Rust Notes #5: Using Constructors as Iterator Adapters

When using the map() iterator adaptor, I usually expect to pass in a function or closure. I was already aware that we can pass a constructor such as String::from() directly, as shown in Rust Notes #2.

However, I recently discovered something interesting: when a function-like constructor takes a single argument—such as an enum variant or a tuple struct—it can also be passed directly to map().

For example:

struct Single(i32);

fn main() {
    let v1: Vec<_> = (1..2).map(Some).collect(); // [Some(1)]
    let v2: Vec<_> = (1..2).map(Single).collect(); // [Single(1)]
}

In both cases, Some and Single act as constructor functions:

Some has the type i32 -> Option<i32>
Single has the type i32 -> Single

Because map() expects a function of the form FnMut(Item) -> B, these constructors can be used directly without writing an explicit closure like |x| Some(x) or |x| Single(x).

This makes the code more concise when the transformation is a simple wrapping operation.

Disclaimer

This post was drafted by me, with AI assistance to refine the content.