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[No Audio]

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In this tutorial, we will be talking about

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trait items. Some of these items are already

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somewhat covered in some of the previous

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tutorials, so we will go through them

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quickly. The trait items are any items that

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are part of a trait declaration. This means

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that all the functions, associated types or

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generics that are declared as part of the trait

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declaration are trait items. Within the

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context of trait items, the first concept is

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that of the Self which generally confuses

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beginner level programmers. Let us develop a

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good understanding of what exactly do we mean

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by Self, I will define a simple trait

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containing a couple of functions.

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[No Audio]

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The first function returns a simple i32

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value, and the second function returns the

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implementation type which is indicated by the

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Self keyword. Within this context, it means the

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type for the simple trait which is being

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implemented. Let us now define a couple of unit structs.

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[No Audio]

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A unit struct is a special

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struct in Rust programming language with no fields.

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Next, let us define the simple trait for the SomeType.

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[No Audio]

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The fn_2 will return an

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instance of the Self, which

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refers to the implementing type, that is the

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SomeType itself within this implementation block.

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[No Audio]

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This means that, the Self means the

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type for which we are implementing the trait

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and is context dependent. Let us see it more

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clearly by implementing the trait for the

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other type. For the fn_1, we will

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again return some arbitrary value.

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[No Audio]

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For the second function, we will return other type.

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[No Audio]

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As you may have spotted, the meaning of Self

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in this context is now the other type, which

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is the type for which we are implementing the

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trait. Now, let us move to the second

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topic, which is with regards to the trait

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items of functions and methods. First, I will

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comment out the previous code.

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[No Audio]

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We will start by highlighting

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the key difference between functions and methods

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within the context of traits.

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A trait bound function is any

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function, whose first parameter does not use

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the Self keyword. A trait_method is any

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function whose first parameter uses the Self.

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Let us go through some examples to clearly

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understand the difference, I will define a

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simple trait of Default.

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I will add the default function which will return the Self.

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[No Audio]

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You may note that this is a function, since the

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first parameter is not that of Self. Let us

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implement this for an i32 value.

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[No Audio]

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The function will return a Default value of

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zero in this case. Let us use the

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implementation now in the main. I will call the function.

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[No Audio]

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The Rust compiler will complain

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if we do not provide the type, since there is

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no way to tell Rust that I am going to call

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this method for an i32 type, let us provide

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the type now. The function with no Self

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argument cannot be called like methods, for

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instance, I will define an i32 variable.

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Now, let us use the dot notation to

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access the function. As expected, the compiler

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is not happy. Let us inspect the error,

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it says, to be used as methods function and must

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have a self parameter. Now, let us look at

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some examples of trait_methods, I will add another trait.

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[No Audio]

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As pointed out earlier, a trait_method

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is any function whose first parameter

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uses the Self. The Self can appear as an own

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type, a reference to Self, or a mutable

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reference to Self. Let me add some functions to explain this.

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[No Audio]

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The first function is using

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an owned Self type, the second one is using a

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reference to Self, and the third is using a

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mutable reference to Self. The first function

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can be desugared into Self with a type of Self.

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[No Audio]

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Similarly, the second function can be

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desugared into Self with a type of reference to Self.

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The third one is similarly desugared

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into Self with a type of mutable reference to

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Self, and similarly, this small self refers to

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instance for which the function is being

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called when the Self with a capital refers to

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the type itself which can be owned reference

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or unmutable reference. We have used

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both the methods and functions in the

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BlockChain implementation section of the

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course. Let us see some examples of that in

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the code. I will comment out this code first.

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[No Audio]

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Next I will paste the code

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of the BlockChain implementation.

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[No Audio]

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In the BlockChain implementation,

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you may have spotted that the

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new is a function and not a method, however,

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the starting block is a method. Similarly

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try_add_block is also a

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method. In the same way, is_block_valid

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is also a method, the remaining functions in the same

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implementation blocks are also methods.

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However, in the block implementation, the new

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and mine, are both functions and not

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methods. Now, let us move to the most

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important and key concept, that is generic

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parameters and associated types and their

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relationship within the context of traits.

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I will comment out the code first and we'll start fresh.

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[No Audio]

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Both generic types and

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associated types defer the decision to the

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implementer on which concrete types should be

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used in the trait functions and methods.

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In these examples, we seek to explain when to use

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one over the other within the trait. The

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general rule of thumb is, use associated types

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when there should be only a single

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implementation of the trait per type and use

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generic types when there can be many possible

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implementations of the trait per type. Now,

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let's see an example to explain this in some

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detail. Let us define the add trait

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with some associated types.

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[No Audio]

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The trait will also contain an add function

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with the inputs of Self, and Rhs or right hand

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side, and the output type will be Self::Output.

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[No Audio]

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This is a general implementation of the trait

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add, which will let us add some sort of two

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things where one will be called the left hand

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side, and the other one will be called a right

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hand side. The Self for the associated type

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means that, these associated types will be

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defined based on the type for which the trait

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is being implemented, let us define a point struct.

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[No Audio]

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This structure will contain the x and

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y coordinates as fields.

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[No Audio]

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We will now implement the add trait for the point type.

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[No Audio]

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First, we will set the type of the right hand

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side or Rhs to that of a point. The output

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will also be a point, so let us set its type also.

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[No Audio]

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Finally, we will add a function with

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suitable inputs and outputs.

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[No Audio]

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We will return a point

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with the x axis value of Self

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being added to that of the right hand side x

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axis value, and the y axis value of Self

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being added to that of the right hand side of

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y axis value. We can also use the Rhs to be,

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to be of type point since Self::Rhs

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has a type of point in this case.

