Comments (5)
davegurnell
commented on July 19, 2024
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] .
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ghisvail
commented on July 19, 2024
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.
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cchphotobox
commented on July 19, 2024
Thanks for your response. It seems I can not close this issue (No "Close" button). Sorry for the misunderstanding
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davegurnell
commented on July 19, 2024
@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!
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davegurnell
commented on July 19, 2024
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!
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