[Haskell-cafe] Ratification for my understanding of functors

Eugene Kirpichov ekirpichov at gmail.com
Sat Jan 8 11:50:08 CET 2011

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 <ckkashyap at gmail.com> 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
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