Can you define "contra-" and "co-" variance for me in the context of structural (not nominal) typing?
Specifically, can you show a situation where contra- or co- variance are problems when using structural typing without mutable data structures? There isn't really a concept of super- and sub- types in structural typing as presented by DJS's presentation or in Ashlar, so we seem to be porting over problems from nominal typing (really, from Java) without being clear how they apply in this context. ~~ Robert. Love Your Enemy: A Campaign to Regain Human Dignity Through Nonviolence http://www.mettacenter.org/mc/projects/love-your-enemy On Mon, Aug 1, 2011 at 2:26 PM, Daniel Spiewak <djspie...@gmail.com> wrote: > >> I find this sentiment- expressed now twice in this discussion- more >> than a little bit surprising. I mean, even in Ocaml, whose user base >> includes a huge percentage of type theorists, the general method for >> handling co-/contra-variance issues is to just avoid them like the >> plague. Even the people who understand the subtleties of the issue >> (and I don't count amount that number) tend to avoid it. The >> introduction of co-/contra-variance issues into Java with generics >> caused a huge amount of sturm und drang in the Java community- the >> resolution of which, as near as I can tell, was for the vast bulk of >> Java developers to simply ignore the issues, and revert to subverting >> the type system and using run-time checking instead (i.e. explicit >> casting). > > Having given it more thought, I'm willing to admit that yes, variance > may be necessary (or very near to it). This is more than a little > annoying though. Covariance is not a problem in and of itself, and > follows nicely by the standard subtype checker. Contravariance screws > everything up. Use site variance is essential if you want declaration > site variance to have any positive impact on your language, and now > you're getting back into dangerous waters. > > I would be very tempted to just avoid the variance issue altogether. > Some things may not be worth sacrificing for. The question is whether > or not this will lead to a *lot* of downcasting in common use, or if > it can be avoided in practice. > > Daniel > > -- > You received this message because you are subscribed to the Google Groups > "JVM Languages" group. > To post to this group, send email to jvm-languages@googlegroups.com. > To unsubscribe from this group, send email to > jvm-languages+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/jvm-languages?hl=en. > > -- You received this message because you are subscribed to the Google Groups "JVM Languages" group. To post to this group, send email to jvm-languages@googlegroups.com. To unsubscribe from this group, send email to jvm-languages+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/jvm-languages?hl=en.
