Rust
impl Trait
Specify parameter and return types with impl Trait.
impl Trait
In return position, impl Trait means "I return a single concrete type implementing this trait, but I won't name it". It's used to return types whose names are very long or unwritable, like iterators and closures:
fn ciftler(n: u32) -> impl Iterator<Item = u32> {
(0..n).filter(|x| x % 2 == 0)
}
fn carpan(k: i32) -> impl Fn(i32) -> i32 {
move |x| x * k
}
The real type of ciftler is Filter<Range<u32>, {closure}>; closure types can't be named at all.
What does the caller see?
The caller can only use what the trait offers; this gives the freedom to change internal details:
let v: Vec<u32> = ciftler(10).collect(); // [0, 2, 4, 6, 8]
let uc_kat = carpan(3);
println!("{}", uc_kat(7)); // 21
move makes the closure take ownership of k by copying it; it's needed because k is gone once the function ends.
Exercise Hidden return types
Write the ciftler and carpan functions above. Collect ciftler(10) into a vector and multiply 7 with carpan(3); print both separated by a space.
Show example solution
fn ciftler(n: u32) -> impl Iterator<Item = u32> {
(0..n).filter(|x| x % 2 == 0)
}
fn carpan(k: i32) -> impl Fn(i32) -> i32 {
move |x| x * k
}
fn main() {
let v: Vec<u32> = ciftler(10).collect();
let uc_kat = carpan(3);
println!("{:?} {}", v, uc_kat(7));
}
Test yourself
-
Why doesn't <code>fn f(b: bool) -> impl Display { if b { 5 } else { "beş" } }</code> compile?
- if isn't an expression
- It compiles
- impl Trait must be a single concrete type; i32 and &str differ
- Display can't be returned
Show answer
impl Trait must be a single concrete type; i32 and &str differ — Returning different types needs dynamic dispatch, like Box<dyn Display>.
-
Return an iterator
fn ciftler(n: u32) -> ____ {
(0..n).filter(|x| x % 2 == 0)
}
Show answer
fn ciftler(n: u32) -> impl Iterator<Item = u32> {
(0..n).filter(|x| x % 2 == 0)
}
-
Return type and meaning
- -> impl Iterator<Item = u32>
- Hidden iterator
- -> impl Fn(i32) -> i32
- Hidden closure
- -> Box<dyn Display>
- Can be different types
- -> Vec<u32>
- Concrete, owned
Next lesson: dyn Trait
