# Posts in the category Computation

### Variations on Weihrauch degrees (CiE 2023)

- 28 July 2023
- Talks, Computation

I gave a talk “Variations on Weihrauch degrees” at Computability in Europe 2023, which took place in Tbilisi, Georgia. The talk was a remote one, unfortunately. I spoke about generalizations of Weihrauch degrees, a largely unexplored territory that seems to offer many opportunities to explore new directions of research. I am unlikely to pursue them myself, but will gladly talk with anyone who is interested in doing so.

**Slides:** `CiE-2023-slides.pdf`

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### The new and improved Programming languages zoo

- 07 September 2016
- Computation, Programming languages, Software, Teaching

It is my pleasure to announce the new and improved Programming languages Zoo, a potpourri of miniature but fully functioning programming language implementations. The new zoo has a decent web site, it is now hosted on GitHub, and the source code was cleaned up. Many thanks to Matija Pretnar for all the work.

The purpose of the zoo is to demonstrate design and implementation techniques, from dirty practical details to lofty theoretical considerations:

- functional, declarative, object-oriented, and procedural languages
- source code parsing with a parser generator
- recording of source code positions
- pretty-printing of values
- interactive shell (REPL) and non-interactive file processing
- untyped, statically and dynamically typed languages
- type checking and type inference
- subtyping, parametric polymorphism, and other kinds of type systems
- eager and lazy evaluation strategies
- recursive definitions
- exceptions
- interpreters and compilers
- abstract machine

There is still a lot of room for improvement and new languages. Contributions are welcome!

→ continue reading

### Hask is not a category

- 06 August 2016
- Computation, Programming

This post is going to draw an angry Haskell mob, but I just have to say it out loud: I have never seen a definition of the so-called category Hask and I do not actually believe there is one until someone does some serious work.

→ continue reading (66 comments)In a paper accepted at POPL 2016 Matt Brown and Jens Palsberg constructed a self-interpreter for System $F_\omega$, a strongly normalizing typed $\lambda$-calculus. This came as a bit of a surprise as it is “common knowledge” that total programming languages do not have self-interpreters.

Thinking about what they did I realized that their conditions allow a self-interpreter for practically any total language expressive enough to encode numbers and pairs. In the PDF note accompanying this post I give such a self-interpreter for Gödel's System T, the weakest such calculus. It is clear from the construction that I abused the definition given by Brown and Palsberg. Their self-interpreter has good structural properties which mine obviously lacks. So what we really need is a better definition of self-interpreters, one that captures the desired structural properties. Frank Pfenning and Peter Lee called such properties **reflexivity**, but only at an informal level. Can someone suggest a good definition?

**Note:** self-interpreter-for-T.pdf

Here are the slides of my TYPES 2015 talk “The troublesome reflection rule” with fairly detailed presenter notes. The meeting is taking place in Tallinn, Estonia – a very cool country in many senses (it's not quite spring yet even though we're in the second half of May, and it's the country that gave us Skype).

**Download slides:** The troublesome reflection rule (TYPES 2015) [PDF].

### Seemingly impossible constructive proofs

- 08 May 2014
- Computation, Constructive math, Guest post, Type theory

In the post Seemingly impossible functional programs, I wrote increasingly efficient Haskell programs to realize the mathematical statement

$\forall p : X \to 2. (\exists x:X.p(x)=0) \vee (\forall x:X.p(x)=1)$

for $X=2^\mathbb{N}$, the Cantor set of infinite binary sequences, where $2$ is the set of binary digits. Then in the post A Haskell monad for infinite search in finite time I looked at ways of systematically constructing such sets $X$ with corresponding Haskell realizers of the above *omniscience principle*.

In this post I give examples of infinite sets $X$ and corresponding constructive *proofs* of their omniscience that are intended to be valid in Bishop mathematics, and which I have formalized in Martin-Löf type theory in Agda notation. This rules out the example $X=2^\mathbb{N}$, as discussed below, but includes many interesting infinite examples. I also look at ways of constructing new omniscient sets from given ones. Such sets include, in particular, ordinals, for which we can find minimal witnesses if any witness exists.

