Monad

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.

Exercise 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.

Show example 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())
}

Test yourself

Complete the Result.map call

val r = Result.success(10).____ { it * 3 }

Show answer

val r = Result.success(10).map { it * 3 }

Match the monad parts

Result<T>

Context
map { }

Transform
success(v)

Success case
getOrNull()

Value or null
What does Result.failure(e) represent?
- A successful value
- A null value
- An empty list
- A failure case
Show answer

A failure case — Result.failure wraps an error.

Next lesson: Summary - Functional programming

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