# PDF Category theory at work

A diagram. And as we saw a couple of weeks ago, a diagram is really a functor. Last week was the start of a mini-series on limits and colimits in category theory. We began by answering a few basic questions, including, "What ARE co limits? For more on this non-technical answer, be sure to check out Limits and Colimits, Part 1.

Towards the end of that post, I mentioned that co limits aren't really related to limits of sequences in topology and analysis but see here. There is however one similarity. In analysis, we ask for the limit of a sequence. In category theory, we also ask for the co limit OF something. But if that "something" is not a sequence, then what is it? I'd like to embark on yet another mini-series here on the blog. The topic this time? Limits and colimits in category theory! But even if you're not familiar with category theory, I do hope you'll keep reading. Today's post is just an informal, non-technical introduction.

And regardless of your categorical background, you've certainly come across many examples of limits and colimits, perhaps without knowing it! They appear everywhere--in topology, set theory, group theory, ring theory, linear algebra, differential geometry, number theory, algebraic geometry. The list goes on. But before diving in, I'd like to start off by answering a few basic questions.

Welcome to our third and final installment on the Yoneda lemma! Last week we divided this maxim into two points And w hat are the actual corollaries? In this post, we'll work to discover the answers. But what if we replace "set" by "group"? Can we view group elements categorically as well? I hope you have enjoyed our little series on basic category theory. I know I have! This week we'll close out by chatting about natural transformations which are, in short, a nice way of moving from one functor to another.

Next up in our mini series on basic category theory: functors! You'd be right, too! You see, it's very different than other branches of math Yes, I agree. The title for this post is a little pretentious. Because I'd like to tell you about an overarching theme in mathematics - a mathematical mantra, if you will.

September July May March January November October February December August June April Category Theory. What is an Adjunction? Part 1 Motivation September 19, Limits and Colimits Part 3 Examples January 21, Announcing Applied Category Theory January 5, Upon first seeing those words, I suspect many folks might think either one of two thoughts: Applied category theory?

Isn't that an oxymoron? Applied category theory? What's the hoopla? I think a dynamic language like Python or Javascript or even pseudo-code would be the lowest common denominator for programmers. Thanks for writing this!! Where are the proofs of correctness? Amazing work so far! Please, keep up the excellent work! The following website has quite a bit of information what Concept Maps are, how they help in encapsulating and passing knowledge, how they can be developed, and also has tool that can be used to develop Concept Maps—.

I need to have one copy of it, yes, for my nightstand also, and for my professional growth. Your style of writing is very lucid. I am very thankful about this stuff! Reblogged this on My Thoughts. I was just about to ask Bartosz to keep the book posts on GitHub for easy commenting and review. Just came across this and this is absolutely great.

Diving into the chapters already available.

## Notes on Applied Category Theory

I think that is a great choice of language for the topic. And yeah, do NOT put it on Github or any other bitbucket please.. Hopefully it picks back up, I was really enjoying following along! And here it is! Thank you for this effort. Ihre formale Basis ist das Lambda Calculus und die Kategorientheorie. Another wonderfully intuitive and thorough introduction to monads in programming is You Could Have Invented Monads! And Maybe You Already Have. What a great read!

### You are here

I really enjoyed this and more than anything loved the gothic cathedrals as an example. Thank you. I earnestly hope that the recent date of the last post means the monad post is coming soon! This looks great! Hi, about that book on your night stand; is it accessible without a solid bqckground in group theory? Or is it something I should wait with. Your book is another leap in clarity just as great as the gulf between these two books. Excellent insight and enthusiasm. Have you considered the MEAP program?

I wonder whether a print book be available. Great job! I found this book very conceivable for a programmer. Thanks for this! It was the perfect next step, for me. Or were you born with that knowledge? What did you do in the absence of blog posts by Bartosz Milewski? Constantine: It took me a very long time. Not so much category theory itself — that I started studying just a few years ago — but the general background in mathematics.

I went to a high school with extended program in mathematics, where I learned the basics of logic, axiomatic theory, geometry and calculus. Then I studied physics, which involved a lot of higher math. I was taught calculus through Banach and Hilbert spaces, and algebra through group theory and fibre bundles. So I had a very strong background in math when I switched to programming. Do I struggle reading math books and articles?

You bet! The trick is not to think about the ultimate goal, but to enjoy the process of learning. This has become so much easier nowadays, with all the resources available through the internet. Thank you very much. Wonderful trove of information. Would have wished if we could have had this course in our under grad!

