Inline, noinline and crossinline example in Kotlin

inline_fun.kt

/**
 * Demonstrates inline, noinline, and crossinline in one function.
 *
 * - onSuccess: regular inline lambda - you can use 'return' to exit the calling function
 * - noinline onError: can be passed to other functions that expect function types
 * - crossinline onComplete: inline but called later (e.g., in a callback), no 'return' allowed
 */
inline fun String.validateEmail(
    onSuccess: () -> Unit,
    noinline onError: (String) -> Unit,
    crossinline onComplete: () -> Unit
) {
    println("→ Starting validation for: $this")

    // Regular inline lambda - called directly
    if (this.contains("@")) {
        println("  Valid format detected")
        onSuccess() // Can use 'return' inside this lambda
    } else {
        println("  Invalid format detected")
        // noinline lambda - can be passed to another function
        // This only works because onError is noinline!
        logErrorToAnalytics(
            email = this,
            message = "Missing @ symbol",
            errorCallback = onError
        )
    }

    // crossinline lambda - called at the end if no non-local return happened
    // Called from a different context (inside an object)
    // If onSuccess contains 'return', this won't be reached because the return exits validateEmail()
    println("  Executing completion callback...")
    val callback = object {
        fun execute() {
            onComplete() // Must be crossinline to use here (no 'return' allowed)
        }
    }
    callback.execute()
    println("→ Validation finished for: $this\n")
}

// This function accepts a function type parameter
// We can pass the noinline lambda here
fun logErrorToAnalytics(
    email: String,
    message: String,
    errorCallback: (String) -> Unit
) {
    println("✗ Invalid email: $email. Logging to analytics: $message")
    errorCallback(message) // Invoke the callback
}

fun validateUsersList(emails: List<String>): List<String> {
    val validEmails = mutableListOf<String>()

    println("Validating ${emails.size} emails...\n")

    // forEach is inline, so we can use non-local return
    emails.forEach { email ->
        println("━━━ Checking: $email ━━━")

        var isValid = false

        email.validateEmail(
            onSuccess = {
                println("  [onSuccess] Valid! Checking if already registered...")
                isValid = true
                // Regular inline lambda - CAN use 'return' to exit forEach iteration
                // Example: early exit if email is already registered
                if (email in validEmails) {
                    println("  [onSuccess] Already registered, skipping... \n")
                    return@forEach // Exit immediately - skips adding and onComplete!
                }
                println("  [onSuccess] New email, will add to list")
            },
            onError = { error ->
                // noinline lambda - CANNOT use 'return' here
                // This is passed to logErrorToAnalytics(), so it must be a real function object
                println("  [onError] $error")
            },
            onComplete = {
                // crossinline lambda - runs at end if no non-local return happened
                // It's called from inside an object callback, so it needs crossinline
                // CANNOT use 'return' here because it would be unclear what to exit
                println("  [onComplete] Validation process finished")
            }
        )

        if (isValid) {
            validEmails.add(email)
            println("  ✓ Added to valid list\n")
        } else {
            println("  ✗ Not added\n")
        }
    }

    println("━━━━━━━━━━━━━━━━━━━━━━━━━━━━━")
    println("Result: ${validEmails.size} valid emails")
    return validEmails
}

fun main() {
    val testEmails = listOf(
        "[email protected]",
        "[email protected]", // Duplicate - will trigger early return
        "[email protected]",
        "user455",
        "[email protected]"
    )

    val validEmails = validateUsersList(testEmails)
    println("\nFinal result: $validEmails")
}