WEBVTT 1 00:00:00.120 --> 00:00:02.720 Have you ever felt, you know, like programming languages are 2 00:00:02.720 --> 00:00:05.519 handed down from on high by these unapproachable wizards. 3 00:00:05.599 --> 00:00:08.919 Oh. Absolutely, Like there's this mystical council of language designers 4 00:00:08.919 --> 00:00:09.880 somewhere right. 5 00:00:09.880 --> 00:00:12.359 And you, the lowly programmer, you just have to accept 6 00:00:12.359 --> 00:00:13.480 their arbitrary rules. 7 00:00:13.599 --> 00:00:17.640 Yeah, the rules about type systems or memory management or 8 00:00:17.719 --> 00:00:18.640 scope exactly. 9 00:00:18.679 --> 00:00:22.320 And it's a super pervasive mindset in software engineering. 10 00:00:22.519 --> 00:00:24.480 Yeah, it really is. I mean we're conditioned from our 11 00:00:24.559 --> 00:00:28.719 very first Hello World tutorial to view a programming language 12 00:00:28.760 --> 00:00:31.440 as this, well, this unyielding law of physics. 13 00:00:31.519 --> 00:00:34.359 Yeah. You just memorize the syntax, you parse the documentation, 14 00:00:34.600 --> 00:00:37.719 and you try your best to operate inside the sandbox 15 00:00:37.759 --> 00:00:38.679 that somebody else. 16 00:00:38.479 --> 00:00:41.479 Built for you, which is incredibly limiting, honestly it is. 17 00:00:42.240 --> 00:00:46.159 So today's deep dive is about entirely shattering that illusion. 18 00:00:46.520 --> 00:00:51.679 We're using the classic computer science text Essentials of Programming Languages, 19 00:00:51.719 --> 00:00:55.920 specifically the third edition by Daniel P. Friedman and Mitchell Wan, 20 00:00:56.079 --> 00:00:58.520 a total classic. Yeah, and we are going back to 21 00:00:58.600 --> 00:01:03.000 the foundational mechanism of language design. We're exploring how an 22 00:01:03.039 --> 00:01:05.480 interpreter is actually, when you get down to it, just 23 00:01:05.560 --> 00:01:09.359 another program, right, It's not magic exactly, and mastering that 24 00:01:09.560 --> 00:01:13.359 concept shifts your mindset from being a mere consumer of 25 00:01:13.400 --> 00:01:17.400 a language to being an architect. Okay, let's unpack this. 26 00:01:18.439 --> 00:01:20.879 Where do we even begin with a paradigm shift like. 27 00:01:20.840 --> 00:01:24.719 That, Well, we start by dismantling that artificial wall, you know, 28 00:01:24.799 --> 00:01:27.159 the one between a program and a programming language. 29 00:01:27.280 --> 00:01:27.400 Right. 30 00:01:28.000 --> 00:01:31.200 In the forward to the book, hal Abelson shares this 31 00:01:31.400 --> 00:01:35.040 really revealing story about his early career. He was building 32 00:01:35.040 --> 00:01:39.319 this massive data entry and information retrieval system using PLI. Yeah, 33 00:01:39.439 --> 00:01:42.920 using PLI, which was this notoriously heavy, feature rich language 34 00:01:42.920 --> 00:01:43.599 at the time. 35 00:01:43.480 --> 00:01:46.480 Right, and he viewed PLI as this rigid rule book like. 36 00:01:46.560 --> 00:01:48.840 His job, as he saw it back then, was to 37 00:01:48.959 --> 00:01:52.680 pick through this massive manual, select predefined features, and then 38 00:01:52.680 --> 00:01:55.959 painstakingly fit them together into a monolith exactly. 39 00:01:56.280 --> 00:01:59.719 He describes a resulting program as a mosaic. Mosaic yeah, 40 00:02:00.000 --> 00:02:02.959 meaning every individual piece of logic had to be carefully 41 00:02:03.040 --> 00:02:06.439 chiseled to fit exactly into place. It was fragile, it 42 00:02:06.519 --> 00:02:08.080 was inflexible. 43 00:02:07.560 --> 00:02:11.400 Because he never considered that there was an underlying architectural 44 00:02:11.439 --> 00:02:15.400 logic to how PLI was organized, or that he actually 45 00:02:15.479 --> 00:02:19.319 had the agency to override those original designer's decisions right to. 46 00:02:19.280 --> 00:02:22.039 Better suit his own domain. But you don't question the 47 00:02:22.080 --> 00:02:23.360 wizards who wrote the compiler. 48 00:02:23.520 --> 00:02:26.840 No, you just pray you don't trigger a syntax error, right. 49 00:02:27.439 --> 00:02:31.360 But the revelation Ableson ultimately shares is that the interpreter 50 00:02:31.439 --> 00:02:35.879 for a computer language is literally just another program executing instructions. 