Um,why exactly does all this matter? So. It's not immediately clear. No. So typically the function is defined the global environment so that values of the free variables are just found in the user's workspace. So this is kind of the. The right thing to do is kind of what most people are expecting. If there's no, if, if there's, you can't find a value inside the function itself, you just look in the global environment. So this is the, the idea here is that you can define things like global variables, that will be common to a lot of different functions. That you might be defining in your workspace. so, but the key difference in R is that you can define functions inside of other functions. 'n so for example a function can return a function as the return value. So, in most functions they'll return a list, or a vector, or a matrix, or a data frame or something like that, but it is possible for a, for a function to return another function and then that, if that's the case then the, then the function that gets returned. It was defined inside of another function. So, it's an, the environment in which it was defined Is not the global environment. It's really the, the, the insides of this other function. So this is when things get interesting and this is when the scoping rules really have an impact on what you can do. So, I am going to define a very simple function here and often these kinds of functions come [UNKNOWN] where you might think of constructive functions. So, the idea that the function is constructing another function. So, here's what I want to, I want to create a function that that defines another, called make.power. And what make.power takes as input is a number n. Okay? So, and inside the make.power function I define another function called pow. And pow is going to take an argument called x. And and so what's going to happen is that the power function is going to take the, then the, take the argument X and raise to their power N, okay, and so make that power returns, with a function power as its return value and so you see inside the power function X is a, X is a function argument but that's not a problem, but n is a free variable because its not defined inside the power. Function. However, N is defined inside the make.power function and so since that's the environment in which the pow is defined. It will find the value of N. The pow, the power function will find the value of n inside this, it's other environment. So what happens is that I can call make.power and pass it a number like 3. And then, it will return a function, which I'll sign to be called cube. And, similarly, I can pass 2 to make that power and create a function that I'll call square. So, now, when I, when I pass cube, the number 3 What is it does is it raises 3 to the 3rd power, so I get 27. If I call square on the number 3, it, it raises three to the 2nd power, so it gives me 9. And so, so, so now, I've cons, I've got one function that can, that's capable of constructing many different types of functions, and by raising to pow, to various powers. So, how do you know what's in a function's environment? So you can, you can at the function, so, excuse me. You can look in the environment in which the function was defined, by calling the LS function. So if I call, if I call LS on On the environment for cube. You can see that inside the cube function, there's, there's something, there's an, there's an object called N. And if I use get on N you'll see that the value of N is equal to 3. So that's how the power function knows to raise it to the 3rd, to the 3rd power. Excuse me, that's how the cube function knows how to, knows to raise the argument to the 3rd power because it's already defined. In it's, in it's, in it's, closure environment. Similarly the environment for square, you can see it has the exact same objects in it. But now the value of n is equal to 2, in the square function. So, so, I want to make one brief comparison between lexical scoping, which is what R does, and dynamic scoping, which is what maybe some other function, some other programing languages implement. So here I've got, I'm assigning the value of Y equal to 10. Then create a function F, which takes, as an argument, X. And then, it assigns, there it assigns Y equal to 2, it squares Y and then adds G of X. So, what's G? G is another function, which takes as an argument called X, and it multiplies X times Y. So, in the F function, Y is a free variable, and G is also a free variable. So, the G function is not defined. Inside of F of or, it's, it, of, argument to F. Then in the G function, then the var-, the symbol Y is a free variable. And so the question is if I call f of 3 what gets returned? So with lexical scoping, the value of Y and the function G is looked up in the environment in which the function was defined. Which in this case was the global environment. So that the value of Y and the G function is 10. So with dynamic scoping the value of Y is looked up in the environment from which the function was called; sometimes called the calling environment. So in the R the calling environment is known as is what's called the parent frame. In this case the calling environment Y was defined to be 2 and so the value of Y would be 2. So. Calling the function F would produce different answers depending on whether you use lexical scoping or dynamic scoping. So, the one thing that, that, that will make lexical scoping and dynamic scoping look the same is that when a function is defined in the global environment and is subsequently called from the global environment, then the defining environment and the calling environment are exactly the same and so this can sometimes give the appearance of dynamic scoping even when It doesn't exist. So here I've got a function called G. It takes an argument X. It assigns A to be equal to 3. And then it adds X plus A plus Y. So, in this case, X is a function is a formal argument. A is a local variable so it's not a formal argument, but I defined it inside the function. Then so, that's okay. And then Y is a free variable, okay? So if I call G of 2, the function G is going to look for the value of Y in the global environment. If I haven't yet defined Y then there has to be an error because it doesn't know what value to assign to the symbol of Y. So that's what I get in this line here. Now if I define what Y is, say I assign it to be 3, if I call it G of 2, then it returns 8 because now it's able to find Y in the global environment. So even though it looks like the value of Y was looked up in the calling environment, it's actually the defining environment because G happened to be defined in the global environment so, there are a number of other languages that support lexical scoping. Some examples are things like Scheme, Perl, Python, and Common Lisp. And of course there's a, a well known computer science theorem which is that all languages eventually converge to Lisp. And so it's, it's not a, it's not an obscure type of feature. It's actually very common in a number of other programming languages. So, one of the main consequences of lexical scoping in R is that all the objects have to be stored in memory. So, if you're working with a programming language that has very small objects this generally speaking not a big problem. but. Because of nature of the scoping rules and because of the complexity of the environment and the, the way they are all linked together, it's difficult to implement this type of model outside of physical memory, and so. So the consequence was that, when R was originally designed. Everything was stored in memory. Things are getting complicated now, because of very large types of data sets. And, being able to read them into R. It is a challenge. Everything has to be stored in memory. Second now, so every function has a carrier pointer to its respect, to its defining environment. and, and that defining environment could literally be anywhere because there could be functions within functions and then the, and if you do, if one function returns another function, then there has, there has to be a pointer to that piece of memory where the defining environment is stored. And so this makes the model a little bit more complex but but, but all the more useful. So, the, in S plus, which was kind of the original implementation of the S language, the free variable were always looked up in the workspace. Everything could be stored on the disk, because the defining environment of all the functions was the same.
