Mutability and Assignment

Desi has a clear distinction between immutable and mutable variables, and between initial binding and mutation.

Variable Declaration

# Immutable variable (cannot be changed)
let name = "Alice"
let count = 10

# Mutable variable (can be changed)
let mut counter = 0
let mut data = [1, 2, 3]

Assignment Operators

Operator	Usage	Description
=	Initial binding	Used with let to create new variables
:=	Mutation	Used to change existing values

Initial Binding with =

let x = 10              # Bind x to 10
let message = "Hello"   # Bind message to string
let nums = [1, 2, 3]    # Bind nums to list

Mutation with :=

let mut x = 10
x := 20                 # Mutate x from 10 to 20

let mut data = [1, 2, 3]
data[0] := 99           # Mutate list element

# Also works in class methods
self.value := new_value

Important

Always use := for mutation, never =. Using = for mutation will cause a compiler error.

Immutable vs Mutable

Immutable Variables

let data = [1, 2, 3]
data[0] := 99  # ERROR: cannot assign to element of immutable list

Mutable Variables

let mut data = [1, 2, 3]
data[0] := 99  # OK
print(data)    # [99, 2, 3]

Class Fields

Fields can be mutable or immutable:

class Person:
    pub name: str           # Immutable field
    pub mut age: int        # Mutable field

    pub def __new__(self, name: str, age: int):
        self.name = name
        self.age = age

    pub def have_birthday(self):
        self.age := self.age + 1  # OK - age is mutable
        # self.name := "Bob"      # ERROR - name is immutable

In Loops

let mut sum = 0
for x in [1, 2, 3, 4, 5]:
    sum := sum + x
print(sum)  # 15

Or with augmented assignment:

let mut count = 0
while count < 10:
    count += 1  # Augmented assignment works too