51 00:02:36.120 --> 00:02:39.319 It's just code, just code. I mean, almost every complex 52 00:02:39.360 --> 00:02:42.719 software system you interact with today, whether it's a sophisticated 53 00:02:42.759 --> 00:02:46.439 CAD tool, or a modern web browser's rendering engine, or 54 00:02:46.479 --> 00:02:49.280 even the logic loop of a video game engine, they 55 00:02:49.319 --> 00:02:51.039 all contain an embedded interpreter. 56 00:02:51.199 --> 00:02:53.080 Oh wow, I guess I never thought about it like that. 57 00:02:53.360 --> 00:02:57.400 Yeah, that interpreter acts as the dynamic glue, allowing users 58 00:02:57.400 --> 00:03:02.439 to combine individual operations into customs patterns. And what's fascinating 59 00:03:02.439 --> 00:03:05.879 here is that once you internalize this, you realize that 60 00:03:05.919 --> 00:03:09.599 advanced programmers don't just write monolithic programs in one fixed 61 00:03:09.919 --> 00:03:14.120 general purpose language they don't know they view themselves as 62 00:03:14.199 --> 00:03:20.479 creators of custom embedded sub languages, like domain specific languages 63 00:03:20.879 --> 00:03:23.439 tailored specifically for the problem they're trying to solve. 64 00:03:23.560 --> 00:03:26.759 Okay, I picture the traditional mindset like building a Lego 65 00:03:26.840 --> 00:03:29.840 set using only the instruction manual that came in the box. 66 00:03:29.919 --> 00:03:30.960 That's a great way to look at it. 67 00:03:31.039 --> 00:03:33.560 Yeah, you follow the predetermined steps, you build the specific 68 00:03:33.639 --> 00:03:37.000 spaceship on the cover, and you stop. But this master 69 00:03:37.120 --> 00:03:40.199 programmer mindset they're describing, it's like having a plastic molding 70 00:03:40.240 --> 00:03:42.000 machine in your living room exactly. 71 00:03:42.080 --> 00:03:43.840 You're making your own pieces, right. 72 00:03:43.719 --> 00:03:48.000 You're inventing entirely new Lego shapes with custom interlocking mechanisms 73 00:03:48.039 --> 00:03:51.120 to solve a problem the original Lego designers never even anticipated. 74 00:03:51.599 --> 00:03:55.080 But wait, if we're inventing our own Lego bricks, how 75 00:03:55.080 --> 00:03:58.360 do we systematically describe their shapes? I mean, we can't 76 00:03:58.360 --> 00:04:00.439 just throw melted plastic at the wall and hope a 77 00:04:00.520 --> 00:04:01.759 valid expression emerges. 78 00:04:01.960 --> 00:04:04.800 Well, no, you definitely can't do that. To build a 79 00:04:04.879 --> 00:04:08.439 language that a computer can actually execute, we first need 80 00:04:08.520 --> 00:04:12.919 a mathematically rigorous way to define the data, the new 81 00:04:13.039 --> 00:04:16.480 brex that our language is going to parse and manipulate. 82 00:04:16.600 --> 00:04:18.160 Okay, and this is where the math comes in. 83 00:04:18.360 --> 00:04:21.680 Yeah. The book grounds this in the concepts of inductive 84 00:04:21.720 --> 00:04:23.600 definitions and grammars. 85 00:04:23.279 --> 00:04:26.639 Which involves defining complex structures from the ground up using 86 00:04:26.759 --> 00:04:29.240 rules of inference. So, if you're listening to this and 87 00:04:29.240 --> 00:04:32.639 trying to visualize it. Picture like a horizontal line on 88 00:04:32.680 --> 00:04:36.040 a talkboard. Above that line you write your hypotheses, the 89 00:04:36.079 --> 00:04:40.040 axioms or previously proven facts you know are true. Below 90 00:04:40.079 --> 00:04:43.560 the line, you write your conclusion. If the conditions above 91 00:04:43.600 --> 00:04:46.120 the line are met, the statement below the line is 92 00:04:46.160 --> 00:04:48.800 mathematically guaranteed to be true exactly. 93 00:04:49.240 --> 00:04:51.439 And by using these rules of inference, you build these 94 00:04:51.480 --> 00:04:55.600 complex nested data structures out of simpler atomic ones. And 95 00:04:55.800 --> 00:05:01.519 to handle this systematically for programming languages, the book Formal Grammars, 96 00:05:01.680 --> 00:05:04.560 right the grams. Yeah, So a grammar consists of non 97 00:05:04.720 --> 00:05:09.000 terminal symbols, which represent the abstract sets of things being defined, 98 00:05:09.480 --> 00:05:11.000 like an expression or a statement. 99 00:05:11.160 --> 00:05:11.560 Got it. 100 00:05:11.600 --> 00:05:14.800 Then you have terminal symbols, which are the literal strings 101 00:05:14.879 --> 00:05:16.439 or characters in the source code. 102 00:05:16.480 --> 00:05:20.040 So a terminal symbol is your literal parenthesis, your comma, 103 00:05:20.279 --> 00:05:23.560 or your your let keyword yes exactly. 