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In the same way, we can also use point as the

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return type, since Self::Output

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means a type of point in this case.

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[No Audio]

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Let us now use this in the main,

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I will create a couple of points.

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[No Audio]

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Next, I will call the add function

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[No Audio]

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That is now confirmed that the x axis value

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and the y axis values are equal to 3.

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[No Audio]

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Let us cargo run this now.

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[No Audio]

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This program is fine. However, let's say we

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want it to add the ability to add i32

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values two point, where i32 would

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be added to both the x and y members.

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Let us go ahead and create another implementation

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for the Add for Point, where Rhs will be an i32 value.

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[No Audio]

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We will set the type of RHS to date of an

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i32 value and the output will be a point.

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[No Audio]

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The right hand side or Rhs will be added to

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the x axis value, and y axis value inside the Add function.

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[No Audio]

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You may note that the compiler is not happy,

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it says conflicting implementation of trait

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add for type point. Since the add trait is

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not parameterised by any generic type, so we

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can only implement it once per type, which

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means, we can only pick the types for both Rhs

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and output once. To allow editing both points

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in i32 values to point, we have to

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refactor Rhs from an associated type to a

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generic type, which would allow us to

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implement a trait multiple times for point

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with different type arguments for Rhs.

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So, let us change the Rhs from an associated type

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to that of a generic type.

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First, we will change the definition of the trait by

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including a generic parameter of RHS.

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[No Audio]

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Next, we will comment out the Rhs type, since it is

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now being treated as generic. Next, we will

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remove the Self from the Rhs since it is now

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being treated as generic and not tied with

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the type itself for which we are implementing

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the trait anymore. Next, in the first

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implementation of the add, we will set the

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generic to that of a point, since this

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implementation is for the type point, where

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the generic should be treated as point.

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[No Audio]

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We will also remove the type Rhs equals to

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Point since that trait is no more using the

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Rhs associated type. In the same way, in the

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second implementation of the add, we will set

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the generic type to that of an i32 to let

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the compiler know that in this

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implementation, a generic of i32 should

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be used. Again we will remove the associated

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type, as it is no more required. You may have

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noted that, the generic allows us to have

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multiple implementation of the same trait for

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a single type, which is the type of point in

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this case. With associated type, it was not

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possible. In general, we will use the

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associated types when there is only a single

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implementation of the trait per type.

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However, if there are multiple

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implementations of the same types for a

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trait, then we will use the generics. Let us

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now use the implementation in the main, I will

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define a point first.

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[No Audio]

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Next, I will add an

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integer value to the point by calling the

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add function which is defined in the new

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implementation. Finally, let us use the

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assert to check if the value is updated correctly or not.

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[No Audio]

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Let us cargo run this now.

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[No Audio]

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Everything is fine up till now. Let us

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further extend the implementation by

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considering another case. Let's say we want

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to add a new type called line which contains

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two points. And now, there are there are some

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contexts within our program where adding two

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points should produce a line instead of a

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point. This means that the output is not

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necessarily, is not necessarily a point

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anymore, but can also be aligned. This is not

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possible, given the current design of the a

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trait where output is an associated type, but

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we can satisfy these new requirements by

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refactoring output from an associated type

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into generic type. Let us see how we can do this.

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Since the output can now be different

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in the different implementations, so

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therefore, we will consider it a generic

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type. So, let us change the definition of the trait.

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[No Audio]

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We will delete it as associated

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type from the trait definition.

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[No Audio]

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Finally, we will also remove

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the cell from the output.

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In the implementation now, we will use two generics

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instead of one. In the first implementation,

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both the Rhs and the output are points.

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[No Audio]

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We do not need the associated type of

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output in the trait implementation.

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In the second implementation, the Rhs

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is i32 and the output is a point.

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[No Audio]

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We will also remove the

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associated type of output.

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Now, to take care of the new requirements,

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we will first add a line struct

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with two fields of start and

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end, both of which will be of type Point.

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[No Audio]

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Next, we will add another implementation for

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adding a point to another point, the Rhs in

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this case will be a Point and the output will be a line.

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We will next add the definition for the add function.

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The input will be Self and

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the rhs and the output will be a line.

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[No Audio]

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We will return a Line with the start being equal

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to the point of Self, and the end being equal

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to the point and indicated by the right hand

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side. Let us now use it in the main.

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I will declare a couple of points.

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Next, I will call the add function.

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[No Audio]

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Please note that in this case, when I pass the

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parameter of p2, which is a point to the

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function, so both the implementation number

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one and implementation number three matches,

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where the right hand side is a point. This is

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because, in both the implementations, the

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function accepts a Self and a Point, so

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therefore, the compiler is confused. We need

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to somehow tell the compiler that I need to

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invoke the implementation number three, where

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I should get back a Line and not a Point.

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This tells the compiler now, that the user of

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the implementation wants to invoke the

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implementation, where the input is a point, and

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the output he is expecting is that of a line.

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In the same way, we will also need to mention

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the output type in the first call to the

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add function to help the compiler identify

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the correct implementation that needs to be

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invoked. Finally, I will add some assertion

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to make sure that our code is correct.

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[No Audio]

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Let us cargo run this now.

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With this, we end this

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tutorial. The key point to note is that, we

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need to use the associated types

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when there should only be a single

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implementation of the trait per type.

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However, we will prefer to use the generic

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types, when there can be many possible

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implementations of the trait per type. See you again

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and until next tutorial, happy Rust programming.