Expand the Function1 example in the variances page

[justinhj]

https://github.com/scala/docs.scala-lang/blob/67f117529298432e799cd88982240bb13a38301a/_tour/variances.md#other-examples

At the end of the page on variances the reader has to do a lot of work to figure out the "what does the animal eat example". I think it would be great if it included working code like the previous examples. However, it's a bit confusing in terms of what the "what does an animal eat" function should return. Should it return a type or an instance of animal?

One way to make it more concrete and clear would be to write a function that takes an Animal and a List[Animal] and filters out the ones that cannot be eaten. This way you could show example code that takes a Cat and a list of different animal instances, some of them being mice, and returns a List[Animal] that is clearly containing SmallAnimal.

This way we have shown contravariance and covariance in Function1

[martijnhoekstra]

I have a branch somewhere reworking that entirely, but I too was stuck here. A function from an animal to another animal that was eaten by the first is kind of weird, especially as a subtype of, eh, what exactly? A clear concrete example where the contravariance of the parameter is used at the same time as the covariance of the returned result would be very welcome, but if that proves just too hairy, showing its co and contravariance separately may be easier.

[SethTisue]

agree there's room for improvement here. pull requests with attempts are welcome. @justinhj it's hard for me to predict from your description how a concrete attempt would turn out, whether it would work and be a good improvement

showing its co and contravariance separately may be easier

yeah, agree, using them both at once would be more like a party trick than something needed for teaching this