5.7 Lambdas and closures

let inc = x => x + 1
let add = (a: i32, b: i32) => a + b
users.map(.name)                     // = u => u.name
users.filter(.isActive())            // = u => u.isActive()
retryWith(3) { fetch() }             // a closure as the last argument
cluster.lock("k", ttl: LOCK_TTL) { migrate() }

• .field and .method(args) in argument position are shorthand for a lambda with one parameter.
• Trailing closure: if the last parameter of a function has a function type, the block after the call is passed as that argument: f(a) { … } or with parameters f(a) { x => … }.
• Capture: a non-escaping closure (not stored and not passed to another task) captures by reference; an escaping one — by move. move x => … is an explicit move. The compiler determines "escaping" from the parameter type: fn(…) escaping or passing to spawn.

Source: docs/spec/oxl-standard.en.md