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From: Eric M. Ludlam
Subject: Re: EIEIO
Date: Thu, 13 Mar 2014 23:35:17 -0400
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.3a1pre) Gecko/20091222 Shredder/3.1a1pre

On 03/13/2014 10:26 PM, Daniel Colascione wrote:
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.

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?

I was taking a C++ class and learning about OO systems and ORBs back when ORBs were new. It sounded like something useful for Emacs where I was having trouble building a precursor to CEDET, so I wrote EIEIO. I never did create the ORB though.

I have vague recollections of trying to use defstructs back then, but they stymied me.

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.

EIEIO is not the best solution for all problems. It sounds like you had a problem that could be solved in a self contained system, so it didn't need to be organized with an extra level of abstraction that EIEIO provides.

If you have a problem where you want to provide interfaces for external developers to code against, you will probably find EIEIO more useful. The learning curve is higher, sometimes debugging is more challenging if your have a lot methods overloading each other, but I've found it has served the goal of enabling several other developers to extend different parts of CEDET as intended, enabling them to implement complex behaviors (ie - wide interfaces) with no impact to the core.

As for method dispatch, that is a long topic. Suffice it to say that EIEIO does not implement the CLOS pattern matching mechanism. It only does first argument dispatching which simplified the implementation, and was sufficient for what I was doing in CEDET. There are also 3 dispatch algorithms, so you can pick how you would like (call-next-method) to behave. There are also fun things like :BEFORE and :AFTER methods which are nifty. The more of those features you use, the longer it takes to execute those methods... naturally. If you only have one method with a particular name, then it is quite fast, ie - about 5 extra lines of code to execute, mostly error checking.


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