Lexical Closures

Functions can reference variables outside of their definition. These variables are captured by the function definition, and remain valid no matter where the closure is called. For example,

let f = {
  let v = cast<i64>(net::subscribe("/local/foo")$)$;
  |n| v + n
};
f(2)

f captures v and can use it even when it is called from a scope where v isn't visible. Closures allow functions to encapsulate data, just like an object in OOP.