7. Functional programming, closures

Rust features the main mechanisms of functional programming, including closures and the iterators seen above.

Closures are functions that, when defined (and unlike ordinary functions), capture the values of variables in the enclosing environment. They are linked to a variable, which means they can be treated as a value, and in particular passed as an argument to a function; in this way, the closure code can be used in the new environment of the function that received it as an argument, with access to the values of the function's local variables.

A closure is defined as an ordinary variable, with the = sign followed by two vertical bars that play a role analogous to parentheses in the definition of a function: between these vertical bars are the names of any formal parameters of the closure, such as :

let double =...