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Consider this example from book Programming in Scala, 5th Edition by Martin Odersky (Chapter 18 · Type Parameterization)

Here - the following class Cat is described:

abstract class Cat[-T, +U]:
def meow[W−](volume: T−, listener: Cat[U+, T−]−)
: Cat[Cat[U+, T−]−, U+]+
  • the concepts of covariance and contravariance for the class level type parameters T and U perfectly make sense.

But the fact that the method level type variable W is being classified as -ve ie as Contravariant - is not very clear. Since we are neither using it in the method argument determination - nor are we using it in the return type.

Why is the example quoting it as contravariant and not invariant?

本文标签： genericsVariance Annotation for an independent Method level type parameters in scalaStack Overflow

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