Name: Code vs Data (Metaprogramming) - Computerphile
Uploaded: 2020-03-27T18:23:35.000Z
Duration: 7 min 38 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

So I talked about how lisp lets you grow the language in such a way and let you add syntax and so on.

Adverbs of manner

And I want to unpack that a little bit and talk about this really fundamental concept, which is this duality between data code to traditionally separate things are actually more related than they kind of first appear.

We don't need Thio Super formally talk about it there.

Concepts that are that are pretty well understood by most people.

A few examples of data thieves could be numbers, for instance, like integers.

Almost every programming language has these almost every programming language rights on the same like 15 100 you could have symbols.

Some do recently was just such a the Julia programming language.

That's basically anything that you can name.

So the variable X, the variables e maybe even something that's not just letters like the multiplication symbol, maybe the addition symbol, just a quantity that's named in some way.

These are very simple, sort of atomic piece of data and list.

There are composite piece of data like lists, lists go together in, for example, Python.

You write a list with square brackets, one comma, two comma three and list.

You write it with the parenthesis and just separated out with spaces.

Um, there's sin Texan in most languages for that.

Anyway, we could go through a bunch of different examples, but data are sort of knowns.

They're they're just pieces of their just objects that are that are sort of just there.

Code, on the other hand, describes usually some type of computation that you want to dio.

So, for example, you want to do arithmetic or calculate an example of doing arithmetic is like computing one plus two times three.

This equals seven, and there's some evaluation process to it.

Another might be just displaying something.

For example, in C, we can print a string by using the print F function, but let's say we want to print the value of X and all these other things lots and lots of different code.

You can think of four loops, etcetera, etcetera is different for different ways of doing something of computing, something enlist.

There's a duality between these two things.

Both these things can be thought of at the same plane, and this is part of the reason why List gets its parentheses.

So in lisp, we write out a computation or a piece of data in actually the same way.

So maybe we want to look at this arithmetic here.

This enlist was written as plus one times 23 enlist.

Everything is written in prefix notation.

You always write the operator first, the verb that you're doing it in the operation's second.

So it's sort of similar to our p n, if you've heard of it, except it's reversed.

The special thing about this is that we can interpret it in two ways.

It looks like we're doing one plus two times three, but it can also be looked at his data than list.

Here we write out symbols just as their names here and so enlist.

We can look at it as data by using something in list called quote.

If we write enlist quote followed by this, then we get data.

We can write something called Evil followed by this, and it will be interpreted as code.

So really the same written representation of something.

The same parenthetical expression could be interpreted in both ways, and you can freely intermix both of these things in the same source file in the same source code.

So at any time you can sort of switch between the data representation, code representation, what does that give you?

That means you can then talk results of the computation and then made that part of the competition.

Yeah, I think it's I think it's very powerful.

We can start thinking about building data, building expressions, using functions to manipulate data.

And once we've built it, we can turn it into code to evaluated.

So really, the power we get out of this is we can write programs that themselves right programs.

We can start automating the work of a programmer by having our programs right programs.

Once it becomes a tool in your tool belt, it becomes a very extraordinarily powerful thing that, like you yourself is a programmer can start programming.

The way in which you think about programming, If that makes sense, is they're really super simple example.

So one example is is back in the day before four loops existed.

For instance, the usual four loop Britain, and see is something like this.

I equals zero eyes, less than 10 I plus plus.

What this will do is we'll repeat the body.

Whatever code we have here 10 times languages didn't always have a four loop in them.

I'm just going to say, said I to zero here.

Then the next thing you do is start executing your body.

And then you say, Like, if I is less than 10 then jumped to start.

And then you have a little label here called Start.

You can already see that There's a lot of busywork and even re unpacking.

The definition of a four loop was back in assembly days.

There is no way to write this in lists or any language that has met a programming.

You could imagine that if you have all these things that you can do in the language that we could just write a program that writes these out for us because we can write out data.

And then now we can re interpret this as code by using evil.

So enlist, you could have something like for eye from 0 to 10 and then some body of code right here.

And what we can have is this itself gets interpreted as a list of instructions like set I to zero a label here, start it's under etcetera.

I'm not gonna write it all out, But this idea that we can write programs that right programs weaken, build data and turn it into code so we can actually extend here.

I mean, four loops exist in every language, but you had to wait for the language designer to add the four loop for you.

Enlist, with this duality between data and code allows you to add these features yourself.

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