早期的 Kotlin 版本函數參數是可以定義成 var 的，不過后來官方明確了「函數參數都是不可變」這一點。在這篇更新日志里可以找到說明：

The main reason is that this was confusing: people tend to think that
this means passing a parameter by reference, which we do not support
(it is costly at runtime). Another source of confusion is primary
constructors: “val” or “var” in a constructor declaration means
something different from the same thing if a function declarations
(namely, it creates a property). Also, we all know that mutating
parameters is no good style, so writing “val” or “var” in front of a
parameter in a function, catch block of for-loop is no longer allowed.