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Hey there.

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In this lecture we're going to take a look at typecasting.

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The purpose of typecasting is to specify or convert the type of an expression.

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When that expression is too generic, and an expression is simply just a value or something that executes

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and returns a value.

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So this would include variables, parameters, and return values from functions.

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There may be times when an automatically inferred type is too generic, and you want to narrow it down

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to be a more specific type.

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And in order to do this, we have to use typecasting.

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However, in order to typecast the original type and the type of the typecast have to be related.

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So let's go ahead and take a look at some examples for typecasting.

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So I'm going to create a table.

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I'm just going to call it my table.

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And inside we're going to have a key value pair.

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I'll just call it names.

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And it's going to be equal to a table.

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Now let's say I wanted to be able to say that only this table can contain strings.

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I don't want to be able to insert anything else into this table.

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Well, in its current state, when I go to insert something into this table, like my table names and

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I can insert a number, we're not going to have any issues here.

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It's not going to underline anything red for us.

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However, if I copy this I'll call this my table two.

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And then I put a typecast on this table to make it only contain, um, values of a specific type.

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then we're going to get a red underline.

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If we try to insert a value that isn't of that type.

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So in order to create a type cast we need to use two colons instead of one.

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So this is not type annotation but this is type casting.

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So we do two colons after we assign the value.

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And then we can put a table in here.

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And inside this table we can define that.

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This names table is going to contain values of type string.

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And that means now our number one here is going to be underlined.

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Red type number could not be converted into string.

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And that's because we've explicitly defined here.

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Or we've cast the type of string to be contained within our names table.

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Now another important thing to note is that type casts are also automatically checked by the type engine,

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meaning you can't cast unrelated types.

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So as an example, if I created a variable, I'll call it sum num and assign it a number like one.

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And then I try to create another variable.

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I'll call it value one and set it equal to some numb.

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I wouldn't be able to do something like set it to a boolean or a string, because those types are unrelated.

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I could set it to something to be something more generic, like any.

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So if I do this with a typecast, this is okay, we're not going to get any red underline here.

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And now that means value one exists as any because we've casted it to be of type any from some num.

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But we can't do something like this.

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If I created a value, set it equal to some num, and then I try to cast the type of like boolean on

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it.

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We're going to get a red underline here.

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And that's because we cannot cast number into boolean because the types are unrelated.

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So again this is the type checking engine checking for us and seeing hey you can't do this.

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These types are unrelated.

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Now another example we're going to take a look at is this function right here.

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And you might remember this function from our previous lectures.

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So basically we had this function here that took two numbers.

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And one number was a and it has to be a number.

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And the other parameter was b.

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And this was an optional parameter.

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We don't know if uh whoever uses this function is going to pass a number to be or not.

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So we set a default right here.

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However, the type checking engine isn't smart enough to realize that we've set a default for B, so

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it says right here type number.

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Question mark could not be converted into number.

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So we're going to have this error here.

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And you might be wondering well how do we get rid of this error.

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Well we can use typecasting in order to fix this error because number question mark is an optional parameter,

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right.

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It is still of the type number.

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And that means number question mark and number are related types.

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That means we can cast B to be of type number.

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And that means this error here would disappear.

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So what I could do right here is I could put two colons and type cast number for B here for this return

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value.

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And now as you can see that red underline has completely disappeared.

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And this is because we've specified the type more clearly for the type checking engine.

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We've narrowed it down to be of the type number.

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Now these were just some basic examples of type casting.

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And you you probably won't use them that often, but they do exist to help to make your code more readable

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to both you and the type checking engine.

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Solving errors like the one we just did here.

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So again, you won't likely use type casting that often, but it is very useful in those situations

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where you have to use it like right here.

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Otherwise, I hope this lecture helped you out a little bit with type casting, and I'll see you in

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the next lecture.