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- So we're gonna have a look at a demo.

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For this lesson, all the
demos are in this project,

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lesson04_enums.

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I created this project using
Cargo, let's take a look.

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So here it is on my file
system in rustdev, lesson04.

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It's a Cargo project,

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so if you open that
project in the code editor,

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I'm gonna open that project in VS Code.

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So here it is, here's
my project in VS Code.

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And if you look in the source folder,

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ignore this file for now,

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we'll discuss that later on.

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Just for now, notice mytypes.rs,

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I define an enum type called color

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with the variance red, green, and blue;

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and that enum is public.

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I define another enum type
called house location,

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let's ignore that for now,

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we'll discuss that later on in this lesson

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so imagine that that didn't exist for now.

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And we'll just concentrate
on the color enum type.

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So mytypes.rs is a module file,

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in order for that module to
become part of my application,

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even though it's part of
the directory structure

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it isn't actually part of my
application unless I load it.

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And you load it from the top level,

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in your top level file
main.rs, it looks like this.

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You load mytypes, like so.

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It effectively will look for mytypes.rs,

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and that code in there gets
imported at this point here,

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it's like an import statement
in other languages, okay?

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And then I've introduced
the color enum type

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and the house location
enum type into scope.

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It means I can use these enum
types directly in my code now.

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I could have split this into
two separate statements,

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I could have written it like this

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and I just do a bit of
copying and pasting.

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I could've imported the color
name if you like, symbol name,

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from mytypes into scope,

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and then separately I could have imported

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the house location also into scope.

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Okay, so you can input
that type into scope,

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into the current scope from that module,

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and you can input that type
into scope from that module.

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Because they're located
in the same module,

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then it makes sense to
group them together,

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so you would do that.

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From the mytypes module
introduce that name

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and that name into scope,

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it means I can use these
names directly now in my code.

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There's some other examples
we're gonna look at later on,

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we're gonna be talk looking at mytypes

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for demo purposes later on,

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which is kind of similar
but slightly different.

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So I'm gonna ignore that for now,

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and we'll ignore this for now,

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we'll come back and look at that later on,

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okay, when we're ready.

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Okay, let me just show you
what functions I've got.

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These are all the
functions for this lesson,

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so like we had in the previous lesson

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I've got a main function

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which is kind of like
the driver for the demo,

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and then it calls one of
several other functions

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depending on which part
of the demo I want to run.

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So there's a function here
which demonstrates simple enums,

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I'm going to uncomment that,

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and let's have a look at the code in here.

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Basically the code in here

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is what we showed in the PowerPoints.

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So it demonstrates simple enums,

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it declares a variable of
type color and it's color red,

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and then we match on the color,

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we output either coch, gwyrdd, or glass.

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Right, so I'm gonna run this
application several times

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because I've got several
different things I wanna show,

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so let me open up the terminal window.

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Now I'm gonna warn you in advance,

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when I run the application

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I'm gonna get quite a lot of
warnings about dead code, okay?

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So I'll run it

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and then I'll explain
what the warnings are,

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and then we'll see how to kind of look

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at some other techniques as well.

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So let's just do a
cargo run, first of all.

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So I'm gonna make this a bit bigger,

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I'm just gonna show you
what the warnings were.

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There were six different
warnings, I think.

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Some of the warnings we
kind of know about already,

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I've got a function demo enum with data,

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it's complaining that
I've got that function

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but I've never used it.

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I've got another function
called demo using option enum,

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option is one of the built-in enum types,

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it's complaining that I've
not used that function.

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"Why have you written it?"

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Well, you know, because
it's just a simple demo.

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Oh, I've got another
function here, second of day.

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That's a curious one, second of day.

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We'll have a look at that later.

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And other functions as well,

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which is also complaining about

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this demo using result enum,

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result is another enum type in Rust.

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So it's complaining about
the functions that exist

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but haven't yet been called,

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but also it's complaining

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I've got an enum type
called house location.

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If I remind you, right at the top,

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I imported the house location.

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Well, actually what I did, I
imported the module, mytypes,

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and in there it has the
color enum, which I'm using,

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it has the house location
enum which I'm not using yet,

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I will do later, but I'm
not using it just yet.

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In the code it's saying, "Why
have you defined this enum

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if you're not using it?"

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Well, because I haven't got
round to it yet is the answer,

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but it's still giving me a warning.

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So, well, yeah, so there we are.

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And then it actually output
what I was hoping to output,

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so back in main what I called
my simple function here,

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it outputs the kind of header message,

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a demo of simple enums.

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I declare my enum variable,

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I test its type, and it's red,

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it outputs coch like so.

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Right, so those warnings,

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I'm gonna tidy my code up a bit later

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to get rid of those warnings,

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but for now we'll just
kind of ignore them.

