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In this video, let's add an extension to the Int type and define a function that accepts a math operation.

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Let's take a look at another example.

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Now keep in mind these examples aren't particularly useful bits of code, but

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we're going to use them to reinforce our point.

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What is the point of writing a function that accepts another function

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as an argument?

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Well, it allows us to write a general function, on a type,

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where we customize and defer the implementation when we call the function.

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This concept is also known as a higher order function.

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So let's write an extension on the int type.

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I want to define a method on int

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that applies a math operation to the integer value.

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What math operation are we applying?

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Well I don't know right now, and that's exactly the point.

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We'll define this method in such a way that we can apply any math operation we

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can conceive of when we want, as long as it involves integers.

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So I'll say extension Int, and

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we're going to write a new function, we'll call this applyOperation.

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And this function will have a single parameter operation

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without an external argument label.

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The type of operation will be a function type,

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because we want to be able to pass in any math operation when using apply operation.

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We'll define this parameter, this argument, as a function that takes

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an integer, remember the parameters have to be enclosed in parentheses.

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And then it returns an integer as well, okay.

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So applyOperation here, takes an operation,

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which is a function that takes an int and returns an int.

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When we apply this operation to an integer value we're going to get an int back,

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as we can see from the return type.

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And we want to return this Int as well, so the final return type for

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the entire function will be an Int inside the body of apply,

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we're simply going to call the function that we provide as an argument.

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Now operation, which is a constant holding on to that function,

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expects an integer value as an argument.

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And since we're doing this as an extension on Int itself,

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we're going to pass in self as the argument.

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Now the result of calling operation on self is an integer as well, so

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we can go ahead and return it.

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So pretty simple, we've defined a function that applies some math operation

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on an integer, where we haven't actually defined any particular math operation.

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Since we can now provide a function that matches the signature as an argument,

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we've deferred defining the actual operation until we call this function.

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What apply operation does is ensure that the function

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we provide as an argument is called and a value is returned.

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We can see this in action by defining a simple math operation that

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doubles an integer value, so say func double.

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This is going to take a value of type Int and

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return an integer, inside the body we'll say return value * 2.

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Notice that this signature Int to Int of double matches the parameter

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type of the applyOperation function.

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It is a function that takes an integer and returns an integer.

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We can now use this with the applyOperation function.

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Remember this is an extension on the Int type, so we first write out an integer,

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and then we call applyOperation and we can pass in double

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as the argument and notice in the results area we get 20.

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The double method is really simple, of course, and

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we don't even need a method for this, but

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it highlights how we can pass in a function to apply and get a result.

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The beauty though of this applyOperation function,

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is that it can take any function or any operation that matches the signature.

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Allowing us to define the operation we implement when we call the function.

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So for example, we can define a new function closestMultipleOfSix,

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this function takes an integer and returns an integer,

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and in here we'll just do a bit of math.

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We'll say loop over the first 100 multiples of 6.

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We'll grab that multiple by doing x * 6,

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and if the multiple is in range.

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We'll just do a bit of math here, what I'm doing doesn't, isn't really important.

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All that matters is that it is a different kind of operation.

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Return multiple else if multiple == value returned value and

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then finally down here I'll return 0.

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You can just copy this as it is.

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Okay, now that we have this new operation we can use it with the apply method, so

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I can save 32.applyOperation closestMultipleOfSix and

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it;ll find me the closest multiple of six, which is 36.

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I can do that on 12, closestMultipleOfSix should be 12, there we go.

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And we'll do it on 200, and we should get a correct value.

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Look at that, now you know that you can use functions as

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arguments to other functions, and more importantly this should give you

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an example of why you would want to do that in the first place.
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