51. Result & runCatching

Instead of propagating an exception, runCatching { } runs a block and captures the outcome in a Result<T> — either a success holding a value or a failure holding the thrown exception. You then inspect or transform it without a try/catch.

fun parse(text: String): Int = text.toInt()  // throws on bad input

fun main() {
    // runCatching wraps success or failure into a Result<T>.
    val ok: Result<Int> = runCatching { parse("21") }
    val bad: Result<Int> = runCatching { parse("nope") }

    // map transforms a success; getOrElse supplies a fallback.
    println(ok.map { it * 2 }.getOrElse { -1 })   // 42
    println(bad.map { it * 2 }.getOrElse { -1 })   // -1

    // getOrNull returns null on failure.
    println(ok.getOrNull())    // 21
    println(bad.getOrNull())   // null

    // onSuccess / onFailure run side-effects and return the Result.
    bad
        .onSuccess { println("got $it") }
        .onFailure { println("failed: ${it.message}") }
}

map applies only on success and leaves a failure untouched, while getOrElse, getOrNull, and getOrThrow extract the value. onSuccess and onFailure return the same Result, so they chain naturally for logging or recovery.

Running it:

$ kotlin run
42
-1
21
null
failed: For input string: "nope"

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