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A couple of things I wanted to say.

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First of all, what would happen...

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I tell you what I'm gonna do,
actually, I'm gonna delete,

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I'm gonna temporarily delete
these other functions, okay,

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because we don't need them just yet.

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We'll just have this simple function

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just to get rid of some of those warnings.

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Okay, so what if I didn't
include the mod statement?

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Okay, let's comment that line out,

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and I'll comment this line out as well.

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So if you don't declare the module,

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then it never bothers

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processing the code in that module, okay?

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So this keyword, this here is required

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to kind of import the code.

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If I don't import the module,

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if I basically comment out line two,

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then I try to run it, cargo run,

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watch what error message I'm gonna get.

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It complains here on line 23,

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it says, "What is this color type?

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I've never seen it before."

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Okay, to which I might
say as a programmer,

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"But I defined it over here, color enum."

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But the compiler would reply back to me

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because we're having a dialogue here,

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me and the compiler,
the compiler would say,

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"Yeah, but you never told me

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to input that module, did you Andy?"

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And I would say, "Oh, thank you very much.

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That's very polite of you, dear compiler.

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Now I see what I've done wrong."

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Now I have imported that code, okay,

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so that code will now be processed.

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You have to actually import it like so.

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So, good, I've still got this line

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commented out though, notice.

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Okay, whoops. So that line is still

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not kind of being processed.

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So what would happen

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if I tried to run the application again?

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Well, another error.

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Now it's saying, what's it saying here,

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what's the first error message?

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Again it's saying, "Use
of undeclared type color."

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So the problem is, a module
is kind of like a namespace.

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When you declare a module at mytypes,

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this name is effectively in
a namespace, mytypes, okay?

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So if you wanted to use it

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you'd have to qualify
it by its module name.

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So what I could do is I could say,

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"Okay, everywhere I'm using the color type

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I need to tell it that that's
part of the mytypes module,"

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like a namespace, and the
:: is like a scope operator.

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So go to this module,
which is like a namespace,

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in there you'll find the color type, yes,

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and it has the red variant.

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That's good, but I'd have to
use this prefix everywhere

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where I've referenced the type name,

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and already I'm starting to
get a bit fed up with this,

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having to qualify it by
its module name everywhere.

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"But that would actually work," he says.

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Let's that a quick run,
that will actually work.

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Okay, so that has actually worked there.

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Couple of warnings but it
has actually worked, okay.

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So, yeah, that's basically
the idea of the use keyword,

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rather than having to fully
qualify color by its typing

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you can bring it into scope.

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The use keyword basically
brings that symbol name

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into top level scope,

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00:09:55,320 --> 00:09:57,870
so the color symbol is now, from mytypes,

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is now directly in scope.

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00:09:59,970 --> 00:10:03,780
So it's kind of almost like
a global symbol name now

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00:10:03,780 --> 00:10:06,150
in my code, and I don't any longer

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00:10:06,150 --> 00:10:08,010
need to use the mytypes prefix,

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00:10:08,010 --> 00:10:11,640
it's already been dragged
into scope by the use keyword.

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00:10:11,640 --> 00:10:15,840
So the mod keyword actually
physically imports the code file

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00:10:15,840 --> 00:10:19,470
and the used keyword
introduces a name into scope,

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00:10:19,470 --> 00:10:21,390
so you can just use that name directly

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00:10:21,390 --> 00:10:25,440
without attempt to prefix
by its module name, like so.

211
00:10:25,440 --> 00:10:29,430
Okay, so that hopefully
explains the mod keyword

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00:10:29,430 --> 00:10:33,720
to import a module, to
basically drag the code in,

213
00:10:33,720 --> 00:10:35,850
and the use keyword to give you

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00:10:35,850 --> 00:10:37,680
direct access to those names

215
00:10:37,680 --> 00:10:40,770
directly in the current
scope of your current file,

216
00:10:40,770 --> 00:10:44,223
without the name, for the module
name as a prefix anywhere.

217
00:10:45,660 --> 00:10:48,750
Okay, I feel like I should run it again.

218
00:10:48,750 --> 00:10:52,920
So it's working again and it outputs coch.

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00:10:52,920 --> 00:10:53,880
Couple of other things,

220
00:10:53,880 --> 00:10:55,930
which I'll just finish off by mentioning.

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00:10:56,880 --> 00:11:01,350
Here, when you declare a
variant, a variable, sorry,

222
00:11:01,350 --> 00:11:05,100
and you give it an enum variant,

223
00:11:05,100 --> 00:11:08,430
you don't really need to type it.

