Contracts

Learn compile-time guarantees with Kotlin contracts.

Contracts

Contracts are commitments that tell the compiler which conditions hold after a function is called. They are written with Kotlin's kotlin.contracts API; it is an experimental API.

The most common use is to declare that a value is non-null once the function returns. Example: contract { returns() implies (x != null) }. Then the call site needs no extra !! or if (x != null) block. Contracts only inform the compiler; they emit no runtime code.

returns() implies

A contract block opens with contract { ... }. The most common clause is returns() implies (condition).

Example: contract { returns() implies (x != null) }. This says: when this function returns normally, x is not null. After a requireNonNull(name) call the compiler treats name as String. A wrong contract causes runtime errors, so write them with care.

Exercise Write the requireNonNull function

Define requireNonNull(x: String?): String. Inside it, place a contract { returns() implies (x != null) } clause. In main, after calling requireNonNull(name), write println(name.length).

Show example solution

fun requireNonNull(x: String?): String {
    contract { returns() implies (x != null) }
    return x!!
}
fun main() {
    val name: String? = "Ayse"
    requireNonNull(name)
    println(name.length)
}

Test yourself

What does a contract do?
- Throws an error at runtime
- Awaits an async call
- Generates a new class
- Gives the compiler a behaviour commitment
Show answer

Gives the compiler a behaviour commitment — A contract informs the compiler; it emits no runtime code.

Complete the contract block

fun requireNonNull(x: String?): String {
    ____ { returns() implies (x != null) }
    return x!!
}

Show answer

fun requireNonNull(x: String?): String {
    contract { returns() implies (x != null) }
    return x!!
}

Match contract parts

contract

Contract block
returns()

Return clause
implies

States result
x != null

Non-null guarantee

Next lesson: Inline functions

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