104 00:05:23.800 --> 00:05:26.600 And then finally you have productions, which are the actual 105 00:05:26.680 --> 00:05:29.759 rules connecting them. They're typically denoted with a double colon 106 00:05:29.879 --> 00:05:32.959 and an equal sign along with notation like the clean 107 00:05:33.040 --> 00:05:35.600 star for you know, zero or more repetitions. 108 00:05:35.680 --> 00:05:38.240 Okay, So a non terminal is the overarching concept like 109 00:05:38.279 --> 00:05:41.480 a list of integers, and the production rule dictates that 110 00:05:41.600 --> 00:05:44.079 a list of integers can be an empty list, or 111 00:05:44.120 --> 00:05:46.639 it can be a single integer attached to another list 112 00:05:46.639 --> 00:05:47.040 of injuries. 113 00:05:47.199 --> 00:05:50.600 Exactly. It's inherently recursive, right, And that recursion brings us 114 00:05:50.600 --> 00:05:53.800 to the first really monumental takeaway slogan of the book, 115 00:05:53.879 --> 00:05:55.800 which is follow the grammar. 116 00:05:55.959 --> 00:05:57.720 Follow the grammar. It sounds like a mantra. 117 00:05:58.160 --> 00:06:01.759 It is a mantra, but it's also the architectural secret 118 00:06:01.800 --> 00:06:08.199 to writing code that processes complex nested data safely. How So, well, 119 00:06:08.199 --> 00:06:11.959 when you write procedures that operate on inductively defined data, 120 00:06:12.600 --> 00:06:16.120 the control flow of your interpreter must perfectly mirror the 121 00:06:16.160 --> 00:06:17.920 recursive structure of your grammar. 122 00:06:18.319 --> 00:06:19.959 Oh perfectly mirror it. 123 00:06:20.199 --> 00:06:24.560 Yes, For every single non terminal in your language's grammar, 124 00:06:24.839 --> 00:06:29.079 you write exactly one corresponding procedure, and inside that procedure, 125 00:06:29.120 --> 00:06:32.319 you write one specific branching alternative, like a case in 126 00:06:32.360 --> 00:06:36.879 the switch statement, for every single production rule that defines 127 00:06:36.879 --> 00:06:38.240 that nonterminal. 128 00:06:37.720 --> 00:06:40.800 And the authors pair that with a second equally strict slogan. Right, 129 00:06:41.240 --> 00:06:44.680 no mysterious auxiliaries, yes, meaning, if you introduce a helper 130 00:06:44.680 --> 00:06:47.639 function into your interpreter, like maybe something tracking the current 131 00:06:47.680 --> 00:06:51.160 index position. While you're traversing an abstract syntax tree. You 132 00:06:51.199 --> 00:06:54.240 are required to formally specify the behavior of that function 133 00:06:54.399 --> 00:06:57.560 on all possible inputs, not just the initial starting state. 134 00:06:57.680 --> 00:06:59.120 Precisely, you have to be exhausted. 135 00:06:59.240 --> 00:07:02.120 I have to admit, as developer, my instinct is to push. 136 00:07:01.879 --> 00:07:03.000 Back on this. Oh really, why? 137 00:07:03.160 --> 00:07:06.199 It just feels unbelievably pedantic, Like if I write a 138 00:07:06.240 --> 00:07:09.680 complex grammar for a binary search tree, are you telling 139 00:07:09.720 --> 00:07:12.519 me I must write a dedicated function for the interior 140 00:07:12.560 --> 00:07:16.120 nodes and a completely separate dedicated function just for the 141 00:07:16.199 --> 00:07:18.439 leaf nodes, even if they share some logic. 142 00:07:18.680 --> 00:07:20.560 Yes, that's exactly what they're saying. 143 00:07:20.720 --> 00:07:23.199 It feels like baking a three tier cake and being 144 00:07:23.240 --> 00:07:27.399 told my recipe must have explicitly separate standalone chapters for 145 00:07:27.519 --> 00:07:29.759 the bottom tier, the middle tier, and the top tier, 146 00:07:30.040 --> 00:07:32.240 otherwise the kitchen will literally catch fire. 147 00:07:32.879 --> 00:07:36.759 This raises an important question, why demands such strict, pedantic 148 00:07:36.879 --> 00:07:38.199 rigidity from the programmer? 149 00:07:38.319 --> 00:07:38.879 Yeah? Why? 150 00:07:39.279 --> 00:07:42.160 The answer lies in the mechanics of a mathematical concept 151 00:07:42.199 --> 00:07:47.560 called structural induction, which is honestly your only defense against catastrophic, 152 00:07:47.600 --> 00:07:49.920 hard to trace bugs in compiler design. 