path: root/2021/captions/emacsconf-2021-form--old-mccarthy-had-a-form--ian-eure--main.vtt

WEBVTT

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My name is Ian Eure,

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and welcome to my talk

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"Old McCarthy Had a Form".

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In this talk,

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I'm going to be discussing EIEIO,

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which is an Emacs Lisp implementation

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of the Common Lisp object system.

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CLOS is a way of writing

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object-oriented Common Lisp code,

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and with EIEIO you have much of that same

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power but inside Emacs.

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I'm going to be using those two names

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interchangeably throughout this talk,

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since they're nearly equivalent.

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You might wonder,

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"Why would I want to write 

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object Emacs Lisp code?".

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I like it because I like writing

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in a functional programming style,

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or I like to do an imperative style

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that captures interactive key sequences

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that I might enter manually.

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Well, I think, different kinds of programs

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need different kind of programming paradigms,

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and sometimes OOP is the one that fits.

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Also, if you've done much OOP before,

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you might be surprised by 

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how EIEIO works and the sorts of power

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that it brings to your programs.

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So, let's talk about that model now.

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Classes are pretty much what 

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you would expect if you've done

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OOP programming before.

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In the CLOS model, 

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they only have fields,

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they only encapsulate values,

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they don't have anything to do with methods.

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So, in this case, we have a base class for

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an EMMS player backend,

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and it has one field (a slot is what

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it's called in CLOS terminology),

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which indicates whether it's playing or not,

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and it's declared abstract,

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so you can't create an instance

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of an object based on this class,

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you can only extend it with another class,

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that's an EIEIO extension,

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but I think it's a pretty good one.

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You can also see there's a class

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that implements an mpv player back-end,

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and it extends the base class,

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it doesn't add any slots,

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and those get inherited from the base class.

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If you want these to do much more

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than encapsulate data,

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you need to start writing methods.

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The CLOS model is to have 

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a generic function.

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A generic function is

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kind of like an interface, 

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it's just a name and an argument list,

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and there's no implementation,

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you have to write a method

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in order to have an actual implementation.

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When you call the generic function,

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the system will do a dynamic dispatch

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to a method that matches based on

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its argument types and how the method

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has declared that it should be invoked.

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Here's an example of some methods

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for our mpv player backend,

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you can see that it'll play anything

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other than something

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with a keyword of `unplayable`,

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and it just has dummy start and stop methods

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that return "Started" and "Stopped" text.

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A method is just one implementation

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of a generic function,

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and you can have as many as you need.

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In order to determine

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which method gets dispatched

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when you call a generic function,

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they're specialized based on

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their argument type.

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In this case, you can see that

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first argument says

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`player talk/emms-player-mpv`,

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that means that if that first argument

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is an instance of the mpv player

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or a subclass of it,

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then those methods will be invoked,

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unless there's a more specific one

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based on that argument type.

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You don't have to define 

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the generic functions.

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If you define a method,

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then it'll implicitly define

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the generic function for you.

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Specialization is really powerful.

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It lets the methods define 

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how they get invoked

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If you've done much programming in Clojure,

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this sounds a little bit like multi-methods,

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but with multi-methods, only the main method,

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the equivalent of the generic function,

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can determine how they're dispatched.

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So, as the writer of an interface,

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you might not be able to foresee 

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every way that someone who's implementing

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a version of that interface

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would like to dispatch.

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CLOS doesn't have that problem,

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you get to define your

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own invocation semantics.

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You're also not limited to

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dispatching based on the value

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being a subclass of an EIEIO type.

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You can dispatch based on

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a primitive type like integer,

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you can dispatch based on the value,

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you can dispatch on multiple arguments,

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and there's also a default dispatch

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that will get applied

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if there's no others that are defined.

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If you don't have a default,

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then you'll just get an error

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from the system,

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and if that doesn't cover it,

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you can even define your own.

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Definition is with the

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`cl-generic-generalizers`,

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which is itself a generic function.

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Much of CLOS is built in CLOS,

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which I think is really cool.

