Yep, but it's still an unfortunately big difference in expressiveness between the two languages, where one might think that they would be more or less on par, given the languages' similarities (both are more or less contemporary in their design, statically typed, with similarly powerful type systems, etc).

My question is then: is there something fundamentally preventing Rust from achieving similar levels of expressiveness to this Scala example without incurring in unnecessary runtime overhead? Given that both languages are statically typed, my inclination would be to say no: the information is there, the compiler should be able to figure it out. But, alas, i know very little about these things, hence my question :)

Update: pcwalton gave some nice insight on why Rust needs some of these constructs on an uncle comment.

Actually Rust already has izip and IntoIterator that covers the early differences. The type annotation `Vec<_>` is also optional since it can be inferred from the context.

The `map` is uglier in Rust because

* the comparison was against a constructor with positional, rather than named, arguments

* the Scala code didn't need to dereference any arguments, and

* and Scala's `Zipped` is a special type with a special `map` function that takes three arguments, unlike a normal iterator.

The first and last points could be easily copied in Rust: you'd build a constructor for HuffmanCode and augment iterators of tuples with with a starmap method (that can be done in a library). The middle point can be done before the zipping. The result would be

    let codes =
        izip!(data_table.iter().cloned(), code_lengths, code_table)
            .starmap(HuffmanCode::new)
            .collect();

Rust's collect is never implicit, like Scala's CanBuildFrom. This prevents accidental collects, which helps writing fast code, but in principle I don't see why it couldn't be implicit - it would just require the whole standard library to be overhauled.

Actually the last two `.iter()` aren't needed at all because of IntoIter. All three can all be removed if you use Itertools' izip!

    let codes: Vec<_> =
        izip!(&data_table, code_lengths, code_table)
            .map(|(&value, length, code)| {
                HuffmanCode {
                    length: length,
                    code: code,
                    value: value,
                }
            })
            .collect();

(If HuffmanCode was a tuple type, this could even be

    #![feature(fn_traits)]

    let codes: Vec<_> =
        izip!(data_table.iter().cloned(), code_lengths, code_table)
            .map(|args| (&HuffmanCode).call(args))
            .collect();

but now I'm just playing around.)

  > (except the obvious `:Vec<_>` type that is not required)

It is required, otherwise, `collect()` doesn't know what type of collection to collect into.

  > It may be possible to implement `.zipped` on tuples though.

This is impossible without varargs, no?

> This is impossible without varargs, no?

Nah, you'd just implement it as an impl over and over on tuples of reasonable size (up to 16 or so).

It wouldn't be elegant, but it'd work in practice.

Which is what itertools does I believe https://bluss.github.io/rust-itertools/doc/itertools/struct....

Oh right, I see now.

This technique is used a lot by specs: https://github.com/slide-rs/specs/blob/master/src/join.rs#L5...

> `collect()` doesn't know what type of collection to collect into.

Yes, unless there is some other mention of Vec, i.e. it is returned from the function.

> This is impossible without varargs, no?

Varargs would definitely help, and would allow Rust to borrow more ideas without workarounds. But one can do it manually for each tuple variant.

Haskell just brute force implements zipWith3, zipWith4 etc

Not elegant, but it works.

The un brute force way is to use the Applicative instance of ZipList.