153 00:07:50.040 --> 00:07:52.360 Okay, lay the mechanics of structural induction on me. 154 00:07:52.519 --> 00:07:56.519 So when your code structure perfectly mirrors your grammar's structure, 155 00:07:56.920 --> 00:07:59.920 you unlock the ability to mathematically prove your program's care. 156 00:08:00.600 --> 00:08:00.800 Yeah. 157 00:08:00.959 --> 00:08:04.560 Yeah. Structural induction works in two steps. First, you prove 158 00:08:04.600 --> 00:08:07.839 your procedure works on the base cases, the simplest, smallest 159 00:08:07.839 --> 00:08:11.160 pieces of data, like an empty list or the absolute 160 00:08:11.240 --> 00:08:12.600 leaf of a binary tree. 161 00:08:12.720 --> 00:08:14.480 Right, the absolute boto right. 162 00:08:14.800 --> 00:08:19.040 Second, you prove the combination step, meaning, assuming the procedure 163 00:08:19.079 --> 00:08:21.720 works on a small subset of the tree, you prove 164 00:08:21.759 --> 00:08:24.959 it will correctly process a node that combines those subsets. 165 00:08:25.120 --> 00:08:28.160 Okay, I see. By forcing the interpreter's code to look 166 00:08:28.199 --> 00:08:32.919 exactly like the data's grammar, you automatically eliminate those bizarre 167 00:08:33.039 --> 00:08:36.960 edge cases where nested structures mysteriously cause a stack overflow 168 00:08:37.159 --> 00:08:38.559 or just you know, silently fake. 169 00:08:38.720 --> 00:08:41.320 Exactly, if the base cases are sound and the combination 170 00:08:41.440 --> 00:08:44.200 logic is sound, the entire infinite tree is sound. 171 00:08:44.240 --> 00:08:47.600 But once we have defined all this incredibly detailed data, 172 00:08:47.639 --> 00:08:50.279 I certainly don't want to look at those messy structural 173 00:08:50.320 --> 00:08:53.279 details every time I write an evaluation step. I mean, 174 00:08:53.360 --> 00:08:56.639 when building a language. We need a mechanism to hide 175 00:08:56.679 --> 00:08:58.399 that underlying complexity, don't. 176 00:08:58.240 --> 00:09:01.600 We we do, and that requirement pivots us directly into 177 00:09:01.720 --> 00:09:06.279 data abstraction. Data abstraction is the practice of strictly dividing 178 00:09:06.279 --> 00:09:09.879 our data types into a public interface and a hidden implementation. 179 00:09:10.720 --> 00:09:14.480 So the interface dictates what the data can do, the 180 00:09:14.519 --> 00:09:17.720 constructors used to build it in memory, and the observers 181 00:09:17.799 --> 00:09:21.200 used to extract information from it, and the implementation is 182 00:09:21.200 --> 00:09:24.519 the raw memory management and pointer logic operating under. 183 00:09:24.320 --> 00:09:27.879 The hood, exactly. And to demonstrate the power of this separation, 184 00:09:28.519 --> 00:09:32.039 Friedman and Wand used this great conceptual example of natural numbers. 185 00:09:32.120 --> 00:09:35.279 Oh yeah, the interface for a natural number requires very little. 186 00:09:35.559 --> 00:09:38.440 You just need a constructor for zero, an observer to 187 00:09:38.519 --> 00:09:41.360 check if a value is zero, and constructors to generate 188 00:09:41.399 --> 00:09:43.679 the successor or predecessor of a given number. 189 00:09:43.840 --> 00:09:46.240 Right, So the client code just calls successor five and 190 00:09:46.320 --> 00:09:48.440 expects six exactly, But. 191 00:09:48.399 --> 00:09:51.840 The underlying implementation of those natural numbers can be wildly different. 192 00:09:52.200 --> 00:09:56.120 The book introduces a unary representation where numbers are essentially 193 00:09:56.200 --> 00:09:59.200 represented as linked lists of boolean values. 194 00:10:00.039 --> 00:10:02.440 Low my mind. So under the hood, the number two 195 00:10:02.519 --> 00:10:05.960 isn't a binary integer it's literally a list containing two 196 00:10:06.279 --> 00:10:07.080 true values. 197 00:10:07.159 --> 00:10:09.240 Right and zero is just an empty list. 198 00:10:09.159 --> 00:10:11.360 Which is incredibly inefficient for computation. 199 00:10:11.759 --> 00:10:16.320 Oh terribly inefficient, but it flawlessly satisfies the required interface. 200 00:10:16.840 --> 00:10:20.519 A more practical implementation they discuss is big nums. So 201 00:10:20.639 --> 00:10:24.519 instead of relying on the host machine's standard memory registers, 202 00:10:24.759 --> 00:10:28.360 which might overflow if an integer exceeds sixty four bits, 203 00:10:29.080 --> 00:10:33.320 big numbs represent massive integers as lists of smaller digits 204 00:10:33.360 --> 00:10:36.519 in a very large base. Wow. This allows your language 205 00:10:36.559 --> 00:10:40.360 to handle astronomical calculations without the client code ever knowing 206 00:10:40.399 --> 00:10:42.559 the memory allocation strategy actually changed. 