# Posts in the year 2010

### Subgroups are equalizers, constructively?

- 10 November 2010
- General

[**Edit 2010-11-12:** Given the gap in my “proof”, I am changing the title of the post to a question.]

I would like to record the following fact, which is hard to find on the internet: *every subgroup is an equalizer, constructively*. In other words, all monos in the category of groups are regular, constructively. It is interesting that this fact fails if we work in a meta-theory with “poor quotients”.

### Delimited continuations in eff

- 30 September 2010
- Eff

**[UPDATE 2012-03-08: since this post was written eff has changed considerably. For updated information, please visit the eff page.]**

****Let's keep the blog rolling! Here are delimited continuations in eff, and a bunch of questions I do not know the answers to.

### How eff handles built-in effects

- 28 September 2010
- Eff, Guest post

**[UPDATE 2012-03-08: since this post was written eff has changed considerably. For updated information, please visit the eff page.]**

From some of the responses we have been getting it looks like people think that the `io`

effect in eff is like `unsafePerformIO`

in Haskell, namely that it causes an effect but pretends to be pure. This is *not* the case. Let me explain how eff handles built-in effects.

### Programming with effects II: Introducing eff

- 27 September 2010
- Computation, Eff, Guest post, Programming, Software, Tutorial

**[UPDATE 2012-03-08: since this post was written eff has changed considerably. For updated information, please visit the eff page.]**

****This is a second post about the programming language eff. We covered the theory behind it in a previous post. Now we turn to the programming language itself.

Please bear in mind that eff is an academic experiment. It is not meant to take over the world. Yet. We just wanted to show that the theoretical ideas about the algebraic nature of computational effects can be put into practice. Eff has many superficial similarities with Haskell. This is no surprise because there is a precise connection between algebras and monads. The main advantage of eff over Haskell is supposed to be the ease with which computational effects can be combined.

→ continue reading (7 comments)### Programming with effects I: Theory

- 27 September 2010
- Computation, Eff, Programming, Software, Talks, Tutorial

I just returned from Paris where I was visiting the INRIA ?r² team. It was a great visit, everyone was very hospitable, the food was great, and the weather was nice. I spoke at their seminar where I presented a new programming language * eff* which is based on the idea that computational effects are algebras. The language has been designed and implemented jointly by Matija Pretnar and myself. Eff is far from being finished, but I think it is ready to be shown to the world. What follows is an extended transcript of the talk I gave in Paris. It is divided into two posts. The present one reviews the basic theory of algebras for a signature and how they are related to computational effects. The impatient readers can skip ahead to the second part, which is about the programming language.

A side remark: I have updated the blog to WordPress to 3.0 and switched to MathJax for displaying mathematics. Now I need to go through 70 old posts and convert the old ASCIIMathML notation to MathJax, as well as fix characters which got garbled during the update. Oh well, it is an investment for the future.

→ continue reading (18 comments)### Random Art and the Law of Rotten Software

- 17 August 2010
- Programming

Since the death of my old web server my Random Art has not worked. Bringing it up to date and installing it on the new server was a nightmare in software management. But it was worth it. The new Random Art runs the random art program inside your browser!

→ continue reading (14 comments)### An amazing functional

- 29 July 2010
- Computation, News

Martin Escardo and Paulo Oliva have been working on the *selection monad* and related functionals. The selection monad `S(X) = (X -> R) -> X` is a cousin of the continuation monad `C(X) = (X -> R) -> R` and it has a lot of useful and surprising applications. I recommend their recent paper *“What Sequential Games, the Tychonoff Theorem and the Double-Negation Shift have in Common”* which they wrote for MSFP 2010 (if you visit the workshop you get to hear Martin live). They explain things via examples written in Haskell, starting off with the innocently looking functional `ox` (which i I am writting as `ox`

in Haskell for “crossed O”):

ox :: [(x -> r) -> x] -> ([x] -> r) -> [x] ox [] p = [] ox (e : es) p = a : ox es (p . (a:)) where a = e (\x -> p (x : ox es (p . (x:))))

It is just four lines of code, so how complicated could it be? Well, read the paper to find out. If you are ready for serious math, have a look at this paper instead.

→ continue reading (3 comments)After more than 1300 days of uninterrupted service, the good old PC that served the blog started to spontaneously reboot every 4 minutes or so. It looks like a hardware failure. I moved the site to a temporary machine. I am seriously considering renting a private virtual server and just forget about buying my own hardware in the future.

On top of that I discovered that evil forces planted a phishing attack on the blog about two weeks ago. The strategy was this:

- Create an account on my blog (I stupidly left registration open to everyone).
- Elevate account privileges to administrator by exploiting a WordPress security hole (I do not know which one).
- Upload evil files to the upload area.
- Direct phishing victims to the uploaded files.

So, keep your WordPress as closed as possible.

→ continue reading (3 comments)### Random art in Python

- 21 April 2010
- Programming, Software, Tutorial

I get asked every so often to release the source code for my random art project. The original source is written in Ocaml and is not publicly available, but here is a simple example of how you can get random art going in python in 250 lines of code.

**Download source:** randomart.py

I am discovering that mathematicians cannot tell the difference between “proof by contradiction” and “proof of negation”. This is so for good reasons, but conflation of different kinds of proofs is bad mental hygiene which leads to bad teaching practice and confusion. For reference, here is a short explanation of the difference between proof of negation and proof by contradiction.

→ continue reading (24 comments)It was time I changed the old blog style to something a bit more modern. I hope you like it.

Now I just have to figure out how to port 60 blog posts from ASCIIMathML notation to something a bit friendlier that can use MathML but does not require it. What is out there? I know about jsMath. I am open to suggestions.

→ continue reading (11 comments)### Tutorial on exact real numbers in Coq

- 07 January 2010
- Computation, Constructive math, Programming, Tutorial

Already a while ago videolectures.net published this **tutorial on Computer Verified Exact Analysis** by Bas Spitters and Russell O'Connor from Computability and Complexity in Analysis 2009. I forgot to advertise it, so I am doing this now. It is about an implementation of exact real arithmetic whose correctness has been verified in Coq. Russell also gave a quick tutorial on Coq.

### Metric Spaces in Synthetic Topology

- 06 January 2010
- Constructive math, Publications

With Davorin Lešnik.

**Abstract:** We investigate the relationship between the synthetic approach to topology, in which every set is equipped with an intrinsic topology, and constructive theory of metric spaces. We relate the synthetic notion of compactness of Cantor space to Brouwer's Fan Principle. We show that the intrinsic and metric topologies of complete separable metric spaces coincide if they do so for Baire space. In Russian Constructivism the match between synthetic and metric topology breaks down, as even a very simple complete totally bounded space fails to be compact, and its topology is strictly finer than the metric topology. In contrast, in Brouwer's intuitionism synthetic and metric notions of topology and compactness agree.

**Download paper:** csms_in_synthtop.pdf