DSL scope control with @DslMarker in Kotlin - Mini Project: Build & Apply

DSL Scope Control with @DslMarker in Kotlin

📖 Scenario: Imagine you are building a simple Kotlin DSL (Domain Specific Language) to create a menu for a restaurant. You want to organize the menu into sections like Starters and Main Courses. To avoid confusion when adding items to different sections, you will use Kotlin's @DslMarker annotation to control the scope and prevent mistakes.

🎯 Goal: You will build a Kotlin DSL that lets you define a menu with sections and items. You will use @DslMarker to keep the scopes clear so that you cannot accidentally add a menu item to the wrong section.

📋 What You'll Learn

Create a @DslMarker annotation called MenuDsl

Define classes Menu, Section, and Item with DSL builder functions

Use @MenuDsl on the DSL classes to control scope

Build a menu with two sections: Starters and Main Courses

Add items to each section using the DSL

Print the menu showing sections and their items

💡 Why This Matters

🌍 Real World

DSLs help create readable and easy-to-write code for specific tasks like building menus, UI layouts, or configuration files.

💼 Career

Understanding Kotlin DSLs and scope control with <code>@DslMarker</code> is useful for Android developers and Kotlin backend developers who build custom DSLs or use libraries with DSLs.

1

Create the DSL marker and data classes

Create a @DslMarker annotation called MenuDsl. Then create three classes: Menu, Section, and Item. Each class should have a name property of type String. The Menu class should have a mutable list of Section called sections. The Section class should have a mutable list of Item called items. Annotate all three classes with @MenuDsl.

Kotlin

# Your code here

Need a hint?

Start by defining the @DslMarker annotation. Then create the classes with the required properties and annotate them with @MenuDsl.

2

Add builder functions for Menu and Section

Add a function section inside the Menu class that takes a name: String and a lambda with receiver Section. Inside this function, create a new Section with the given name, apply the lambda to it, and add it to the sections list. Also add a function item inside the Section class that takes a name: String and creates an Item with that name, adding it to the items list.

Kotlin

@DslMarker
annotation class MenuDsl

@MenuDsl
class Menu(val name: String) {
    val sections = mutableListOf()

    // Add section function here
    # Your code here
}

@MenuDsl
class Section(val name: String) {
    val items = mutableListOf()

    // Add item function here
    # Your code here
}

@MenuDsl
class Item(val name: String)

Need a hint?

Inside Menu, write a section function that creates and adds a Section. Inside Section, write an item function that adds an Item.

3

Create the menu using the DSL

Create a variable menu by calling Menu("Dinner Menu") and use the section function to add two sections: "Starters" and "Main Courses". Inside the "Starters" section, add items "Soup" and "Salad". Inside the "Main Courses" section, add items "Steak" and "Pasta".

Kotlin

@DslMarker
annotation class MenuDsl

@MenuDsl
class Menu(val name: String) {
    val sections = mutableListOf()

    fun section(name: String, block: Section.() -> Unit) {
        val section = Section(name)
        section.block()
        sections.add(section)
    }
}

@MenuDsl
class Section(val name: String) {
    val items = mutableListOf()

    fun item(name: String) {
        items.add(Item(name))
    }
}

@MenuDsl
class Item(val name: String)

// Create the menu variable below
# Your code here

Need a hint?

Create the menu variable using Menu("Dinner Menu") and add two sections with their items using the DSL functions.

4

Print the menu with sections and items

Write code to print the menu name, then for each section print its name, and for each item in the section print its name indented. Use a for loop with variables section and item to iterate over menu.sections and section.items. The output should look like this exactly: Dinner Menu Starters Soup Salad Main Courses Steak Pasta

Kotlin

@DslMarker
annotation class MenuDsl

@MenuDsl
class Menu(val name: String) {
    val sections = mutableListOf()

    fun section(name: String, block: Section.() -> Unit) {
        val section = Section(name)
        section.block()
        sections.add(section)
    }
}

@MenuDsl
class Section(val name: String) {
    val items = mutableListOf()

    fun item(name: String) {
        items.add(Item(name))
    }
}

@MenuDsl
class Item(val name: String)

val menu = Menu("Dinner Menu").apply {
    section("Starters") {
        item("Soup")
        item("Salad")
    }
    section("Main Courses") {
        item("Steak")
        item("Pasta")
    }
}

// Print the menu below
# Your code here

Need a hint?

Print the menu name first. Then use nested for loops to print each section and its items with indentation.