Why is it necessary to wrap the

data class

with parentheses?

StringAction("...") {}

Means that the last parameter of the constructor is a lambda, that's not the case here, as you have 2 method calls : constructor followed by invoke on the Action instance.

The parentheses tell the compiler that the constructor invocation is complete before the {}

For these cases, you can define factory functions on the companion object which look like a constructor, but are more flexible. For instance, define a factory that applies accepts that trailing lambda and applies it, so you can use the intended syntax of

newString

data class StringAction(val value: String) : Action {
    companion object {
        operator fun invoke(value: String, f: ()->String): String {
            return StringAction(value).invoke(f)
        }
    }
}

Looks a bit weird to me to have that on a data class, do you have a good use case for that? (Is it not better to use SAM interface then?)