Turbofish
- Rust course
- Intermediate
- Lesson 11/115
- 4 min
Help type inference with the ::<> turbofish.
Turbofish
When the compiler can't infer a generic type from context, the type is given after the function or method name with ::<T>. This syntax is called the "turbofish":
let n = "42".parse::<i32>().unwrap();
let v = (1..4).collect::<Vec<i32>>();
let t = [1, 2, 3].iter().sum::<i32>();
let bos = Vec::<String>::new();The same information can be given by annotating the variable: let n: i32 = "42".parse().unwrap();. In the middle of a chain there's no variable, so the turbofish is needed.
Why is it needed?
parse and collect behave differently depending on the return type: parse can produce i32, f64, bool or IpAddr; collect can produce a Vec, String, HashSet or HashMap. The compiler can't generate code without knowing which you want:
let harfler = "abc".chars().collect::<String>(); // "abc"
let kume = [3, 1, 3].into_iter().collect::<std::collections::HashSet<_>>();
println!("{harfler} {}", kume.len()); // abc 2Practice Extract numbers
Split the text let s = "1 2 3 4"; at spaces, convert each piece with parse::<i32>() and collect with collect::<Vec<_>>(). Compute the sum with sum::<i32>(); print the vector and the sum separated by a space.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let s = "1 2 3 4";
let v = s.split(' ').map(|x| x.parse::<i32>().unwrap()).collect::<Vec<_>>();
let t = v.iter().sum::<i32>();
println!("{:?} {}", v, t);
}Expected output
[1, 2, 3, 4] 10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does the <code>_</code> in <code>collect::<Vec<_>>()</code> mean?
- An unused value
- Let the compiler infer the element type Correct answer
- An empty vector
- Any type is accepted
Explanation: You choose the container type (Vec); the compiler infers the element type from the iterator.
2 Give the target type
let n = "42".parse____().unwrap();let n = "42".parse::<i32>().unwrap();Explanation: ::<T> is written after the method name.
3 Expression and result
| Term | Meaning |
|---|---|
"7".parse::<u8>() |
Ok(7) |
"ab".chars().collect::<String>() |
"ab" |
[1, 2].iter().sum::<i32>() |
3 |
Vec::<i32>::new().len() |
0 |