Module four, Interfaces for Abstraction. Topic 2.2, Type Assertions. So a lot of the point of an interface is to conceal differences between types. So if you think about it, an interface can hide the differences between two types. It basically highlights the two similarities. So while in this fit and yard implementation, there are rectangles and there are triangles. But from the inside fit in yard, they're all treated the same, right? As long as they both satisfy Shape2D, I can call s.Area, s.Perimeter, right? So what you're doing is, what interface allow you to do is to treat different types that have some similarities, some similar methods, treat them the same. All right, so you are hiding differences by using interfaces. But sometimes you need to disambiguate, sometimes you need to treat different types in different ways, okay? So sometimes, like in this function, we don't, right? We can just say s.Area, s.Perimeter. We treat them exactly the same because they have the same methods. But sometimes you do need to differentiate based on the type. You need to be able to figure out what is the concrete type, right? So in this example, s, since you're just calling area and perimeter, it doesn't matter exactly what the concrete type is, right? The concrete type could be rectangle, it could be triangle. It doesn't matter, either way area and perimeter do what you think. So in this case, the concrete type that underlies the interface value, that doesn't matter, okay? But there are definitely cases where the concrete type matters. In those cases, you're going to have to expose those type differences. So you're going to have to take this interface, which is hiding the differences between the types, and peel it apart again, and say okay, actually, this is really a rectangle, this is really a triangle. So situations like that might be a graphics program, okay? So I got my graphics program which I've used many times, but in my graphics program, this time I want to write a function called DrawShape. And it should draw any shape. So I want to be able to pass it as an argument any two dimensional shape. So I declare it sort of the top line I'm showing right there, func DrawShape. It takes Shape2D, that's the type of it's argument, Shape2D. So it can take any two dimensional shape as an argument. So that's good, right? I've used my interface to generalize and to hide the differences between the types, rectangle, triangle, circle, doesn't matter, for passing it as an argument to draw shape anyway. Now, inside DrawShape, though, in this case, I'm going to have to disambiguate. I'm going to have to determine this s, is it a rectangle, is it a triangle, what is it? Because maybe in the underlying API there's some kind of drawing functions that I'm using in this API, right? And the underlying drawing functions, they actually are specific to the type of shaping drawing. So for instance, maybe the underlying API gives me a draw rectangle, DrawRect, right? And then another draw triangle, and a draw circle, and so on, which is not uncommon in these drawing APIs, right? So you got draw rectangle, draw triangle, draw circle, all those. Now, these API functions, my DrawShape is going to have call these, right? When it wants to draw a rectangle, it's going to have to call DrawRectangle. DrawTriangle to draw triangle. And these underlying API functions, they don't take just any shape. They won't take Shape2D. DrawRect only takes rectangles. DrawTriangle only takes triangles. And so on. So this is the case where I want to use my interface so my DrawShape can take any argument, any type of reasonable shape. But inside my DrawShape, I'm going to have to differentiate. I'm going to say, look, if you're a rectangle, call DrawRect. If you're a triangle, call DrawTriangle, and so on. So in this case, inside DrawShape, I'm going to have to determine the concrete type that s is based on, that the shape is based on. So for that, I use what's called a type assertion. So type assertions can be used to disambiguate between the different concrete types that actually satisfy a particular interface. And you can see that here with DrawShape. It needs to actually disambiguate. So if it's a rectangle that's being passed, it needs to call DrawRect. If it's a triangle, it needs to call it DrawTraingle. So you can see us doing that here. At the top, you've got that first type assertion, where it says rect, ok:=, so that will return a rectangle, if the s is actually a rectangle. So if ok is true, and it found a rectangle, it will call DrawRect with that rectangle. Otherwise, the next type assertion actually checks to see if the type of the interface of s is a triangle. So it says, tri, ok:= s.(Triangle) this time. And so, ok will be true if s is actually a triangle. And in that case, tri is going to equal that triangle. And so, you call DrawTriangle with a triangle. So either way, we use this type assertion to disambiguate, to determine the actual underlying concrete type for this Shape2D interface. Now, another way to do this sort of a common thing that you need to do is what we just did in the last slide, we went down a list of possible types. So rectangle and triangle in this case. But note that an interface can actually be satisfied by many different types. There's an interface that's satisfied by 10 different types, you might need to disambiguate all 10. So to run down the list. If you're this type, then do this. If you're that type, then do that, and so on. And so, there's a switch construct type switch which is just for that purpose. So you got one case for every different type that you need to deal with. So in this case, you got two cases, case Rectangle, case Triangle. And in each case, case Rectangle draws the rectangle, case Triangle draws the triangle. But right before that, you start with the switch. So notice the type Type assertion says s.type. In parentheses you just say type, the generic word type. And so, what happens is sh will be whatever the, it'll be the concrete type that s represents. So if s is actually a rectangle, then sh will be that rectangle, right? If s is a triangle, then sh will be that triangle. And you'll hit the appropriate case. So if sh is a rectangle, then you'll execute the case rectangle. If sh is a triangle, then you'll execute the case triangle. So this is just a more convenient way to sort of run down a list to disambiguate a whole set of types that all satisfy a particular interface. Thank you.