207 00:10:42.600 --> 00:10:45.840 The rest of the interpreter remains blissfully ignorant. It asks 208 00:10:45.879 --> 00:10:48.159 for a number, and it gets a number. But here's 209 00:10:48.200 --> 00:10:50.919 where it gets really interesting, at least for me. It 210 00:10:51.000 --> 00:10:55.360 was when the authors applied this abstraction concept to environments. Oh. 211 00:10:55.440 --> 00:10:58.960 Yes, environments are crucial. They're the mechanical backbone of lexical 212 00:10:59.000 --> 00:11:00.039 scoping in any. 213 00:11:00.559 --> 00:11:03.679 So, an environment is simply the data structure that associates 214 00:11:03.759 --> 00:11:07.440 variable names with their bound values. Like when your language 215 00:11:07.440 --> 00:11:10.600 evaluates an expression containing the variable X, it queries the 216 00:11:10.679 --> 00:11:13.200 environment to retrieve the value currently bound to X. 217 00:11:13.639 --> 00:11:16.759 Right and you could implement an environment as an a list, 218 00:11:17.080 --> 00:11:20.919 which is just a linear list of paired variable value temples, okay. 219 00:11:21.120 --> 00:11:24.080 Or you could use a ribcage structure where you maintain 220 00:11:24.159 --> 00:11:27.600 one list of variables running parallel to a separate list 221 00:11:27.639 --> 00:11:28.919 of corresponding values. 222 00:11:29.080 --> 00:11:32.159 But then the book introduces the procedural representation. 223 00:11:32.720 --> 00:11:36.200 Break down the mechanics of the procedural representation because it's brilliant. 224 00:11:36.399 --> 00:11:40.360 Okay. So instead of representing an environment as passive data 225 00:11:40.399 --> 00:11:43.919 sitting in memory, an environment can be represented by its action. 226 00:11:44.799 --> 00:11:49.399 Invoking the environment actually executes a first class procedure that 227 00:11:49.519 --> 00:11:53.240 was constructed dynamically for the sole purpose of remembering a 228 00:11:53.279 --> 00:11:55.080 specific variable binding. 229 00:11:55.200 --> 00:11:55.799 It's so cool. 230 00:11:55.919 --> 00:11:58.960 Think of it like this. The traditional data structure approach 231 00:11:59.240 --> 00:12:02.679 is writing a friends addressed down on a physical Rolodex card. 232 00:12:02.799 --> 00:12:05.679 When you need it, you manually iterate through the kurds. 233 00:12:05.519 --> 00:12:06.480 Right flipping through them. 234 00:12:06.559 --> 00:12:10.679 Yeah, but the procedural representation is like hiring a highly 235 00:12:10.720 --> 00:12:14.120 trained smart assistant whose single purpose in the universe is 236 00:12:14.159 --> 00:12:16.120 to stand in your office and wait for you to ask, 237 00:12:16.320 --> 00:12:17.320 what is John's address? 238 00:12:17.480 --> 00:12:18.320 Huh? Exactly? 239 00:12:18.399 --> 00:12:22.039 When queried, they immediately execute their function and speak the 240 00:12:22.080 --> 00:12:25.759 answer you aren't storing the raw string data. You are 241 00:12:25.799 --> 00:12:29.840 storing an executable action that yields the data, and that. 242 00:12:29.840 --> 00:12:34.960 Assistant is literally a closure holding onto state. You are 243 00:12:35.120 --> 00:12:39.799 exploiting the advanced features of your host language, specifically higher 244 00:12:39.919 --> 00:12:43.799 order functions, to create a living computational mapping. 245 00:12:44.039 --> 00:12:48.360 But suppose your underlying implementation language like C doesn't support 246 00:12:48.399 --> 00:12:50.519 higher order functions, you can't hire the. 247 00:12:50.480 --> 00:12:52.039 Smart assistant, right, You're stuck. 248 00:12:52.240 --> 00:12:54.679 You have to take the exact instructions that assistant would 249 00:12:54.679 --> 00:12:58.519 have followed, encode them into a concrete structor record, and 250 00:12:58.679 --> 00:13:02.440 pass that passive data strepsture to a dedicated apply procedure function. 251 00:13:02.840 --> 00:13:07.200 And that mechanical transformation is the technique called defunctionalization. 252 00:13:07.519 --> 00:13:11.639 So by defunctionalizing, we convert dynamic procedural behavior back into 253 00:13:11.639 --> 00:13:12.960 static data structures. 