Variables

Variables in Do are dynamically typed and lexically scoped. They must be declared before use.

Declaration with let

Introduce new variables with let:

let x = 42
let greeting = "Hello, world!"

The right-hand side of let is a compact expression context, so variables and calls work without $:

let doubled = func(x)
let items = [1, 2, 3]

It can also be a complete command statement:

let response = http.get https://google.com

Assignment

Reassign an existing variable with =:

let count = 0
count = 1
count = (count + 1)

Assignment does not declare a new variable; the variable must already be bound by let, a function parameter, for, bind, or another binding form.

Scoping

Variables are lexically scoped to the block where they are declared:

let x = 1
if true
  let y = 2
  echo $x $y  # both visible
# y is no longer in scope here
echo $x

Functions close over their defining scope:

let x = 42
let f = do
  echo $x  # captures x
f()  # prints: 42

Closures can mutate captured variables:

let counter = 0
let inc = do
  counter = (counter + 1)
inc()
inc()
echo $counter  # 2