224
00:11:08,430 --> 00:11:11,100
You don't need to specify it's a color,

225
00:11:11,100 --> 00:11:13,350
it can guess that it's a color.

226
00:11:13,350 --> 00:11:14,940
By virtue of the fact that you've given it

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00:11:14,940 --> 00:11:18,600
one of the color variants,
it knows that it's a color.

228
00:11:18,600 --> 00:11:21,213
Okay, so that would actually
be a little bit better.

229
00:11:22,470 --> 00:11:24,393
Okay, so that is working still.

230
00:11:25,320 --> 00:11:27,780
And just one last thing,

231
00:11:27,780 --> 00:11:31,230
you still need to specify the enum type

232
00:11:31,230 --> 00:11:34,590
in front of the variant,
he can't do that, okay,

233
00:11:34,590 --> 00:11:37,020
because red, green and blue

234
00:11:37,020 --> 00:11:41,910
might, for example, be
variants on another enum type.

235
00:11:41,910 --> 00:11:45,930
You might have like a
political party in the UK,

236
00:11:45,930 --> 00:11:49,110
green is a political party in the UK.

237
00:11:49,110 --> 00:11:52,620
So that wouldn't work, you're
gonna get an error here.

238
00:11:52,620 --> 00:11:57,543
If I run the compiler again it will say,

239
00:11:58,687 --> 00:12:00,840
"I don't understand what
you're talking about here,

240
00:12:00,840 --> 00:12:05,610
I've lost it. This symbol
name is not understood."

241
00:12:05,610 --> 00:12:09,393
Okay, oh, right, so basically
I need to scope it like so.

242
00:12:11,100 --> 00:12:13,980
Okay, so the actual process

243
00:12:13,980 --> 00:12:16,380
of defining enum types
is quite straightforward,

244
00:12:16,380 --> 00:12:20,040
remember to make it pub so
it's kind of visible elsewhere.

245
00:12:20,040 --> 00:12:23,850
The variants are always
public when you have an enum.

246
00:12:23,850 --> 00:12:25,710
The interesting part, really,

247
00:12:25,710 --> 00:12:28,740
most of our discussion here
is about organizing our code.

248
00:12:28,740 --> 00:12:31,770
I've put it into a module
file so it can be imported

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00:12:31,770 --> 00:12:34,260
in lots of other
applications, potentially.

250
00:12:34,260 --> 00:12:38,010
In my main code, I input that module,

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00:12:38,010 --> 00:12:40,110
technically I'm declaring the module,

252
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I'm dragging that module
file into my application,

253
00:12:43,890 --> 00:12:48,150
I'm introducing these
names directly into scope

254
00:12:48,150 --> 00:12:50,673
so I can use them without qualification.

255
00:12:51,630 --> 00:12:53,940
Right, okay, so I think
there's one other thing

256
00:12:53,940 --> 00:12:55,290
I want to discuss,

257
00:12:55,290 --> 00:12:57,483
let me just run the
compiler one last time.

258
00:12:59,037 --> 00:13:02,454
(keyboard keys clacking)

259
00:13:03,600 --> 00:13:07,560
Note this, I'm still
getting some warnings.

260
00:13:07,560 --> 00:13:09,510
I'm gonna look at this one in here.

261
00:13:09,510 --> 00:13:14,510
It sees my color enum and my
color enum has three variants,

262
00:13:15,180 --> 00:13:16,830
red, green, and blue,

263
00:13:16,830 --> 00:13:18,540
and it's complaining

264
00:13:18,540 --> 00:13:23,540
that I'm not using the
green or the blue variant.

265
00:13:23,550 --> 00:13:24,930
Technically they never constructed,

266
00:13:24,930 --> 00:13:28,500
it means my code never seems
to make use of color blue.

267
00:13:28,500 --> 00:13:30,300
I'm not actually creating a variable

268
00:13:30,300 --> 00:13:32,100
of color blue or color green,

269
00:13:32,100 --> 00:13:34,200
obviously because it's
such a small example

270
00:13:34,200 --> 00:13:36,060
I don't need to.

271
00:13:36,060 --> 00:13:39,240
In a large application,
hopefully all of these enums,

272
00:13:39,240 --> 00:13:40,170
all of these variants

273
00:13:40,170 --> 00:13:43,050
would kind of crop up
somewhere in my code.

274
00:13:43,050 --> 00:13:44,970
At the moment, it's complaining

275
00:13:44,970 --> 00:13:48,127
that there are variants
here that aren't being used,

276
00:13:48,127 --> 00:13:50,850
"Why are you declaring these variants

277
00:13:50,850 --> 00:13:52,830
if you're not using them?"

278
00:13:52,830 --> 00:13:55,140
Okay, so the solution to that dilemma

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we're gonna look at in the next section.