Type Inference

Rust can infer the type of each closure parameters and its return type, provided its usage is unambiguous.

fn  add_one_v1   (x: u32) -> u32 { x + 1 }
let add_one_v2 = |x: u32| -> u32 { x + 1 };
let add_one_v3 = |x|             { x + 1 };
let add_one_v4 = |x|               x + 1  ;

If the usage is ambiguous, you may explicitly specify the parameter types and return type that the closure must adhere to.

Shared Syntax Does Not Compile

Since each closure is typed, the following will fail to compile, as Rust needs to choose whether x's type is u32 or i32:

#![allow(unused_variables)]
fn main() {
let add_one = |x| x + 1;
add_one(1u32);
add_one(1i32);

println!("🦀 Ok!")
}

Snippet from The Rust Book - Chapter 13.1 Closures