In Kotlin, how do you declare a data class with zero constructor parameters? -

say want declare simple algebraic datatype integer lists:

sealed class intlist data class cons(val head: int, val tail: intlist): intlist() data class nil() : intlist() 

however, last declaration results in error

data class must have @ least 1 primary constructor parameter

1. why limitation present? looking @ documentation, there seems no technical reasons requiring data class constructors non-nullary.

2. is possible express nullary constructors without having write lots of boilerplate code? if change last declaration like

   sealed class nil() : intlist() 

   then lose free implementations of hashcode() , equals() come free data class declarations.

edit

alex filatov gave nice short solution below. obviously, never need more 1 instance of nil, can define singleton object

object nil : intlist() 

however, if our lists parameterized type parameter? is, first 2 lines of our definition be

sealed class list<a> data class cons<a>(val head: a, val tail: list<a>): list<a>() 

we cannot declare polymorphic singleton nil object derives list<a> a, since have provide concrete type a @ time of declaration. solution (taken this post) declare a covariant type parameter , declare nil subtype of list<nothing> follows:

sealed class list<out a> data class cons<a>(val head: a, val tail: list<a>): list<a>() object nil : list<nothing>() 

this allows write

val xs: list<int> = cons(1, cons(2, nil)) val ys: list<char> = cons('a', cons('b', nil)) 

because data class without data doesn't make sense. use object singletons:

object nil : intlist()