Contravariance in Scala: Why Is It So Hard? | Rock the JVM

This video is for the Scala programmer who's only been using generics in their basic form: either with collections, or simply attaching a type argument to a class/trait and minding their own business.

I will introduce the concepts of co- and contravariance and why they are useful, with a few examples and some rules of thumb to follow, without getting into the academic weeds that otherwise make these concepts seem so obscure and confusing to most programmers.

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[kirillgavrilov]

To remember this Java developers used "PECS" mnemonics (Producer - extends, Consumer - super). Kotlin uses in/out keywords (in - for being able to put/pass an object into container, out - to get from the container). Scala has the most concise syntax for that. Thanks for the lesson!

[nao_concordo]

You definitely have a talent for explanation

[awesomebearaudiobooks]

Taking my fedora off to honor the humanity-changing great feat of Laika the space dog 😔🎩
You will forever be in our hearts, comrade Laika ❤
Let's honor her memory by studying hard, so that the people of the world can finally tame the boundless nature of outer space... With the help of Scala, obviously.

[yohaitol]

I have read several explanations about the variances subject and never felt like I fully understood it.
This video is the best explanation I found, now I feel very comfortable with it.
Thanks a lot and please make more videos like this :-)

[_mumbler_]

The best explanation of contravariance I’ve seen.

[hinocenciopaulo]

Finally, someone could make me understand this thing... BTW I came here without any knowledge of Scala (my background is from Kotlin lol). The way that you explained it is so good that I could understand it even with examples in a language that I had never seeing before. Thank you so much.

[jimaaman]

Slowly coming to terms with this concept but the consumes vs. produces rule of thumb is the best summary I've seen so far of when to apply variance. 👍

This is the best explanation of this concept I have seen till now! Thanks, Daniel!

[vetiarvind]

wow, underrated channel, i remember drawing a lot of diagrams explaining contravariance in stackoverflow once so i thought i understood it but you have an incredibly effective way of distilling information. i'm going to watch all your videos just to see how you explain it ==)

[sergeykvartalnov]

Best Scala tutorials evere, I'm so happy I found your channe. Perfect 💯

[mikepictor]

Really really good explanation. It feels quite intuitive now

[UniformDelta]

What a wonderful video, this subject was so vague for me, thank you !

[OmarMendozaKS]

Awesome video man, quite simple, after some research I was almost at the starting point to understand this, your approach rocks, just one complaint, Hachi was not a lucky dog, he is a legend, 👏

[flaviodasilveirapepino]

I've benn waiting for this video!

[temoncher]

The best co- vs contravariance explanation in the world

[Alchimystic]

An interesting natural example where variance and contravariance exist, even if doesn't seem logical: human blood types.
In terms of blood cell compatibility, O is the universal donor (provides for every other), while AB is the universal receiver.
In terms of plasma is the other way around: AB is the universal donor, while O is the universal receiver

[viddyasagarverma]

As always very well explained.
Thank you.

[TJ-hsqm]

so awesomely explained I finally got it. Thank you!

[rheafernandeshallu]

Thanks a lot :). I learnt something completely new today

[mompyish]

I read effective java to thousand articles .Contra-variance remained a puzzle.I think today it sinked in.Thanks.