# Y Combinator, 2

arethuza 7 hours ago [-]

It’s over 30 years since I was taught about the Y combinator and I still find it amazing – it allows you to define recursion in a language which has no concept of recursion or named functions – such as λ-calculus.

The fact that you then define this in terms of amazingly simple functions like the S and K combinators just, in my opinion, adds to how wonderful it is.

— Why Y? Deriving the Y Combinator in JavaScript

— Hacker News

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2022.04.19 Tuesday ACHK

# Category Theory for Dummies

It seems that category theory is the new hype — almost nobody actually understands what it is about, or, more importantly, what it is for. Let me tell you what it is for — it’s an important technical tool in mathematical research, which gives you new, coherent language, sometimes provides you with an additional insight in the structure of the stuff you are researching and makes it easier to notice and classify similarities between different kind of structures. Unfortunately, it is almost completely useless and uninteresting by itself — because, well, what’s interesting in objects and arrows anyway?

What make category theory interesting are its connections with various field[s] and [in] math and computer science. That’s why introducing “category theory for dummies” makes completely no sense — it’s like following Erlangen program to teach kids about points, lines and circles on a plane. The need and the significance of Erlangen program arise when you learn about many different geometries, notice what they have in common and what they do not, and try to find out what the geometry is all about. Without it, the Erlangen program is all about abstract bullshit, and the situation is completely the same with category theory. But nobody writes or posts Erlangen program for dummies on HN. Why? “General theory of everything” hype, that’s why. Erlangen program is “general theory of geometry”, but geometry seems a bit pale when compared to everything.

If you really want to understand the significance of category theory, then learn set theory, then algebra, then topology, then algebraic topology and algebraic geometry, or take abstract programming languages theory path. If you don’t care about all this stuff, because you’re hyped on the category theory, then you’re missing the point — it’s like you wanted to learn about algebraic topology, but did not care about algebra or topology.

Also anything that has “for dummies” in title should invariably remind you of Norvig’s essay (google Peter Norvig 21 days).

— xyzzyz 171 days ago

— Hacker News

2012.03.14 Wednesday ACHK

# Category Theory | Lisp, 4

（安：哪兩樣東西呢？）

Lisp 和「範疇論」都是我幾年前就開始關心的課題。最近才發現，它們竟然是「同一樣」東西，令我十分驚奇。幾天前，我還正正考慮著，好不好暫時放棄其中一門，因為我沒有時間同時深入研究 Lisp 和「範疇論」。現在，問題突然消失。

— Me@2011.08.03

# Quantum gravity 4

Ultimate goal is to merge:

quantum theory (representation theory)

&

general relativity (differential geometry)

by combining notions of

quantum state & geometry of space (spin network)

quantum history & geometry of spacetime (spin foam)

— John Baez

2010.07.25 Sunday ACHK

# Mathematics, physics, logic and philosophy

So, we can only hope that in the future, more interaction between mathematics, physics, logic and philosophy will lead to new ways of thinking about quantum theory — and quantum gravity — that take advantage of the internal logic of categories like Hilb and nCob.

— Quantum Quandaries: A Category-Theoretic Perspective, John C. Baez

2010.05.19 Wednesday ACHK

Category Theory | Lisp 3

Haskell is very obviously a language built by mathematical logicians for mathematical logicians. Category theory lurks behind it in the same way that the lambda calculus lurks behind LISP.

— Eric S. Raymond

2010.03.12 Friday ACHK

# Category | Lisp 2

With Lisp (Scheme/Common Lisp), you can write programs to write programs.

With Category theory, you can unify the mathematics of General Relativity and the mathematics of Quantum Mechanics.

— Me@2010.03.09

# Category Theory | Lisp

My current self-study programme includes

1. read almost everything written by Paul Graham

public domain image
Wikipedia

2. learn Lisp programming language (Common Lisp/Scheme) by reading Paul Graham’s textbooks

3. learn Category Theory so that I can read almost everything written by John Baez

public domain image
Wikipedia

Yester-night and tonight, I discovered that they are related:

“Lists, and recursive operations on them, are an excellent case in point. But the path connecting them to their mathematical underpinnings is a long and winding one, which lays in the realm of Category Theory.” — jao

I had never expected that. They are one thing.

Paul Graham –> Lisp –> Category Theory –> John Baez

I have been learning Lisp since 2000 (Machine Intelligence course), since 2006 (Structure and Interpretation of Classical Mechanics, Structure and Interpretation of Computer Programs), since 2010. I have been learning Category Theory since 2006-2008 (John Baez), since 2008 (Sets for Mathematics), since 2010. I had never expected that they are just two different languages of the same thing.

— Me@2010.03.04