254 00:13:13.000 --> 00:13:16.639 Exactly, we're separating the pure executable meaning of the code 255 00:13:16.919 --> 00:13:19.279 from the way it's actually represented in memory. 256 00:13:18.960 --> 00:13:22.960 Which perfectly maps to the broader architectural separation of abstract 257 00:13:23.000 --> 00:13:27.279 syntax from concrete syntax. Yes, exactly, Concrete syntax is the 258 00:13:27.440 --> 00:13:31.600 literal texts the programmer types, the brackets, the semicolons. The 259 00:13:31.720 --> 00:13:34.679 parser reads that text and converts it into an abstract 260 00:13:34.720 --> 00:13:37.759 syntax tree, stripping away the formatting and leaving only the 261 00:13:37.799 --> 00:13:38.639 structural meaning. 262 00:13:38.919 --> 00:13:43.440 And by combining data abstraction, defunctionalized environments, and abstract syntax trees, 263 00:13:43.960 --> 00:13:47.080 you unlock what the book terms the interpreter recipe. 264 00:13:47.120 --> 00:13:50.080 The interpreter recipe which dictates the control flow of the 265 00:13:50.240 --> 00:13:54.000 entire system. Step one, look at a piece of abstract 266 00:13:54.000 --> 00:13:58.720 syntax data. Step two, determine its grammatical type. Step three 267 00:13:59.159 --> 00:14:03.360 extract its underline components using the observer interface, and recursively 268 00:14:03.399 --> 00:14:04.799 apply the correct behavior. 269 00:14:05.080 --> 00:14:08.120 And armed with procedural environments to manage state and that 270 00:14:08.320 --> 00:14:11.679 interpreter recipe to traverse the syntax trees, we finally have 271 00:14:11.759 --> 00:14:15.120 the architectural runway to dissect the actual mini languages constructed 272 00:14:15.120 --> 00:14:15.519 in the book. 273 00:14:15.639 --> 00:14:17.840 Yes, this is where we bring the theory to life. 274 00:14:18.159 --> 00:14:21.399 The book systematically upgrades our capability by walking through three 275 00:14:21.519 --> 00:14:25.200 progressive languages LET, praio, C, and LITREK. 276 00:14:25.519 --> 00:14:29.720 Let's start with LED. The LED language establishes our baseline 277 00:14:29.759 --> 00:14:33.399 for local scope. It introduces the let expression, allowing a 278 00:14:33.440 --> 00:14:37.120 programmer to bind a variable to a computed value, but 279 00:14:37.200 --> 00:14:39.480 strictly within an isolated block of code. 280 00:14:39.559 --> 00:14:42.519 Right, So, when a let expression is evaluated, the interpreter 281 00:14:42.639 --> 00:14:45.919 takes the current environment, creates a new extended environment containing 282 00:14:45.919 --> 00:14:49.600 the new binding, and evaluates the inner block of code using. 283 00:14:49.399 --> 00:14:53.080 That new environment exactly. And once the block finishes, that 284 00:14:53.200 --> 00:14:57.519 extended environment is just discarded and the temporary variable vanishes. 285 00:14:57.679 --> 00:15:01.159 It's elegant memory management, but lee T is just the foundation. 286 00:15:01.720 --> 00:15:05.240 The real paradigm shift arrives with the PROC language. 287 00:15:05.240 --> 00:15:09.600 Oh PROC is huge. PROC elevates procedures from static top 288 00:15:09.679 --> 00:15:12.559 level definitions into first class express. 289 00:15:12.240 --> 00:15:15.480 Values, meaning in PRC, a function is treated exactly like 290 00:15:15.519 --> 00:15:16.879 an integer or a boolean. 291 00:15:17.080 --> 00:15:19.600 Yes, you can pass a function as an argument to 292 00:15:19.639 --> 00:15:22.320 another function, you can return a function from a computation, 293 00:15:22.840 --> 00:15:24.399 or bind a function to a variable. 294 00:15:24.519 --> 00:15:26.799 But to make first class functions work, we have to 295 00:15:26.799 --> 00:15:31.039 confront the mechanics of lexical scope enclosures. The authors emphasize 296 00:15:31.039 --> 00:15:34.320 that a procedure's behavior must be governed by the environment 297 00:15:34.360 --> 00:15:37.519 that existed at the exact moment the procedure was created, 298 00:15:37.960 --> 00:15:41.159 not the environment where it happens to be executed later, and. 299 00:15:41.120 --> 00:15:44.519 The underlying data structure that enforces this rule is called 300 00:15:44.519 --> 00:15:48.519 a closure. When the interpreter evaluates a procedure definition it 301 00:15:48.559 --> 00:15:50.120 allocates a closure in memory. 