Implementation of Result.flatMap(), missing from Kotlin stdlib

ResultFlatMap.kt

inline fun <R, T> Result<T>.flatMap(transform: (T) -> Result<R>): Result<R> =
    fold({ transform(it) }, { Result.failure(it) })

fun main() {
    getUserInput()
        .flatMap { input -> parseInput(input) }
        .flatMap { numbers -> calculateMax(numbers) }
        .onSuccess { maxNumber -> println("max: $maxNumber") }
        .onFailure { throwable -> throwable.printStackTrace() }
}

// fails on empty input
fun getUserInput(): Result<String> = runCatching { readln().ifEmpty { error("empty") } }

// fails on non-numbers
fun parseInput(input: String): Result<List<Int>> =
    runCatching { input.split(" ").mapNotNull { if (it.isBlank()) null else it.toInt() } }

// fails on " " input
fun calculateMax(numbers: List<Int>) = runCatching { numbers.maxOf { it } }

I find Kotlin Result type very convenient for functional-style error handling and other uses cases described in the corresponding KEEP.

One thing that is missing from the Kotlin standard library is Result.flatMap(). There are specific reasons behind this decision, but nevertheless sometimes it would be useful for combining APIs designed around Result.

Fortunately it can be trivially implemented on top of existing functions. See above for implementation & usage example.