Agda is a dependently typed functional programming language based on Martin-Löf type theory. By the Curry-Howard correspondence, Agda is also a language for formulating mathematical theorems (types) and writing down their proofs (programs). Agda acts as a thorough referee, only accepting correct theorems and proofs. Moreover, Agda can run your proofs. Here is a graph of the main Agda modules for this post, and here is a full graph with all modules.

→ continue reading (1 comment)### Intuitionistic Mathematics and Realizability in the Physical World

- 04 March 2014
- Computation, Publications

This is a draft version of my contribution to “A Computable Universe: Understanding and Exploring Nature as Computation”, edited by Hector Zenil. Consider it a teaser for the rest of the book, which contains papers by an impressive list of authors.

**Abstract:** Intuitionistic mathematics perceives subtle variations in meaning where classical mathematics asserts equivalence, and permits geometrically and computationally motivated axioms that classical mathematics prohibits. It is therefore well-suited as a logical foundation on which questions about computability in the real world are studied. The realizability interpretation explains the computational content of intuitionistic mathematics, and relates it to classical models of computation, as well as to more speculative ones that push the laws of physics to their limits. Through the realizability interpretation Brouwerian continuity principles and Markovian computability axioms become statements about the computational nature of the physical world.

**Download:** real-world-realizability.pdf

### The topology of the set of all types

- 30 March 2012
- Computation, Constructive math, Guest post

It is well known that, both in constructive mathematics and in programming languages, types are secretly topological spaces and functions are secretly continuous. I have previously exploited this in the posts Seemingly impossible functional programs and A Haskell monad for infinite search in finite time, using the language Haskell. In languages based on Martin-Löf type theory such as Agda, there is a set of all types. This can be used to define functions $\mathbb{N} \to \mathrm{Set}$ that map numbers to types, functions $\mathrm{Set} \to \mathrm{Set}$ that map types to types, and so on.

Because $\mathrm{Set}$ itself is a type, a large type of small types, it must have a secret topology. What is it? There are a number of ways of approaching topology. The most popular one is via open sets. For some spaces, one can instead use convergent sequences, and this approach is more convenient in our situation. It turns out that the topology of the universe $\mathrm{Set}$ is indiscrete: every sequence of types converges to any type! I apply this to deduce that $\mathrm{Set}$ satisfies the conclusion of Rice's Theorem: it has no non-trivial, extensional, decidable property.

To see how this works, check:

- A short paper with the proofs in mathematical vernacular, and further discussion of the intuitions, motivations and consequences.
- Literate proofs in Agda of the universe indiscreteness theorem and Rice's Theorem for the universe.
- Agda proofs of related facts.

The Agda pages can be navigated be clicking at any (defined) symbol or word, in particular by clicking at the imported module names.

→ continue reading (2 comments)### A puzzle about typing

- 20 January 2012
- Computation, Programming

While making a comment on Stackoverflow I noticed something: suppose we have a term in the $\lambda$-calculus in which no abstracted variable is used more than once. For example, $\lambda a b c . (a b) (\lambda d. d c)$ is such a term, but $\lambda f . f (\lambda x . x x)$ is not because $x$ is used twice. If I am not mistaken, all such terms can be typed. For example:

