Monad
- Kotlin course
- Advanced
- Lesson 53/83
- 5 min
Learn monad patterns like Result and Either.
Monads: Result and Either
A monad wraps a value in a context and provides chainable operations. In Kotlin Result<T> wraps a success or failure. Either<L, R> (from the Arrow library) represents two possible types.
Example: Result.of { 10 / 2 }.map { it * 3 }. On failure the chain short-circuits. Result provides type-safe error handling instead of throwing. This pattern treats errors as values; it reduces side effects.
Result.success and failure
Result.success(v) creates a success, Result.failure(e) a failure. getOrNull() returns the value or null; getOrElse provides a default.
Example: val r: Result<Int> = Result.success(5); val v = r.getOrNull(). On failure, getOrNull returns null. fold handles both cases with separate lambdas. This pattern manages errors explicitly.
Practice Safe division with Result
Define a function safeDiv(a: Int, b: Int): Result<Int> that returns Result.failure(ArithmeticException()) if b is 0, otherwise Result.success(a / b). In main print the result of safeDiv(10, 2).getOrNull() with println.
Starter code
// safeDiv function: returns failure if b is 0, success otherwise
// in main get safeDiv(10, 2) via getOrNull and printSample solution
fun safeDiv(a: Int, b: Int): Result<Int> {
return if (b == 0) Result.failure(ArithmeticException())
else Result.success(a / b)
}
fun main() {
println(safeDiv(10, 2).getOrNull())
}Expected output
5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the Result.map call
val r = Result.success(10).____ { it * 3 }val r = Result.success(10).map { it * 3 }Explanation: Result.map transforms the success value.
2 Match the monad parts
| Term | Meaning |
|---|---|
Result<T> |
Context |
map { } |
Transform |
success(v) |
Success case |
getOrNull() |
Value or null |
3 What does Result.failure(e) represent?
- A successful value
- A null value
- An empty list
- A failure case Correct answer
Explanation: Result.failure wraps an error.