302 00:15:50.159 --> 00:15:51.000 Okay, what's in it? 303 00:15:51.600 --> 00:15:56.159 That closure captures three distinct pieces of data, the formal parameter, 304 00:15:56.240 --> 00:15:59.759 which is the input variable name, the abstract syntax tree 305 00:15:59.799 --> 00:16:03.320 of the procedure's body, and crucially, a pointer to the 306 00:16:03.360 --> 00:16:06.279 exact active environment at that moment of creation. 307 00:16:06.519 --> 00:16:09.559 You know, while studying this, I initially questioned the necessity 308 00:16:09.559 --> 00:16:13.120 of that captured environment pointer. Really yeah, I kept thinking, 309 00:16:13.159 --> 00:16:16.320 why wouldn't a function just grab the closest available variable 310 00:16:16.360 --> 00:16:19.279 bindings from the call stack when it is actually executed. 311 00:16:19.639 --> 00:16:22.919 That avoids storing the environment entirely? Ah? I see, And 312 00:16:22.960 --> 00:16:26.360 the alternative I was imagining is actually known as dynamic scope. 313 00:16:26.240 --> 00:16:30.279 Right, and the authors include exercises exploring dynamic scope to 314 00:16:30.360 --> 00:16:33.879 demonstrate exactly why it's considered an anti pattern in modern 315 00:16:33.960 --> 00:16:35.200 language design. Yeah. 316 00:16:35.240 --> 00:16:38.120 I came up with an analogy for this. Lexical scoping 317 00:16:38.320 --> 00:16:41.240 is like packing a highly specific lunch box in your 318 00:16:41.240 --> 00:16:42.720 own kitchen before you leave for the day. 319 00:16:42.840 --> 00:16:43.480 Okay, I like this. 320 00:16:43.840 --> 00:16:46.440 It doesn't matter if you travel to a corporate office, 321 00:16:46.559 --> 00:16:49.960 a park bench, or submarine. When you invoke your lunch 322 00:16:50.000 --> 00:16:53.639 break and open that box, you consume the exact sandwich 323 00:16:53.679 --> 00:16:57.600 prepared in your home environment. The function carries its native 324 00:16:57.639 --> 00:16:58.480 context with it. 325 00:16:58.679 --> 00:17:00.240 That makes perfect sense, But. 326 00:17:00.240 --> 00:17:03.559 Dynamic scoping is like walking into whatever room you currently 327 00:17:03.559 --> 00:17:05.880 happen to be in and trying to eat whatever items 328 00:17:05.880 --> 00:17:08.920 are lying around. Oh wow, Yeah, if you're called inside 329 00:17:08.920 --> 00:17:11.519 a bakery, you eat bread. If you're called inside a 330 00:17:11.519 --> 00:17:15.920 hardware store, you eat nails. It's chaotic because merely renaming 331 00:17:15.960 --> 00:17:19.680 a local variable in the calling function can completely hijack 332 00:17:19.720 --> 00:17:21.400 the behavior of the executed function. 333 00:17:21.640 --> 00:17:24.400 That sounds like a debugging nightmare. And if we connect 334 00:17:24.400 --> 00:17:27.279 this to the bigger picture, that lexical lunchbox. The closure 335 00:17:27.680 --> 00:17:30.480 is honestly one of the most mechanically powerful inventions in 336 00:17:30.519 --> 00:17:34.599 the history of computer science. Really yes, Because a function 337 00:17:34.799 --> 00:17:40.759 securely remembers its creation environment, it enables advanced functional programming patterns. 338 00:17:40.640 --> 00:17:45.000 Like currying right named after the mathematician Haskell Curry exactly. 339 00:17:45.400 --> 00:17:48.720 Currying is the mechanical process of taking a function that 340 00:17:48.759 --> 00:17:52.440 requires multiple arguments and breaking it down into a sequence 341 00:17:52.480 --> 00:17:56.200 of functions that each take single argument. You pass the 342 00:17:56.240 --> 00:17:59.680 first argument to the outer function, which returns an inner function, 343 00:18:00.599 --> 00:18:04.519 and because of lexical closures, that inner function inherently remembers 344 00:18:04.559 --> 00:18:06.880 the environment where the first argument. 345 00:18:06.640 --> 00:18:09.759 Was bound, so you can chain these closures together dynamically, 346 00:18:09.920 --> 00:18:14.359 building these complex stataware pipelines without ever relying on global 347 00:18:14.440 --> 00:18:18.839 variables exactly. But there is a glaring mechanical hurdle when 348 00:18:18.920 --> 00:18:22.400 environments and functions operate like this. How do you implement 349 00:18:22.440 --> 00:18:23.440 a recursive function? 350 00:18:23.799 --> 00:18:25.400 Ah the recursive nott. 351 00:18:25.599 --> 00:18:28.759 Right. If I'm writing a function named calculate factorial, its 352 00:18:28.799 --> 00:18:32.119 internal body needs to call calculate factorial, but I am 353 00:18:32.200 --> 00:18:35.039 currently in the middle of defining it. Yep, the environment 354 00:18:35.079 --> 00:18:38.960 doesn't contain the binding for calculate factorial yet because the 355 00:18:39.000 --> 00:18:40.680 closure hasn't finished being built. 