# fun a b c -> (a b) (fun d -> d c) ;; - : ('a -> (('b -> 'c) -> 'c) -> 'd) -> 'a -> 'b -> 'd =→ continue reading (5 comments)# fun a b c d e e' f g h i j k l m n o o' o'' o''' p q r r' s t u u' v w x y z -> q u i c k b r o w n f o' x j u' m p s o'' v e r' t h e' l a z y d o''' g;; - : 'a -> 'b -> 'c -> 'd -> 'e -> 'f -> 'g -> 'h -> 'i -> 'j -> 'k -> 'l -> 'm -> 'n -> 'o -> 'p -> 'q -> 'r -> 's -> 't -> ('u -> 'j -> 'c -> 'l -> 'b -> 'v -> 'p -> 'w -> 'o -> 'g -> 'q -> 'x -> 'k -> 'y -> 'n -> 't -> 'z -> 'r -> 'a1 -> 'e -> 'b1 -> 'c1 -> 'i -> 'f -> 'm -> 'a -> 'd1 -> 'e1 -> 'd -> 's -> 'h -> 'f1) -> 'v -> 'b1 -> 'z -> 'c1 -> 'u -> 'y -> 'a1 -> 'w -> 'x -> 'e1 -> 'd1 -> 'f1 = </pre> What is the easiest way to see that this really is the case? A related question is this (I am sure people have thought about it): how big can a type of a typeable $\lambda$-term be? For example, the Ackermann function can be typed as follows, although the type prevents it from doing the right thing in a typed setting: # let one = fun f x -> f x ;; val one : ('a -> 'b) -> 'a -> 'b = # let suc = fun n f x -> n f (f x) ;; val suc : (('a -> 'b) -> 'b -> 'c) -> ('a -> 'b) -> 'a -> 'c = # let ack = fun m -> m (fun f n -> n f (f one)) suc ;; val ack : ((((('a -> 'b) -> 'a -> 'b) -> 'c) -> (((('a -> 'b) -> 'a -> 'b) -> 'c) -> 'c -> 'd) -> 'd) -> ((('e -> 'f) -> 'f -> 'g) -> ('e -> 'f) -> 'e -> 'g) -> 'h) -> 'h =</pre> That's one mean type there! Can it be “explained”? Hmm, why _does_ `ack` compute the Ackermann function in the untyped $\lambda$-calculus?

### Definability and extensionality of the modulus of continuity functional

- 27 July 2011
- Computation, Tutorial

In an earlier post I talked about the modulus of continuity functional, where I stated that it cannot be defined without using some form of computational effects. It is a bit hard to find the proof of this fact so I am posting it on my blog in two parts, for Google and everyone else to find more easily. In the first part I show that there is no extensional modulus of continuity. In the second part I will show that every functional that is defined in PCF (simply-typed $\lambda$-calculus with natural numbers and recursion) is extensional.

→ continue reading (8 comments)### Running a classical proof with choice in Agda

- 10 May 2011
- Computation, Constructive math, Guest post, Logic, Programming, Tutorial

As a preparation for my part of a joint tutorial *Programs from proofs* at MFPS 27 at the end of this month with Ulrich Berger, Monika Seisenberger, and Paulo Oliva, I've developed in Agda some things we've been doing together.

Using

- Berardi-Bezem-Coquand functional, or alternatively,
- Berger-Oliva modified bar recursion, or alternatively,
- Escardo-Oliva countable product of selection functions,

for giving a proof term for classical countable choice, we prove the classical infinite pigeonhole principle in Agda: every infinite boolean sequence has a constant infinite subsequence, where the existential quantification is classical (double negated).

As a corollary, we get the finite pigeonhole principle, using Friedman's trick to make the existential quantifiers intuitionistic.

This we can run, and it runs fast enough. The point is to illustrate in Agda how we can get witnesses from classical proofs that use countable choice. The finite pigeonhole principle has a simple constructive proof, of course, and hence this is really for illustration only.

The main Agda files are

These are Agda files converted to html so that you can navigate them by clicking at words to go to their definitions. A zip file with all Agda files is available. Not much more information is available here.

The three little modules that implement the Berardi-Bezem-Coquand, Berger-Oliva and Escardo-Oliva functionals disable the termination checker, but no other module does. The type of these functionals in Agda is the J-shift principle, which generalizes the double-negation shift.

