Variance annotations

[ccheneson]

Under the section "Controlling Instance Selections", it recaps 3 cases for addition of variance annotations:

A type with an unannotated parameter Foo[A] is invariant in A .
This means there is no rela onship between Foo[B] and Foo[C]
no ma er what the sub- or super-type rela onship is between B
and C .
• A type with a parameter Foo[+A] is covariant in A .
If C is a subtype of B , Foo[C] is a subtype of Foo[B] .
• A type with a parameter Foo[-A] is contravariant in A .
If C is a supertype of B , Foo[C] is a subtype of Foo[B] .

Should the last statement be

If C is a supertype of B , Foo[C] is a supertype of Foo[B] .

[davegurnell]

Contravariance works the other way around. Take a look in Essential Scala in the section on variance. That should explain it.

…

On Sat, 5 Aug 2017 at 21:21, Cyril Cheneson ***@***.***> wrote: Under the section "Controlling Instance Selections", it recaps 3 cases for addition of variance annotations: A type with an unannotated parameter Foo[A] is invariant in A . This means there is no rela onship between Foo[B] and Foo[C] no ma er what the sub- or super-type rela onship is between B and C . • A type with a parameter Foo[+A] is covariant in A . If C is a subtype of B , Foo[C] is a subtype of Foo[B] . • A type with a parameter Foo[-A] is contravariant in A . If C is a supertype of B , Foo[C] is a subtype of Foo[B] . Should the last statement be If C is a supertype of B , Foo[C] is a supertype of Foo[B] . — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#72>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAOI58saAT5KkB_R1iultd23Yk_HdFCYks5sVM7SgaJpZM4OujhI> .

[ghisvail]

If C is a supertype of B , Foo[C] is a subtype of Foo[B]

This the correct definition of contravariance. See essential-scala section 5.6.1 which briefly mentions that contravariance is used for function arguments. The issue should be closed.

[cchphotobox]

Thanks for your response. It seems I can not close this issue (No "Close" button). Sorry for the misunderstanding

[davegurnell]

@ccheneson @cchphotobox No worries -- thanks for submitting the issue. You're not the first to mention this. Variance is one of the more esoteric parts of Scala's type system -- it confuses loads of people.

In fact, I think I'll add a note to clarify the situation. If it's being picked up by multiple people, it's worth addressing directly. I'll leave this ticket open as a reminder.

Please do post again if you spot anything else -- every bit of feedback is useful!

[davegurnell]

I've made two changes in c7cf137 in an attempt to improve the clarity here:

• I've changed the wording in the definitions so they both say "subtype" in all places, instead of mixtures of "subtype" and "supertype".

• I've added some quick examples.

If you have time, please let me know what you think!