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- This lesson is all about
nested functions and closures.

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To be honest, it's mostly about closures,

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but we'll have a look at
nested functions first,

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which is a kind of simpler syntax.

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It'll introduce some concepts

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which will then lead
us into closures later.

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So you can define nested
functions in Rust.

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In other words, a function
defined inside another.

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The nested function can only be accessed

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inside the function
within which it's defined.

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So here's a simple example.

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I've got an outer function,

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some_outer_function I've called it,

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and inside, I've defined
a nested function sqr.

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It takes an integer,

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a 32 bit integer as an input,

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and it returns the square, like so.

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This is a nested function.

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You can only call this function

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within the scope in
which it's defined, okay?

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So only this outer function
can see this nested function,

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and you call it like a regular function.

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So I've called the nested function once.

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I passed in the value five.

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It returned 25.

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The value 25 gets printed.

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I call the function again.

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I pass in the value seven into here.

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It returns 49.

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The value 49 gets printed, okay?

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So it's very simple,

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and not much you can say about it, really.

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Some fun facts about nested functions.

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You've got to specify full information

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about the parameters and the return type

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for a nested function,

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and that seems quite straightforward.

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Obviously, with a function

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you have to say what's its parameters,

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what's its return type.

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Also, a nested function isn't allowed

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to access variables
defined in the outer scope.

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So your nested function will
live inside an outer function.

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That outer function might
declare some variables,

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but you can't see them
in the nested function.

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The nested function can
only see its parameters.

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So if you had to get any
values into the nested function

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you've gotta pass them
in as regular parameters.

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Okay, so that all seems
perfectly reasonable.

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The reason I make these points

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is because when we look at closures

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for the rest of the lesson,

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closures actually overcome
these constraints.

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As we'll see,

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a closure doesn't have
to specify explicitly

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the types of the parameters in the return.

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And as we'll see, a closure
can access variables

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declared in the outer scope.

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It captures the variables
declared in the outer scope

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and can use them inside a closure.

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Okay, so we'll have a look at closures

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later on for the rest of the lesson.

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But first of all,

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let's just see a quick
example of nested functions

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before we go on any further.

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So here it is.

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The project is
lesson09_nested_functions_closures.

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Okay, so here it is in rustdev,

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lesson09_nested_functions_closure.

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I open that up in a code
editor, and here we are.

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Same arrangement as usual.

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I've got my main code.

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In my main code I'm going
to uncomment the first demo.

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Demo_nested_functions, right?

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So here we are, nested functions.

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Basically, the code that
we saw in the slide.

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Here's my nested function,

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and I call it here,

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and I call it here.

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It is only visible inside this function.

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So I guess we could just run it

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and just confirm that it works.

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Cargo run; So I'm running the application.

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It's downloading some dependencies

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that I need in my application.

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In my TOML I've got a chronodependency.

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I'm going to be doing some
time related bits and pieces.

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When you add the chrono
crate into your project,

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it pulls in various other
bits and pieces as well.

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And then, I called my
demo_nested_functions,

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which called the sqr function
to get the square of five,

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which I can confirm is 25,

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and the square of seven is 49.

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So there we go, a very
simple nested function.

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Quite limited, you know.

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You've gotta give it full type information

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for its parameters and its return.

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It can't access any variables

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that might have been
declared in the outer scope,

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and it's only visible
inside this function.

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What we're gonna do for
the rest of the lesson

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is see a similar kind of approach,

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but instead of having
nested function syntax,

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we're going to replace this with closures,

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which are a bit like
lambdas in other languages

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and much more powerful.