[lltx] why user-defined callbacks?

[Philipp Stephani]

2011/7/21 Arno Trautmann <Arno.Trautmann at gmx.de>:
> Hi, me again,
>
> I forgot to ask that question in the last mail: What is the advantage of a
> user-defined callback (still talking about luatexbase-mcb) over a
> user-defined function? Maybe I still don't get the idea of a callback, but
> in that case I don't see the difference. Is there any and if so, which?

LuaTeX "callbacks" are implementations of the Observer pattern and
therefore make sense whenever using the Observer pattern is
appropriate.