Adrien <camaradetux@gmail.com> wrote in article <666572260710241205x19edbd4ar840811b1d7a7315f@mail.gmail.com> in gmane.comp.lang.caml.inria:
> > 7. They easily understand how the standard library is used (but not the
> > functors), the open statement, the fact that a program may be in several
> > .ml files. The .mli files are a bit more mysterious. Functors are _very_
> > mysterious.

Any tips on how (and perhaps how not) to teach functors?  I'm using a
Haskell equivalent of functors (namely constructor classes) in an AI
class (!) and they seem to be mysterious.  It didn't seem to work to
explain the Java/C# code that I would like to write (but can't, because
these languages have no interface _on_ generics (as opposed to generic
interfaces)).

> I just don't get the importance of functors.

I use them to provide multiple interpretations of the same abstract
construction (each interpretation is a module and each abstract
construction is a functor from interpretation modules to result
modules).  Here is a description of a simple example:
http://conway.rutgers.edu/~ccshan/wiki/blog/posts/Translations/

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One can show you aren't immediately killed by tidal forces once crossing
over the event horizon of Black Hole. Mathematically, your space coordinates
become time-like and vice versa. This causes a shift in your light
cone and your world line can never intersect with someone on the outside.