Structs

Structs are user-defined composite types that group related data together. In EZ, structs are always declared with const since the type definition itself is immutable.

Defining Structs

Use const with the struct keyword to define a new type:

const Person struct {
    name string
    age int
    email string
}

const Point struct {
    x int
    y int
}

Multi-Member Declarations

Fields of the same type can be declared on a single line:

const User struct {
    name, email string      // both are strings
    age int
    active, verified bool   // both are booleans
}

const Point3D struct {
    x, y, z float           // all three are floats
}

Creating Instances

Literal Initialization

Create struct instances using literal syntax with named fields:

temp person Person = Person{
    name: "Alice",
    age: 30,
    email: "alice@example.com"
}

// Single line for simple structs
temp point Point = Point{x: 10, y: 20}

Using new()

Create a struct with default values using new():

temp person Person = new(Person)
person.name = "Bob"
person.age = 25
person.email = "bob@example.com"

Default values:

• int, float: 0
• string: ""
• bool: false
• char: '\0'

Accessing Fields

Use dot notation to read and write fields:

temp user User = User{name: "Alice", age: 30}

// Reading fields
std.println(user.name)   // "Alice"
std.println(user.age)    // 30

// Writing fields (only for temp variables)
user.age = 31
user.email = "alice@newmail.com"

Nested Structs

Structs can contain other structs:

const Address struct {
    street string
    city string
    zipcode int
}

const Employee struct {
    name string
    id int
    address Address
}

do main() {
    temp emp Employee = Employee{
        name: "John Doe",
        id: 12345,
        address: Address{
            street: "123 Main St",
            city: "Austin",
            zipcode: 78701
        }
    }

    // Access nested fields
    std.println(emp.name)              // "John Doe"
    std.println(emp.address.city)      // "Austin"
    std.println(emp.address.zipcode)   // 78701

    // Modify nested fields
    emp.address.street = "456 Oak Ave"
    emp.address.city = "Dallas"
}

Structs in Arrays

const Task struct {
    title string
    done bool
}

do main() {
    temp tasks [Task] = {
        Task{title: "Write docs", done: false},
        Task{title: "Fix bugs", done: true},
        Task{title: "Add tests", done: false}
    }

    for_each task in tasks {
        if task.done {
            std.println("[x]", task.title)
        } otherwise {
            std.println("[ ]", task.title)
        }
    }
}

Structs as Function Parameters

const Rectangle struct {
    width int
    height int
}

do area(rect Rectangle) -> int {
    return rect.width * rect.height
}

do scale(rect Rectangle, factor int) -> Rectangle {
    return Rectangle{
        width: rect.width * factor,
        height: rect.height * factor
    }
}

do main() {
    temp r Rectangle = Rectangle{width: 10, height: 5}
    std.println("Area:", area(r))  // 50

    temp scaled Rectangle = scale(r, 2)
    std.println("Scaled:", scaled.width, "x", scaled.height)  // 20 x 10
}

Returning Structs

Functions can return struct literals directly:

const Point struct {
    x int
    y int
}

do createPoint(x, y int) -> Point {
    return Point{x: x, y: y}
}

do origin() -> Point {
    return Point{x: 0, y: 0}
}

do main() {
    temp p1 Point = createPoint(10, 20)
    temp p2 Point = origin()

    std.println("p1:", p1.x, p1.y)  // 10 20
    std.println("p2:", p2.x, p2.y)  // 0 0
}

Const vs Temp Structs

The struct type definition is always const, but instances can be either:

const Config struct {
    debug bool
    timeout int
}

// Mutable instance - fields can be changed
temp config Config = Config{debug: true, timeout: 30}
config.timeout = 60  // OK

// Immutable instance - fields cannot be changed
const defaults Config = Config{debug: false, timeout: 30}
// defaults.timeout = 60  // Error! Cannot modify const

Important: When a struct instance is declared with const, all of its fields are protected from modification. This includes nested struct fields. Attempting to modify any field on a const struct will produce a compile-time error.

Struct Field Tags for JSON

Struct fields can have tags that control how they are serialized to and deserialized from JSON. Tags use backtick syntax similar to Go.

Basic Syntax

const User struct {
    name string `json:"name"`
    email string `json:"email_address"`
    age int `json:"age"`
}

When this struct is serialized to JSON, the field names in the output will use the tag values instead of the original field names.

Available Tag Options

Examples

Custom Field Names

const Person struct {
    firstName string `json:"first_name"`
    lastName string `json:"last_name"`
}

temp p Person = Person{firstName: "Alice", lastName: "Smith"}
// JSON output: {"first_name": "Alice", "last_name": "Smith"}

Excluding Fields

const Config struct {
    host string `json:"host"`
    password string `json:"-"`  // Never included in JSON
}

temp c Config = Config{host: "localhost", password: "secret123"}
// JSON output: {"host": "localhost"}

Omit Empty Values

const Item struct {
    name string `json:"name"`
    description string `json:"description,omitempty"`
}

temp item Item = Item{name: "Widget", description: ""}
// JSON output: {"name": "Widget"}
// (description omitted because it's empty)

Encoding Numbers as Strings

const Product struct {
    id int `json:"id,string"`
    price float `json:"price,string"`
}

temp p Product = Product{id: 12345, price: 29.99}
// JSON output: {"id": "12345", "price": "29.99"}

Example Program

import @std
import @arrays

const Product struct {
    name string
    price float
    quantity int
}

const Cart struct {
    items [Product]
    discount float
}

do addToCart(&cart Cart, product Product) {
    arrays.append(cart.items, product)
}

do calculateTotal(cart Cart) -> float {
    temp total float = 0.0
    for_each item in cart.items {
        total += item.price * float(item.quantity)
    }
    return total * (1.0 - cart.discount)
}

do main() {
    temp cart Cart = Cart{
        items: {},
        discount: 0.1  // 10% discount
    }

    arrays.append(cart.items, Product{name: "Book", price: 29.99, quantity: 2})
    arrays.append(cart.items, Product{name: "Pen", price: 4.99, quantity: 5})

    std.println("Shopping Cart:")
    for_each item in cart.items {
        std.println(" -", item.name, "x", item.quantity, "@ $${item.price}")
    }

    temp total float = calculateTotal(cart)
    std.println("Total (with 10% discount): $${total}")
}