32K views · Aug 22, 2019 · 1:24:38 · 1 HN recommender · 3 mentions · discussed 2020–2023
Lecture 7A from MIT 6.001 (SICP) by Abelson and Sussman introduces the metacircular evaluator, a Scheme interpreter written in Scheme. It shows how evaluating a program is itself a program.
Why Hacker News points people here: HN holds the SICP lectures up as joyful classics, and this lecture as the moment the evaluator is revealed.
“But mostly it just makes me happy - it's infectious and weird.”HN comment, 2020
“But mostly it just makes me happy - it's infectious and weird.”
SICP lecture revealing the metacircular evaluator, joyful to rewatch
I re-watch this course every bunch of years, and every time I get something different from it. But mostly it just makes me happy - it's infectious and weird. Especially the "revelation" (spoiler!) this video
“I learned best in Scheme because you don't need to parse your input and can just evaluate a tree[0]”
Shows evaluating a tree directly in Scheme
I learned best in Scheme because you don't need to parse your input and can just evaluate a tree[0] (Parsing is, in my opinion, the most boring part about writing an interpreter!) Chapter 7 of Crafting Interpreters lays out how an evaluator works in a pretty clean way [1]. I've also blogged about evaluating ASTs in JavaScript [2] Lastly, Gary Bernhardt took it a step further and instead of "evaluating" code, "compiled" it to JavaScript (this tutorial uses Ruby) [3]. Very much a mind-blown moment for me. Have fun!! Writing interpreters remains one of my favorite things to do when learning a new programming language. [0] this video [1] link [2] link [3] link
“We're getting really close to the spirit of the computer at this point, so I have to show a certain amount of reverence and respect”
Lecturer's reverent remark on nearing the spirit of the computer
"We're getting really close to the spirit of the computer at this point, so I have to show a certain amount of reverence and respect" this video
