EIEIO

[Daniel Colascione <dancol@dancol.org>]

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On 03/13/2014 05:36 PM, Eric M. Ludlam wrote:
> The key thing that EIEIO lets me do is define interfaces that allows
> modules to work together.  For example, there is a tag-table concept in
> the semantic system for managing lists of symbols found in the source
> code it parses.  The parser system all knows how to populate and
> maintain a table.  There is also code that searches tables so you can
> find a tag to jump to, for example.  By defining the core interface as a
> table class with EIEIO, I can also create other classes that manages tag
> tables from GNU Global, and just stick it in a list of other tables to
> search.  The code searching tables doesn't have to know  about GNU
> Global.  The Global person doesn't have to know about jumping to tags.
> And no-one has to write some weird bit of code that reaches into a plist
> to get a function symbol to call.  I was able to move the Java
> completion in CEDET from in-file only to surprisingly robust for Android
> in an afternoon just by writing a database that parses a few.jar files.
>  Nifty.

Thanks for putting work into CEDET. I expect to be writing some Java in
the near future, so I'll probably be taking a much closer look at it soon.

I agree that polymorphism is useful when implementing and extending a
system like CEDET. Emacs has traditionally used dispatch functions in
cases like this, though: look at file-name-handler-alist. Consequently,
EIEIO feels a bit foreign. What motivated the choice of EIEIO over
dispatch functions or defstructs with function slots?

A while ago, I considered using EIEIO for one of my projects; I decided
to use plain defstructs instead. I didn't like how EIEIO required each
object to have a name (requiring that EIEIO allocate a new string for
each object instance), and I had very simple interface requirements, and
found calling funcall on a struct slot more straightforward than a
generic function. I still don't know how method dispatch actually works
or what the performance characteristics of the various combination
methods are. It's also not clear what happens on method redefinition,
package unloading, and so on.

CLOS is a comprehensive OO system, but I'm not sure we're dealing with a
problem that actually requires its power.


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