It's covariant if the parameter is to the left of an even number of arrows. For instance (-> a) is co, (a ->) is contra, (a -> b) -> b is co wrt a and neither co nor contra wrt b, etc. Read the c#-related stuff on variance, I am sure it is completely relevant.
08.01.2011, в 13:23, C K Kashyap <[email protected]> написал(а): > Hi, > > Thanks to Luke Palmer's explanation. I got a better idea of functors. > I just want to ratify my understanding of what a functor is - > according to category theory, functors are arrows between categories > ... these arrows map objets and arrows from one category into another. > > So, would it be right to say that the list type constructor + the map > function form the functor? Saying "list is a functor then is a > convenient approximation right? > > In this context, could someone please point me to some writeup on > co-variant/contravariant type constructors? Googling for it seems to > only give a lot of C++/Java/C# hits. > > Regards, > Kashyap > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
