WEBVTT captioned by bhavin192

NOTE Introduction

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Hello, and welcome to EmacsConf 2023!

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My name is Musa Al-hassy,

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and I will be talking about "REPL-driven development."

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I like programming languages so much.

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I want to use them all over the place.

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I got a PhD in the topic.

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So let's just dive in.

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This is a lightning talk,

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so the details will be available in the repository.

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Links to longer videos and GIFs

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for those who are interested.

NOTE The wonders of C-x C-e

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So the wonderful thing about Emacs

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is that you can execute Lisp anywhere.

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For example, I can go to this piece of Lisp,

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press Ctrl-x Ctrl-e (`C-x C-e`)

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and get a little pop-up.

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Alright, so here I pressed `C-x C-e`

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and this is what it ran.

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And what this package gives you

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is the same ability

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for any language of your choosing.

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So for example, here is some Java,

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and I bound it to Ctrl-x, Ctrl-j (`C-x C-j`).

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So I say `C-x C-j`, and this runs some code,

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and over here I get a little pop-up.

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The wonderful thing about

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being able to run code from anywhere

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is that you can keep it in normal text files

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or any kind of file you like.

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For example, you can have hyperlinks,

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as it were…, like this one.

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`C-x C-e`, and now we can see this down here.

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Learn about it.

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But being able to run other languages besides Emacs

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lets us do some interesting things.

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For example, in the middle of a JavaScript program,

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you might write a line like this. `C-x C-n`.

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And that says down here

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`javascript-eval` in the mode line,

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and so this just checks that some endpoint

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is working as intended.

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So you don't need to use an external tool

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to check endpoints.

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You can just use Emacs in your favorite language.

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You know, you can keep your spirit up.

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So, `C-x C-a` runs `applescript-eval`.

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[Computer]: You can do it, buddy.

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[Musa]: Maybe you heard that in the background?

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The cool thing is, your code immediately takes form.

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Right away, you see it doing things, you know,

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altering your environment.

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For example, this is JavaScript,

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and JavaScript here is gonna change Emacs for us.

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So `C-x C-n`. And you can see down here

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`javascript-eval` was invoked.

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It returned true, and this line of JavaScript

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altered our Emacs. So that's really nice.

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So you don't need to use just Emacs Lisp if you like.

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You can use other systems.

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As you saw, the output is shown in overlays.

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So here's, for example, `C-x C-p` to run

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some Python code.

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Notice it blinked in red

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because I thought red was nice.

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You can configure as you like.

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And if you hover over it,

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you can see the solution there,

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and you can see it in a variety of familiar ways

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if you use `C-x C-e` regularly.

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You can insert the results.

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You can find documentation about them.

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The idea here is that

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you have this familiar Lisp workflow

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with `C-x C-e`, and this package ports it over

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to your favorite language out.

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So you can eval things,

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you can insert them, and do as you like.

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I think that's really neat.

NOTE An overview of REPL Driven Development

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This package allows you to do

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REPL driven development.

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In that, you can use it to grow your program.

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You don't need to restart it.

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You can see changes live.

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So here's, for example, this is in a Java runtime.

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You can see some balls bouncing around.

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I don't really know which one is the second one,

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but let's increase its size. `C-x C-j`.

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And this one increased in size,

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and you can see down here I pressed `C-x C-j`.

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Let's change the color to blue,

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`C-x C-j`, the color is blue.

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So all these changes are happening live.

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Three balls isn't really a ball pit.

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Let's go to 50 balls. There you go.

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So it's nice that you can do this

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without leaving your environment,

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without building, you can rapidly modify and see.

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I think that's nice.

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Unfortunately, I'm at the halfway point,

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so I won't be able to work through

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some of these nice problems,

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but you can see the videos

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or GIFs on the associated repo.

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Likewise for this one.

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This one would have been a lot of fun,

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but unfortunately, we're running short on time.

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Apologies.

NOTE REPL Driven Development with Java

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The neat thing here is: this package tries to

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bring the feeling of Lisp to other languages.

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So the idea of a REPL, or a Read Eval Print Loop

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is R and P are data interchange protocols.

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Unfortunately, not every language has those,

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but this package kind of encourages us to

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implement them if we don't have them.

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Let me show you an example.

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What do I mean here?

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This is a Java file.

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I've loaded this definition in.

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We can press `C-x C-j`,

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and you see it's a list of person.

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I didn't give this a name, so its name is $59.

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If I hover over it, we can see the definition again.

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So it's a person called Hamid, age 5,

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another person called Jaafar, age 6.

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This pretty printing is nice for me as a human,

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but I can't execute this. This isn't valid Java.

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So I could do `C-u C-x C-j`.

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If I have the help of a read protocol inserted.

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Sorry, `C-x C-j`. There it is.

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So look, it gives me a `new Person`

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with the construction everything.

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Now I can work with.

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Java can work with this.

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So this can be quite useful for regression testing

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or just to find out what your method spits out.

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We can do the same thing. Here's a bigger example.

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Trying to see it in the overlay is a bit cramped.

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We can say `C-u C-x C-j`.

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All right, and now we have executable code.

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Okay, this is neat.

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If you want to do something with it.

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I'm not really interested. I have four new persons.

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If I really want to look at this,

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I can say `M-x java-eval-navigate-output`,

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and now I see my output as this hierarchical tree.

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I go down, I can see what's inside these,

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what's the type of this thing, what's in there.

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All right, so that's nice.

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The idea is that we're not limited to

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just textual output.

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We can render output,

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thanks to the power of Emacs, in any way we want.

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In a browser, in a LaTeX file, in an Org mode (file),

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however we really desire.

NOTE Bring your own Read Protocol

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And if our language doesn't have an easy protocol,

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so what I did for Java in particular was:

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you take a blob, and you use some

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parsing expression grammars, or if you really want

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regular expression pattern matching,

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and you get some property list out,

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and then you construct an executable expression

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out of that. Like this `new Person`.

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It's nice that you can do these kind of things

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and that this software encourages you to do them.

NOTE Use Case: RDD & Job Interviews

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So one use case I actually use

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besides learning things is…

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oops oops oopsies oopsies showing you metadata

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you shouldn't be looking at.

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Don't look at my metadata. Sorry about that.

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One place I actually use this besides learning

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and trying new libraries and APIs

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and stuff like that is,

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for example, when I interviewed for jobs

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last year (I was changing jobs),

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I would share my screen,

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and if the interviewer gave me

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some inputs and outputs to play with,

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I could do essentially

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some sort of test driven development

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and constantly evaluate things

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right in front of the interviewer.

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For some design questions

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rather than sketching out my ideas in a Google Doc,

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I would share my screen and be like,

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"Hey, here's actual executable code,

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we can stub some things out

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and have some types,"

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and the idea is we can still check things

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and run them right there and then,

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which I thought was quite nice, and it helps to

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clarify your thought process, I guess.

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Anyhow, so that's my time,

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I hope you've enjoyed this little package,

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and again, sorry for the rush,

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it's a lightning talk.

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Feel free to find everything on the associated repo.

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And have a wonderful December Saturday.

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Take care! Bye bye, everyone.