TSa Thomas.Sandlass-at-barco.com |Perl 6| wrote:
HaloO,
John M. Dlugosz wrote:
Variance?
Yeah, the standard set of co-, contra-, in- and bivariance ;)
Assume A <: B being subtypes. Then how should Foo[A]
and Foo[B] relate?
Foo[A] <: Foo[B] # covariance
Foo[B] <: Foo[A] # contravariance
Invariance means there's no relation at all. And Bivariance
means both!
Regards, TSa.
The multi-dictionary search didn't show this usage of covariance and
contravariance. But the series of articles on Type Theory in JOT uses
it to mean "in the same direction" and "in the opposite direction" but
doesn't define it as absolute subtyping directions. How is it being
used in this mailing list?
I've thought about the issue, and it depends. The type parameters to
the class might be annotated to indicate whether it is treated like a
contained value type, is ignored, or what. But there are too many cases
and the relationships are complex. Instead, it needs to look at the
resulting public methods that involve the type parameters. It considers
one a subtype of the other if they meet the requirements for method
substitutability.
--John
