Lambdas

// 1) The shapes
val add: (Int, Int) -> Int = { a, b -> a + b }
val square: (Int) -> Int = { it * it }            // 'it' = the single param

add(2, 3)                                          // 5
square(5)                                          // 25

// 2) Standard library — sort / filter / map / fold
val nums = listOf(1, 2, 3, 4, 5)

nums.map { it * 2 }                                // [2, 4, 6, 8, 10]
nums.filter { it % 2 == 0 }                        // [2, 4]
nums.fold(0) { acc, n -> acc + n }                 // 15
nums.maxBy { -it } ?: 0
nums.groupBy { if (it % 2 == 0) "even" else "odd" }

// 3) Trailing-lambda syntax + DSLs
fun retry(times: Int = 3, block: () -> String): String {
    repeat(times - 1) {
        try { return block() } catch (_: Exception) { /* retry */ }
    }
    return block()
}

val body = retry(times = 5) { httpGet("https://api.example.com") }

// 4) Receivers — lambdas with a receiver are how DSLs read so cleanly
fun buildString2(block: StringBuilder.() -> Unit): String {
    val sb = StringBuilder()
    sb.block()
    return sb.toString()
}

val msg = buildString2 {
    append("Hello, ")
    append("world")
    appendLine('.')
}

// 5) Scope functions — let / run / apply / also / with
val len = name?.let { it.length } ?: 0             // smart-cast, only if non-null
val user = User().apply {                            // mutate, return receiver
    name = "Ada"; age = 36
}
val msg2 = with(user) { "$name ($age)" }            // shorter access to receiver

// 6) Function references
val lengths = listOf("Ada", "Bo", "Cy").map(String::length)
val users = ids.mapNotNull(::findUser)

// 7) inline — zero-cost higher-order functions
inline fun measure(block: () -> Unit): Long {
    val start = System.nanoTime()
    block()
    return System.nanoTime() - start
}

// 8) Filter null vs nullable lambda
val names: List<String?> = listOf("Ada", null, "Bo")
val clean: List<String> = names.filterNotNull()

val nums = listOf(1, 2, 3, 4)
val doubled = nums.map { it * 2 }
val evens = nums.filter { it % 2 == 0 }
val total = nums.sum()