Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

// Network result as sealed class hierarchy
sealed class NetworkResult<out T> {
    data class Success<T>(val data: T)        : NetworkResult<T>()
    data class Error(val code: Int, val msg: String) : NetworkResult<Nothing>()
    object Loading                             : NetworkResult<Nothing>()
}

data class Post(val id: Int, val title: String)

fun fetchPost(id: Int): NetworkResult<Post> = when (id) {
    0    -> NetworkResult.Loading
    else -> if (id > 0)
                NetworkResult.Success(Post(id, "Post #$id"))
            else
                NetworkResult.Error(404, "Not found")
}

// Exhaustive when expression — compiler ensures all cases covered
fun render(result: NetworkResult<Post>): String = when (result) {
    is NetworkResult.Loading       -> "Loading..."
    is NetworkResult.Success       -> "Post: ${result.data.title}"
    is NetworkResult.Error         -> "Error ${result.code}: ${result.msg}"
}

// Sealed interface
sealed interface Expr
data class Num(val value: Double)         : Expr
data class Add(val l: Expr, val r: Expr)  : Expr
data class Mul(val l: Expr, val r: Expr)  : Expr

fun eval(e: Expr): Double = when (e) {
    is Num -> e.value
    is Add -> eval(e.l) + eval(e.r)
    is Mul -> eval(e.l) * eval(e.r)
}

fun main() {
    listOf(-1, 0, 5).map(::fetchPost).map(::render).forEach(::println)

val expr = Mul(Add(Num(2.0), Num(3.0)), Num(4.0))
    println("Result: ${eval(expr)}")   // 20.0
}