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In addition to all that,

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you have four different types of methods,

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and those are distinguished by

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what's called a qualifier.

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Every function can have methods

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that have all four different 

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types of qualifiers,

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and based on your class inheritance,

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you might have multiple of each type.

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There's the primary method,

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which is equivalent to the method

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in any other OOP system,

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so we're not going to cover that too much.

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Then there's a `before` method.

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This is evaluated before

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the primary method for side effects,

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and its return value is discarded.

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There's an `after` method,

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which is the same but happens after

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the method has finished evaluating.

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And then there's an `around` method

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that happens around all the other three.

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And by using these types of methods

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and using class inheritance

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to compose them into your classes,

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you can add some really powerful

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mixin type functionality.

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You can use before and after

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to build things like logging,

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and you can use around

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to implement things like memoization.

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If you've done much Emacs Lisp programming,

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those before after and around

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might jog your memory because

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they're the same features you get

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with Emacs's built-in function advice.

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The thing with function advice is that

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it only works on functions

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in the global namespace,

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and there's no kind of conditionality, 

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they always get dispatched

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when that function is invoked.

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The nice thing about the CLOS system is that

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whether they get invoked or not

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depends on whether they exist in

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the class hierarchy,

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so you can add them to your

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class hierarchy if you want

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that extra functionality 

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like logging or memoization,

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and you can exclude it if you don't.

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I think that's really powerful

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and a very interesting way of doing it.

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It also supports multiple inheritance,

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which is the mechanism that you can use

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to compose all these different kinds of

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behaviors into a single object that does

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all the things that you want.

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Here's a quick example of a logger.

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So, you can see the class just has

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a single slot called `messages`,

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it has a `log` method

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that pushes a new message,

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which is a format string, into that,

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and then it will return them back out,

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or it'll just return the latest.

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And there's a simple example

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that shows it logging,

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and then shows it coming back out,

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pretty much what you would expect.

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Here's another class that adapts

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the `emms-player` to the `logger` class.

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It only extends the logger

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because it doesn't need any features

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of the `emms-player` class itself.

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It just implements methods that dispatch

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based on it being that logging player class,

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and you can see it logs whenever

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a track is started or stopped,

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and it also adds some track

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to tell you whether or not 

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the track was playable,

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that is using the around method.

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So, you can see we have all three methods

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before, after, and around in this class,

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so you can see how those work.

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Then you need one more,

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which is the class

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that mixes it all together,

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So, that's the `logging-player-mpv`,

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and it extends both the `logging-player` class

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and the `emms-player-mpv` class.

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What's really interesting about this is

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that even though the logging player is

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part of the `emms-player` hierarchy,

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it doesn't depend on

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a specific implementation,

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so you can combine the two different

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classes that lets the logging class

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only care about logging,

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and the `emms-player` class

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only care about playing,

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and that is a really nice

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way of separating your concerns

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that I think is very powerful.

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Here's a quick example of

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just how that works,

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and you can see the `unplayable`

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track is not playable,

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and it gets logged as such,

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`foo` is playable, and you can see

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the logs in started and stopped.

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So, you can see it's having

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the side effects from those methods,

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and it's also returning 

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the value off the player as well.

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I think this system has a bunch of

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really nice properties.

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First and foremost,

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it feels like a normal Lisp,

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all you're doing is calling functions,

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there's no magic involved.

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Also, you can use either or both of the

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classes or generic functions.

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If you only need to

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encapsulate data into a structure,

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then you can only use classes,

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you don't have to use generic functions.

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And if you only need dynamic dispatch,

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you can only implement

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a generic function and methods.

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You don't get forced into a model

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where you have to use both.

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You can mix and match for

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whatever needs your program has,

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which I think is really amazing.

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Any value can conform to an interface,

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meaning a generic function.

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So, you don't have to use those classes.

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You can even implement

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a generic function over `nil`,

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which gives you those really nice

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properties of Lisp,

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where you have nil-punning, you know,

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if you take the head of a nil list,

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then the output is `nil`,

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but you can do that

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with your object system too.

