Closures
Closures are functions that can capture the enclosing environment. For example, a closure that captures the x
variable:
|val| val + x
The syntax and capabilities of closures make them very convenient for on the fly usage. Calling a closure is exactly like calling a function. However, both input and return types can be inferred and input variable names must be specified.
Other characteristics of closures include:
- using
||
instead of ()
around input variables.
- optional body delimination (
{}
) for a single expression (mandatory otherwise).
- the ability to capture the outer environment variables.
fn main() {
let outer_var = 42;
// A regular function can't refer to variables in the enclosing environment
//fn function(i: i32) -> i32 { i + outer_var }
// TODO: uncomment the line above and see the compiler error. The compiler
// suggests that we define a closure instead.
// Closures are anonymous, here we are binding them to references
// Annotation is identical to function annotation but is optional
// as are the `{}` wrapping the body. These nameless functions
// are assigned to appropriately named variables.
let closure_annotated = |i: i32| -> i32 { i + outer_var };
let closure_inferred = |i | i + outer_var ;
// Call the closures.
println!("closure_annotated: {}", closure_annotated(1));
println!("closure_inferred: {}", closure_inferred(1));
// Once closure's type has been inferred, it cannot be inferred again with another type.
//println!("cannot reuse closure_inferred with another type: {}", closure_inferred(42i64));
// TODO: uncomment the line above and see the compiler error.
// A closure taking no arguments which returns an `i32`.
// The return type is inferred.
let one = || 1;
println!("closure returning one: {}", one());
}