Thanks again for sharing. Very interesting. Bartosz Milewski is writing a fascinating series of blog posts concerning this […]. Thank you for all your excellent material on category theory, I am really looking forward to the book! I am a programmer and — mainly thanks to your blog — I have developed an interest in category theory.

In programming zips are very common, and n-ary zips can be defined in a quite general way. Unfortunately, the text is not nearly as accessible as your material. Keep up the great work! But Bartosz Milewski does a pretty good job sparking my […]. Dear doctor Milewski, I read the material on your website and follow your lectures on YouTube.

### 1. General Definitions, Examples and Applications

You are a good lecturer and, also, a prefect writer. Thank you for your efforts in simplifying things! I have read this blog several times. I love your youtube lessons, too. We need to translate it into other languages german first, please. How do we deal with copyright issues? Regards, Klaus. Just throwing this idea out for your consideration: have you thought about opensourcing your book and moving it over to Wikibooks. Wikibookifying will allow others to contribute to your book, translate it into multiple languages, and maintain your book in the event that you shut down your blog.

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Hello Bartosz, I really like your blog and your videos, I just wanted to ask you if it is possible to have a pdf version of your posts? I write them directly in HTML.

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Bartosz, I add my praise to all the other praise you have received already. Like you, I am a theoretical physicist working in computing with a love for the mathematics underlying both physics and computer science. Have you considered self-publishing?

You could use a crowdfunding site like Kickstarter or Indiegogo to raise money for the book with some sort of premium to funders like a free ebook. I would be willing to contribute, and it sounds like others would as well. Thank you! Writing a book is a very time-consuming process. I […]. Just chiming in to say that your explanations are of top-notch quality. Thanks so much for your hard work in making this content, and for making it available to the public! I made this epic book into a PDF!

Thank you so much for your awesome work sir! Like you said I completely forgot about math — hope you will be gentle — I am just starting! I am rereading your book and I am dedicating some time to finding or coming up with more code examples.

## Category Theory: Lecture Notes and Online Books

I started by wanting to write about Natural Transformations but ended up backpedaling a bit coding Functor Composition Ch 7 now ready for public viewing with some minor TODOs left. I decided to think hard, every step of the way, about more code examples and practical code examples. Many of the concepts are really foundational to understanding of what is coming next but being able to see more code examples or a practical application of something like Functor Composition is very gratifying. At least it is to me. I welcome any comments you might have about my project and examples.

Thanks very much! From the present chapter, I think the biggest source of confusion was a lack of distinction in the minds of some of our members between categories in the abstract on the one hand, and the particular categories Set and Hask on the other. As an example: what to call the arrows arrows, morphisms, or functions was not clear to everyone.

Something else that stood out to me: the basic laws of categories were not clear to all of us. Some examples:. The fact that equality is defined for morphisms this is implicit in the other laws, which make use of that, but not elsewhere. The fact that there may be multiple arrows from an object to itself.

The fact that composition is not optional this one is written in the article, but somehow not clear enough to some. If I may be so bold to propose a slight reorganization of the material: either first talk about categories and their laws in the abstract, and then follow up with the link to the category Set, or vice versa. I came to the book with a basic understanding of Category Theory but still struggle with its use in Haskell and Scala.

I also wish there was a section on Free Monads. There is a paper version available on lulu. May I suggest that you include a Concept Map to show the relationships among the various aspects of Category Theory as it relates to various aspects of Computer Programming Paradigms functional, object-oriented etc. It would be better if you write the meaning of each part of your book in the preface so the ordering of the chapters can be more expressive. Lex Huang: This would be very difficult, since each chapter builds on the previous knowledge.

So how would I explain the meaning of the Yoneda lemma or an end or a topos without the context of previous chapters? In your category theory video 1. You say that id b after f equals f. If the range of f does not cover the full codomain, Id b must select from the codomain to arrive at the range of f. Is this what you intend, or does the left identity only work when the range covers the codomain? If possible, please kindly send an e-mail to my address.

Thank you for reading it. In his excellent series of articles about category theory for programmers, Bartosz Milewski describes free monoids in Haskell as the list […]. Needless to add even more praise. Thanks a million!

## soft question - What's there to do in category theory? - MathOverflow

Is it ok to make a series of posts strongly based in your book with all credits given, of course in Portuguese? There are also two more courses on […]. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account.

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