Summary - Compile-time safety
- Kotlin course
- Expert
- Lesson 6/47
- 5 min
Combine contracts, inline and type-safe builders to write a DSL.
Summary - Compile-time safety
In this group you learned five compile-time safety tools: contracts, inline, reified, tailrec and @DslMarker.
contracts commits behaviour to the compiler; inline cuts lambda overhead; reified bypasses type erasure; tailrec turns recursion into a loop; @DslMarker blocks receiver mixing in DSLs. Now you will write a small DSL that combines them: a box that reads its value's type via reified and avoids overhead with inline.
Combining: Box DSL
Now we will write a Box DSL that combines inline + reified + @DslMarker. A Box holds a value; it reads the type tag via reified.
Example: @BoxDsl class Box<T>(var value: T). inline fun <reified T> tag(): String = T::class.simpleName ?: "?". Box gets fun describe(): String = "${tag<T>()}:$value". But tag<T>() inside Box must be in an inline function, because T is only visible with inline/reified. So describe must be inline too.
Practice Write the Box DSL
Define @DslMarker annotation class BoxDsl. Write inline fun <reified T> tag(): String = T::class.simpleName ?: "?". Inside @BoxDsl class Box<T>(var value: T) add inline fun describe(): String = "${'$'}{tag<T>()}:${'$'}value". In main, write val b = Box(42) and println(b.describe()).
Starter code
// Write the Box DSL
annotation class BoxDsl
// add the tag function
class Box<T>(var value: T) {
// add the describe method
}
fun main() {
val b = Box(42)
// print the describe output
}Sample solution
@DslMarker
annotation class BoxDsl
inline fun <reified T> tag(): String = T::class.simpleName ?: "?"
@BoxDsl
class Box<T>(var value: T) {
inline fun describe(): String = "${tag<T>()}:$value"
}
fun main() {
val b = Box(42)
println(b.describe())
}Expected output
Int:42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which keyword cuts lambda overhead?
- inline Correct answer
- reified
- tailrec
- @DslMarker
Explanation: inline copies the body to the call site for lambda-parameter functions.
2 Complete the describe method
inline fun describe(): String = "${____<T>()}:$value" inline fun describe(): String = "${tag<T>()}:$value"Explanation: tag<T>() returns the type name via reified.
3 Match the five tools
| Term | Meaning |
|---|---|
contracts |
Behaviour commitment |
inline |
Copies the body |
reified |
Bypasses erasure |
tailrec |
Turns into a loop |
@DslMarker |
Blocks receiver mixing |