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[[!meta title="Old McCarthy Had a Form"]]
[[!meta copyright="Copyright © 2021 Ian Eure"]]
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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
- Q2: AFAIK, EIEIO is generally slower than, e.g. cl-defstructs. When
do you think EIEIO is not suitable for performance reasons?
- A: I agree with Dmitry: first make it work, then make it fast.
I don't think there's a blanket reason not to use EIEIO, but
definitely profile if you're using it in a performance-critical
context. EXWM is one project that uses EIEIO extensively and
seems to perform well, so I don't think it's off-limits for
performance-critical code.
- Q3: Do you have any tips about introspection? e.g. IIRC there's an
EIEIO introspection facility, though it may be somewhat primitive.
- A: It is somewhat primitive, but seems to work okay
(<https://www.gnu.org/software/emacs/manual/html_node/eieio/Introspection.html)>.
I haven't found a need for anything fancier (yet).
- Q4: Have you used any of the EIEIO-related serialization tools?
IIRC there are some limitations with regard to printable/readable
values.
- A: I haven't had call for this, but
<https://www.gnu.org/software/emacs/manual/html_mono/eieio.html#eieio_002dpersistent>
is the mechanism (for anyone wondering)
- Q5: I did not get how generic functions can work with non class
objects
- A: Dynamic dispatch is very powerful!
- Q6:So with that Emacs is on pair with Smalltalk development
environments now (?)
- A: Not very familiar
- Q7: Most of what you presented can be done without `defclass`.
AFAICT, the only exception is *multiple* inheritance (since
`cl-defstruct` also supports single inheritance via `:include`).
- A: Yes, you can mix and match structs/objects or any other
type. You need classes if you want the EIEIO customization
editing facility or MI. I think also `initialize-instance` is
class-only, so you need classes if you have to do some kinds of
complex (cross-slot) initializtaion.
- 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.
- This is not a question: Brilliant title for the presentation. :)
Links:
- <https://www.gnu.org/software/emacs/manual/html_mono/eieio.html>
# 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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