Scala's classic 81st: The construction of list in Scala is type constrained inversion, covariance, and the lower bound

:: Appends the object to the head of the list.

The head part of an existing list[subclass X] is placed in another object A, which is a subclass Y, and if both a object and the original list object inherit from the same parent class, the list is displayed as the list[parent class].

The parameters of the function are contravariant so that covariance cannot occur, so the nether is used to solve the problem. The object to which the:: Method returns is programmed as a list of upper bound object B.

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Scala's classic 81st: The construction of list in Scala is type constrained inversion, covariance, and the lower bound