356 00:18:40.799 --> 00:18:44.440 It is the ultimate compiler chicken and egg paradox. A 357 00:18:44.480 --> 00:18:47.559 function cannot be added to an environment until its closure 358 00:18:47.640 --> 00:18:50.759 is created, but the closure needs to capture the environment 359 00:18:50.799 --> 00:18:51.960 containing the function. 360 00:18:52.599 --> 00:18:53.599 So how do they solve it? 361 00:18:54.480 --> 00:19:01.000 Solving this requires the final language progression. The trick introduces 362 00:19:01.039 --> 00:19:05.200 a new specialized environment operation called extend on. 363 00:19:05.400 --> 00:19:08.160 Veriic okay, extend on verik yeah, and. 364 00:19:08.079 --> 00:19:11.720 It solves the paradox by fundamentally mutating the environment structure 365 00:19:11.880 --> 00:19:15.839 to tie a recursive knot. Mechanically, the interpreter allocates a 366 00:19:15.880 --> 00:19:19.839 new environment frame with an empty placeholder or an uninitialized 367 00:19:19.920 --> 00:19:23.240 pointer for the function name okay. It then creates the closure, 368 00:19:23.519 --> 00:19:27.200 pointing the closure's captured environment back to this new frame. Finally, 369 00:19:27.240 --> 00:19:29.839 it reaches back into the frame and overwrites the empty 370 00:19:29.839 --> 00:19:32.279 placeholder with a pointer to the newly minted closure. 371 00:19:32.559 --> 00:19:35.519 So it creates a circular reference in memory. Yes, the 372 00:19:35.680 --> 00:19:38.240 environment points to the closure and the closure points back 373 00:19:38.240 --> 00:19:41.079 to the environment. It allows the function to look up 374 00:19:41.119 --> 00:19:45.279 its own definition within its own native context, safely executing 375 00:19:45.319 --> 00:19:49.000 recursion without unraveling the fabric of the interpreter. It's brilliant, 376 00:19:49.160 --> 00:19:52.240 it really is. So what does this all mean. Let's 377 00:19:52.240 --> 00:19:55.279 bring this vast mechanical landscape back to you, the listener. 378 00:19:56.359 --> 00:19:59.519 We started this deep dive by asking if programming languages 379 00:19:59.559 --> 00:20:03.839 felt like arbitrary magic spells handed down by untouchable wizards. 380 00:20:03.440 --> 00:20:05.720 And hopefully we've proved they aren't exactly. 381 00:20:05.759 --> 00:20:09.119 You've seen that language design is entirely systematic. If you 382 00:20:09.160 --> 00:20:13.920 follow the grammar, you can predictably parse infinite complexity. By 383 00:20:14.000 --> 00:20:18.960 leveraging data abstraction, you separate interface from memory implementation seamlessly, 384 00:20:19.079 --> 00:20:23.400 upgrading from unary lists to massive integer arrays. You've learned 385 00:20:23.440 --> 00:20:27.000 that environments can be passive lists or dynamic procedural closures 386 00:20:27.000 --> 00:20:29.440 that capture their native state like a packed lunch box, 387 00:20:29.759 --> 00:20:32.400 and you understand the memory mutation required to tie a 388 00:20:32.440 --> 00:20:33.319 recursive knot. 389 00:20:33.519 --> 00:20:38.599 Programming languages are not mystical incantations. They're just meticulously structured 390 00:20:38.680 --> 00:20:40.799 data processed by recursive recipes. 391 00:20:41.039 --> 00:20:42.319 Just data and recipes. 392 00:20:42.680 --> 00:20:45.240 Yeah. An interpreter is nothing more than a program that 393 00:20:45.279 --> 00:20:49.200 traverses an abstract syntax tree, resolves variable bindings in an 394 00:20:49.279 --> 00:20:54.119 environment frame, and applies operations. By studying the mechanics and 395 00:20:54.279 --> 00:20:58.240 essentials of programming languages, you acquire the conceptual blueprint to 396 00:20:58.279 --> 00:21:02.039 look at any modern language, whether it's Python's dynamic typing, 397 00:21:02.359 --> 00:21:06.359 Russ's barrochecker, or JavaScript's event loop, and actually understand the 398 00:21:06.440 --> 00:21:08.440 architectural trade offs operating under the hood. 399 00:21:08.720 --> 00:21:11.680 You transition from a mere user of tools into a 400 00:21:11.759 --> 00:21:15.440 designer of systems exactly which leads us, with one final 401 00:21:15.559 --> 00:21:18.640