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Destructuring

Destructuring lets you unpack arrays and dicts directly into named variables in a single let statement.

Array Destructuring

let [a, b, c] = [1, 2, 3]
println(a)   // 1
println(b)   // 2

Missing positions become null:

let [a, b, c] = [10, 20]
println(c)   // null

Provide a default that is used when the position is missing:

let [x, y = 99] = [1]
println(y)   // 99

...name collects all remaining elements into an array. It must be the last element in the pattern.

let [head, ...tail] = [1, 2, 3, 4, 5]
println(head)        // 1
println(len(tail))   // 4

If there are no remaining elements, the rest binding is an empty array:

let [a, ...rest] = [1]
println(len(rest))   // 0

Use _ to skip a position without creating a binding:

let [_, second, _] = [10, 20, 30]
println(second)   // 20

Binds a variable with the same name as the key:

let point = {"x": 3, "y": 7}
let {x, y} = point
println(x)   // 3
println(y)   // 7

Use key: alias to bind the value under a different name:

let config = {"host": "db.internal", "port": 5432}
let {host, port: p} = config
println(p)   // 5432

Provide a default used when the key is absent or null:

let {timeout: t = 30} = {"host": "api.example.com"}
println(t)   // 30

A key that is not present in the dict produces null (or the default if one is given):

let {x, z} = {"x": 1}
println(z)   // null

let {host, port: p = 5432, timeout: t = 30} = cfg

Destructuring works anywhere let does, including function bodies:

fn connect(cfg) {
    let {host, port: p = 5432} = cfg
    return host + ":" + str(p)
}

fn swap(pair) {
    let [a, b] = pair
    return [b, a]
}