Can anyone explain me, the concept of covariance and contravariance in programming languages theory?
+4
A:
Covariance is pretty simple and best thought of from the perspective of some collection class List. We can parameterize the List class with some type parameter T. That is, our list contains elements of type T for some T. List would be covariant if
S is a subtype of T iff List[S] is a subtype of List[T]
(Where I'm using the mathemtical definition iff to mean if and only if.)
That is, a List[Apple] is a List[Fruit]. If there is some routine which accepts a List[Fruit] as a parameter, and I have a List[Apple], then I can pass this in as a valid parameter.
def something(l: List[Fruit]) {
l.add(new Pear())
}
If our collection class List is mutable, then covariance makes no sense because we might assume that our routine could add some other fruit (which was not an apple) as above. Hence we should only like immutable collection classes to be covariant!
oxbow_lakes
2009-07-22 06:56:36
Good definition, but it misses the fact that not only types can be treated as co/contravariant. For example, Java `List<T>` isn't either, but Java wildcards let you treat in either covariant or contravariant fashion at point of use (rather than declaration) - of course, restricting the set of operations on the type to those that are actually covariant and contravariant for it.
Pavel Minaev
2009-07-22 07:48:54
I believe that `List<? extends Fruit>` is a kind of *existential type*: i.e. `List[T] forSome T <: Fruit` - the *forSome T <: Fruit* is itself a type in this instance. Java is still not covariant in this type though. For example a method accepting a `List<? extends Fruit>` would not accept `List<? extends Fruit>`
oxbow_lakes
2009-07-22 14:29:11
I mean "would not accept a `List<? extends Apple>` of course
oxbow_lakes
2009-07-22 14:29:42
Well FWIW, the explanation is from a discussion that i and my colleagues had with an eminent member (former HP) when the discussion veered towards OOSC by Bertrand Meyers in which i think Meyers highlights the importance of contravariance in OOP. He said to us OOSC by B.Meyers is a must read book if one is to practically understand OOP.
Abhay
2009-07-22 10:27:14
This maybe copyrighted material. This appears to be from a HP tech report and i think they have not given permission to republish. http://www.hpl.hp.com/techreports/90/HPL-90-121.pdf
chikak
2009-07-22 10:33:45
Removed content (replaced with link). The original document is clearly marked with copyright (not that it needs to be...). Feel free to add your own original content in **addition** to the link. But don't paste verbatim.
Marc Gravell
2009-07-22 21:08:52
+6
A:
Here are my articles on how we have added new variance features to C# 4.0. Start from the bottom.
http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx
Eric Lippert
2009-07-22 16:28:36
OT: Eric, you had posted a puzzle on one of the questions earlier. I couldn't solve it and asked question about it. Could you please look at it? http://stackoverflow.com/questions/1167666/c-puzzle-reachable-goto-pointing-to-an-unreachable-label
SolutionYogi
2009-07-22 20:00:58
Watch it with those exclamation marks there. You can put someone's eye out with one of those things.
Eric Lippert
2009-07-23 05:10:43
A:
Bart De Smet has a great blog entry about covariance & contravariance here.
Mike Thompson
2009-09-11 03:19:03
+1
A:
For added convenience, here's an ordered list of links to all of Eric Lippert's articles on variance:
- Covariance and Contravariance in C#, Part One
- Covariance and Contravariance in C#, Part Two: Array Covariance
- Covariance and Contravariance in C#, Part Three: Method Group Conversion Variance
- Covariance and Contravariance in C#, Part Four: Real Delegate Variance
- Covariance and Contravariance In C#, Part Five: Higher Order Functions Hurt My Brain
- Covariance and Contravariance in C#, Part Six: Interface Variance
- Covariance and Contravariance in C# Part Seven: Why Do We Need A Syntax At All?
- Covariance and Contravariance in C#, Part Eight: Syntax Options
- Covariance and Contravariance in C#, Part Nine: Breaking Changes
- Covariance and Contravariance in C#, Part Ten: Dealing With Ambiguity
Derek Mahar
2010-03-31 21:37:42