Variance rewrite #2488
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Variance rewrite #2488
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Mostly a re-write. * Begin with invariance, since it's the default without any annotations * Move to covariance, which is the thing that's most intuitive for most people * Introduce contravariance as the opposite of covariance * Remove more contrived examples. Less is more
When you're trying to understand something, jokes are never funny
SethTisue
requested changes
Aug 11, 2022
_tour/variances.md
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| A type parameter `T` of a generic class can be made covariant by using the annotation `+T`. For some `class List[+T]`, making `T` covariant implies that for two types `A` and `B` where `B` is a subtype of `A`, then `List[B]` is a subtype of `List[A]`. This allows us to make very useful and intuitive subtyping relationships using generics. | ||
| By default, type paramters in scala are invariant: subtyping relationships between the type paramter aren't reflected in the parameterized type. To explore why this works the way it does, we look at a simple parameterized type, the mutable box. |
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s/paramters/parameters/ (in more than one place)
SethTisue
approved these changes
Aug 24, 2022
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thank you Martijn! |
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All but a full rewrite. I have always found this section of the tour confusing at least, particularly the part where it does the function.
In my experience, most people start with the false intuition that things should automatically be invariant. That's why I start with invariance, show why things can't be covariant by default. Then I show covariance, and then I show contravariance as the opposite.
I completely omit the profunctor thing with co and contravariance at the same time: I don't think it helps understanding of variance.
It's written a bit more tutorial style than most of the tour, because this tend to be a point where many questions arise, and readers can use a gentle push on understanong.