Variables & Types¶

Learn how to declare variables and use Desi's type system.

Declaring Variables¶

Immutable Variables: `let`¶

Use let for variables that won't change:

let name = "Desi"
let age = 25
let pi = 3.14159

Trying to reassign a let variable is an error:

let x = 10
x = 20  # ❌ Error: cannot assign to immutable variable

Mutable Variables: `let mut`¶

Use let mut when you need to modify a variable, and := to reassign it:

let mut counter = 0
counter := counter + 1  # ✅ OK
counter := counter + 1  # ✅ OK
print(counter)  # Prints: 2

Prefer plain let

Use let by default and add mut only when mutation is needed. There is no var keyword in Desi.

Type Annotations¶

Desi has strong static typing with inference. You can be explicit:

let name: str = "Desi"
let count: int = 42
let price: float = 19.99
let active: bool = true

Or let Desi infer:

let name = "Desi"     # str
let count = 42        # int
let price = 19.99     # float
let active = true     # bool

Basic Types¶

Numbers¶

Type	Description	Example
`int`	Integer	`42`, `-17`, `0`
`float`	Floating point	`3.14`, `-0.5`
`i32`	32-bit signed	`i32(100)`
`i64`	64-bit signed	`i64(1000000)`

let whole = 42          # int
let decimal = 3.14      # float
let big = i64(9999999)  # i64

Strings¶

let greeting = "Hello, World!"
let name = "Desi"
let combined = greeting + " " + name  # String concatenation

Booleans¶

let is_valid = true
let is_empty = false

# Boolean operators
let result = is_valid and not is_empty

Type Conversion¶

Convert between types explicitly:

let x: int = 42
let y: float = float(x)    # int -> float
let z: str = str(x)        # int -> str

let a: float = 3.7
let b: int = int(a)        # float -> int (truncates to 3)

Constants¶

By convention, use UPPER_CASE for constants:

let MAX_SIZE = 1000
let PI = 3.14159265359
let APP_NAME = "MyApp"

Multiple Declarations¶

Declare multiple variables:

let a = 1
let b = 2
let c = 3

# Or compute from previous
let sum = a + b + c

Scope¶

Variables are scoped to their block:

def example():
    let x = 10

    if true:
        let y = 20
        print(x)  # ✅ OK - x is in scope
        print(y)  # ✅ OK - y is in scope

    print(x)  # ✅ OK
    # print(y)  # ❌ Error - y is out of scope

Practical Example¶

def calculate_area():
    let width: float = 10.5
    let height: float = 20.0
    let area = width * height

    print("Width:")
    print(width)
    print("Height:")
    print(height)
    print("Area:")
    print(area)

def main():
    calculate_area()

Output:

Width:
10.5
Height:
20.0
Area:
210.0

Summary¶

Keyword	Mutability	When to Use
`let`	Immutable	Default choice
`var`	Mutable	When value must change

Next¶

Learn about Functions →