't support higher order types because they are missing from the whole language, i.e. it's not a philosophical difference between traits and type classes. Also, Haskell has existential types, which are exactly like Rust's trait objects.

thinkpad20 12y ago
Rust has generics, which are higher order types (you can't have just a vector, it needs to be a vector "of something"). I'm not sure if the rust team would describe them that way, but there doesn't seem to be that much of a theoretical difference. Thus if they wanted to, they could (presumably) allow for a higher-order trait like Functor: trait Functor { fn map<a, b> : (|a| -> b) -> Self<a> -> Self<b> } (I'm not sure if that's valid syntax but you get the idea). As for existential types, they're an area of Haskell I haven't really explored that much, but they seem to be seen as a bit more of an advanced feature than their equivalents in Rust or Java. Not just because they require a GHC extension and additional keywords, but because there's a bit more theoretical overhead to understand how they work, while the presentation in Rust/Java is more intuitive and straightforward.
- dbaupp 12y ago
  > Rust has generics, which are higher order types (you can't have just a vector, it needs to be a vector "of something"). I'm not sure if the rust team would describe them that way, but there doesn't seem to be that much of a theoretical difference. No, there's no first class "type level functions", i.e. you always need `Vec<T>`, and you can't ever write `Vec` by itself, which is what a true higher-order type is (and what Functor needs: impl Functor for Vec { ... } impl Functor for Option { ... } Thus, your example isn't at all possible now or even in the near future.
  - thinkpad20 12y ago
    I'm aware that my example isn't possible, which is why I said, "if they wanted to, they (presumably) could allow for..." My point being that Rust restricts what one can do with higher-order types, but those types still exist and are expressible in the language. From a theoretical point of view, there doesn't seem to be any reason that they couldn't allow for traits on higher-kinded types -- they simply choose not to. Perhaps for performance reasons, or for simplicity, or for syntax or whatever, but I haven't seen any reason why they couldn't. That was the point that I was making.

by dbaupp 12y ago

thinkpad20 12y ago

Rust has generics, which are higher order types (you can't have just a vector, it needs to be a vector "of something"). I'm not sure if the rust team would describe them that way, but there doesn't seem to be that much of a theoretical difference. Thus if they wanted to, they could (presumably) allow for a higher-order trait like Functor: trait Functor { fn map<a, b> : (|a| -> b) -> Self<a> -> Self<b> } (I'm not sure if that's valid syntax but you get the idea). As for existential types, they're an area of Haskell I haven't really explored that much, but they seem to be seen as a bit more of an advanced feature than their equivalents in Rust or Java. Not just because they require a GHC extension and additional keywords, but because there's a bit more theoretical overhead to understand how they work, while the presentation in Rust/Java is more intuitive and straightforward.

dbaupp 12y ago
> Rust has generics, which are higher order types (you can't have just a vector, it needs to be a vector "of something"). I'm not sure if the rust team would describe them that way, but there doesn't seem to be that much of a theoretical difference. No, there's no first class "type level functions", i.e. you always need `Vec<T>`, and you can't ever write `Vec` by itself, which is what a true higher-order type is (and what Functor needs: impl Functor for Vec { ... } impl Functor for Option { ... } Thus, your example isn't at all possible now or even in the near future.
- thinkpad20 12y ago
  I'm aware that my example isn't possible, which is why I said, "if they wanted to, they (presumably) could allow for..." My point being that Rust restricts what one can do with higher-order types, but those types still exist and are expressible in the language. From a theoretical point of view, there doesn't seem to be any reason that they couldn't allow for traits on higher-kinded types -- they simply choose not to. Perhaps for performance reasons, or for simplicity, or for syntax or whatever, but I haven't seen any reason why they couldn't. That was the point that I was making.