Variables

Tao is intentionally strict about names: variables are immutable unless declared mutable, and every name may be defined exactly once in its scope.

let

let declares an immutable binding.

let x = 42
x = 10    // error: cannot assign to immutable variable

let mut

let mut declares a mutable binding that can be reassigned.

let mut x = 5
x = 10    // ok

const

const declares a top-level constant. Like let, it is immutable.

const LIMIT = 100

Destructuring

let a, b = expr unpacks the first two elements of a list into two immutable variables. Extra elements are ignored.

let val, err = readFile("x")   // val = first, err = second

Assignment

The right of = may be an identifier, a field access, or an index. Assigning to anything else is an error.

x = 2          // variable
p.y = 1        // field
xs[0] = 42     // element

Names are defined once

Redefining a name in the same scope is an error, whether it is a variable, a function, or a type.

let x = 1
let x = 2    // error: already defined variable: x

Scopes

Each block opens a new scope. Inner blocks can read outer variables, and closures capture the scope where they were created.

fun f() {
  let mut n = 0
  let c = fun -> () { n = n + 1; n }
  c()
  c()      // 2
}
f()