Variable Declarations

Let

The let binds an immutable value to an identifier for later use:

# Binds 42 to x
let x = 42
# Uses x in an expression
| let y = x + 1
# Binds `add` function to z
| let z = do let z = fn(x): x + 1; | z(1);

Once a variable is declared with let, its value cannot be changed.

The var declares a mutable variable that can be reassigned:

# Declares a mutable variable
var counter = 0
# Reassigns the value
counter = counter + 1
# counter is now 1

Variables declared with var can be modified using the assignment operator (=):

var total = 100
| total = total - 25
# total is now 75

var message = "Hello"
| message = message + " World"
# message is now "Hello World"

Both let and var support destructuring patterns on the left-hand side.

let [a, b] = [1, 2] | a
# => 1

let [head, ..tail] = [1, 2, 3] | tail
# => [2, 3]

let {name, age} = {"name": "Alice", "age": 30} | name
# => "Alice"

Using var allows reassigning destructured variables:

var [a, b] = [1, 2] | a = 99 | a
# => 99

Use let when you want to create an immutable binding (most cases)
Use var when you need to modify the value after declaration (counters, accumulators, etc.)