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# Closures
Closures are functions that can capture the enclosing environment. For
example, a closure that captures the `x` variable:
```Rust
|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.
```rust,editable
fn main() {
// Increment via closures and functions.
fn function(i: i32) -> i32 { i + 1 }
// 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 + 1 };
let closure_inferred = |i | i + 1 ;
let i = 1;
// Call the function and closures.
println!("function: {}", function(i));
println!("closure_annotated: {}", closure_annotated(i));
println!("closure_inferred: {}", closure_inferred(i));
// 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());
}
```
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