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# Old McCarthy Had a Form
Ian Eure



Name Pronunciation: (EE-un YOU-r)

Pronouns: he/him/his

Preferred contact info: ian@retrospec.tv / ieure on Libera

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Most practical languages are multi-paradigm, offering several
abstractions for the programmer.  But did you know that Emacs Lisp
comes with a powerful system for object-oriented programming? Join me
for a discussion of EIEIO, and learn how it can help you write more
modular, flexible Emacs Lisp.

# Discussion

IRC nick: ieure

- I didn't know that custom.el works with EIEIO that way, very nice
- Dang Ian. What a talk, great demos.
- Wow, that's a great talk.
- Great talk. So with that Emacs is on pair with Smalltalk development environments now
- For reference, transient.el, which we all know and love as the engine that drives the magit interface, is written via EIEIO afaik.
- I reckon I should look more into it, I've always avoided it because I was afraid it wouldn't be /quite as nice/ as CLOS or GOOPS.
  - ieure: It's missing a few things (most documented in the manual: https://www.gnu.org/software/emacs/manual/html_mono/eieio.html#CLOS-compatibility), but it's solid and worth using.
    - Yeah when transient.el first came out I was impressed by how naturally it worked as part of that abstraction.
- ieure: EIEIO all the things!  I had to cut it, but you can use dynamic dispatch based on major-mode, like: (cl-defmethod whatever ((mode (derived-mode python-mode)))) and then (whatever major-mode).

- Also really nice for things like 'window-system.  I really like when callsites are clean and not cluttered with conditionals.
- Can eieio do regexp dispatch? 
  - ieure: Not currently, but it's possible to add.
  - okay, so I don't need to feel too bad about coding up my own vtable for those then
  - ieure: This is the thing that implements (thing (eql :whatever)) specialization, should be a good starting point if you want (thing (string-match-p "^foo")): <https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/emacs-lisp/cl-generic.el#n1164>
  - thanks for the pointer, but I think I have some more pressing cl-defgeneric reimplementations to make before I touch that
  - ieure: Extremely fair.  One thing I didn't get to touch on is that you can extend generic functions from anywhere.  So you don't have to patch up cl-generic.el, you can define a new method for a generic function defined anywhere, in any file.  Which rules.

# Outline

-  What is EIEIO?
-  Why OOP?
-  The CLOS Model
    -  Classes
    -  Generic Functions
    -  Methods
    -  Specialization
    -  Method Qualifiers
    -  Multiple Inheritance
    -  Nice Properties
-  Practical Examples
    -  Encapsulation
        -  Example: `transmission.el`
    -  Abstraction
        -  Example: `sql.el`
    -  Extensibility
        -  Example: comint
-  Conclusion


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