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And a really nice feature of that is

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that you have no possibility of

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null pointer exceptions

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like you do in other languages.

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They have a calling convention

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where you call object dot method,

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but in CLOS, you call a generic function,

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and you just give it some arguments.

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Typically, the first one is going to be

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an EIEIO class object,

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but it doesn't have to be,

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but because you're not calling that

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instance of an object,

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there's nothing to be nil in the first place,

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so there's no possibility of

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a nil pointer exception.

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And then the ability to

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have multiple inheritance

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and mix in all of these different

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functionalities into your final object

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is very powerful.

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Because you have multiple inheritance,

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your final object that

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composes all of those things

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is both a player and a logger,

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and it can be substituted into code

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that expects either of them.

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It's not an adapter class

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that only allows you to do one thing,

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it does both of them.

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I think that's amazing.

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So, here are some practical examples

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where I think maybe this

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could be a good idea.

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And I like to think about,

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"What is OOP actually good for?".

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I think it has three really amazing powers,

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encapsulation, abstraction,

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and extensibility,

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so let's look at those.

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Encapsulation is just keeping

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related data together.

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Here's an example from the

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transmission Torrent client.

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In order for it to work,

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it needs to have all four of

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these variables set consistently,

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but if you use the customization interface,

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they're kind of strewn all over the buffer

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because that shows them alphabetically

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by variable name instead of

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grouping them logically by function.

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You also have all these

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in the global namespace,

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so you need this disambiguation in front,

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you have to have a transmission prefix,

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so it's kind of ugly.

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An alternative example would be to

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encapsulate all of that

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in a single class.

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This has one slot for each of those values,

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except the username

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and password is broken out,

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there was only one in the previous example,

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but it works pretty much the same.

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The really neat thing about this is that

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the customization interface understands

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how to customize EIEIO objects,

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so you can set your custom variable

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to the value of an object,

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and in the customization interface,

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it shows you all of the fields,

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and it lets you edit the values

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of those slots directly.

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So, that keeps that logical grouping

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that I think makes things really easy to use.

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Another thing it's really good at is

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abstraction, and this is really core to

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a lot of what Emacs does

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because it runs on so many different systems,

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and it works with so many different

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kinds of similar tools.

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Here's an example from

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the built-in SQL implementation.

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This is the definition of

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the postgres backend,

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there's one of these for

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every supported database backend.

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And you can see, this is a pretty

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classic interface abstraction pattern.

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On the left-hand side,

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you have a symbol that's common

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among all the database backends,

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and the code that doesn't 

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know about the implementation can use,

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no matter what implementation

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is being specified.

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On the right-hand side,

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you have the implementation

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specific values,

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in some cases these are just strings,

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or regexes, and in others

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those are actual functions.

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So, really this already is

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object orientation,

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it's just an ad-hoc system for it,

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but you don't have to use an ad-hoc system

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because there's this

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nice formal one instead.

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Another thing that it's

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really good at is extensibility,

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we saw some of that with

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the emms-player example earlier.

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But here's another thing I think

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it would be interesting to explore.

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Emacs has this idea of derived modes

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where you have a mode that's based on

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another, and that's a pretty clear

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inheritance pattern straight out of OOP

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as far as I'm concerned.

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What would it look like

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if major modes were EIEIO classes,

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and you could extend your mode

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by extending the class.

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I think that's a really interesting idea,

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and I'd like to explore that more.

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In conclusion, I think EIEIO is amazing,

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and I had no idea that such a powerful

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object orientation system

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was available in the Emacs.

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My goal with this talk

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is for anyone who writes Emacs Lisp,

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to look at these classes of problems,

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encapsulation, abstraction,

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and extensibility,

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and if you run into

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those problems in your code,

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instead of immediately reaching

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and building your own system,

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I want you to think:

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"Oh there's a thing for that,

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and I can just use it."

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That's my talk, thanks.

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Hack on!

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[captions by bhavin192 (Bhavin Gandhi)]