→ continue reading (3 comments)### Canonical Effective Subalgebras of Classical Algebras as Constructive Metric Completions

- 24 January 2011
- Computation, Constructive math, Publications

Jens Blanck and I presented a paper at Computability and Complexity in Analysis 2009 with a complicated title (I like complicated titles):

Canonical Effective Subalgebras of Classical Algebras as Constructive Metric Completions

which has been published in Volume 16, Issue 18 of the Journal of Universal Computer Science. I usually just post the abstract, but this time I would like to explain the general idea informally, the way one can do it on a blog. But first, here is the abstract:

**Abstract:** We prove general theorems about unique existence of effective subalgebras of classical algebras. The theorems are consequences of standard facts about completions of metric spaces within the framework of constructive mathematics, suitably interpreted in realizability models. We work with general realizability models rather than with a particular model of computation. Consequently, all the results are applicable in various established schools of computability, such as type 1 and type 2 effectivity, domain representations, equilogical spaces, and others.

**Download paper:** effalg.pdf or directly from JUCS

### Alg

- 22 January 2011
- Computation, Software

**Alg** is a program for enumeration of finite models of single-sorted first-order theories. These include groups, rings, fields, lattices, posets, graphs, and many more. Alg was written as a class project by Aleš Bizjak, a student of mine whose existence I cannot confirm with a URL. I joined the effort, added bells and whistles, as well as an alternative algorithm that works well for relational structures. Alg is ready for public consumption, although it should be considered of “beta” quality. Instructions for downloading alg are included at the end of this post.

### European workshop on computational effects

- 04 January 2011
- Computation, News

Alex Simpson, Matija Pretnar and I are organizing a workshop on computational effects. It will take place in Ljubljana on March 17th and 18th 2011. More information is available at the workshop web page.

→ continue reading### 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

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

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)### 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)### 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.

### Mathematically Structured but not Necessarily Functional Programming

- 29 May 2009
- Computation, Constructive math, Programming, RZ, Talks

These are the slides and the extended abstract from my MSFP 2008 talk. Apparently, I forgot to publish them online. There is a discussion on the Agda mailing list to which the talk is somewhat relevant, so I am publishing now.

**Abstract:** Realizability is an interpretation of intuitionistic logic which subsumes the Curry-Howard interpretation of propositions as types, because it allows the realizers to use computational effects such as non-termination, store and exceptions. Therefore, we can use realizability as a framework for program development and extraction which allows any style of programming, not just the purely functional one that is supported by the Curry-Howard correspondence. In joint work with Christopher A. Stone we developed RZ, a tool which uses realizability to translate specifications written in constructive logic into interface code annotated with logical assertions. RZ does not extract code from proofs, but allows any implementation method, from handwritten code to code extracted from proofs by other tools. In our experience, RZ is useful for specification of non-trivial theories. While the use of computational effects does improve efficiency it also makes it difficult to reason about programs and prove their correctness. We demonstrate this fact by considering non-purely functional realizers for a Brouwerian continuity principle.

**Download:** msfp2008-slides.pdf, msfp2008-abstract.pdf

I have been writing lecture notes on computable mathematics. One of the questions that came up was whether it is possible to simulate the booleans in the simply-typed $\lambda$-calculus. This is a nice puzzle in functional programming. If you solve it, definitely let me know, although I suspect logicians did it a long time ago.

→ continue reading (8 comments)### A Haskell monad for infinite search in finite time

- 21 November 2008
- Computation, Constructive math, Guest post

I show how monads in Haskell can be used to structure infinite search algorithms, and indeed get them for free. This is a follow-up to my blog post Seemingly impossible functional programs. In the two papers Infinite sets that admit fast exhaustive search (LICS07) and Exhaustible sets in higher-type computation (LMCS08), I discussed what kinds of infinite sets admit exhaustive search in finite time, and how to systematically build such sets. Here I build them using monads, which makes the algorithms more transparent (and economic).

→ continue reading (14 comments)### Not all computational effects are monads

- 17 November 2008
- Computation

Lately I've been thinking about computational effects *in general*, i.e., what is the structure of the “space of all computational effects”. We know very little about this topic. I am not even sure we know what “the space of all computational effects” is. Because Haskell is very popular and in Haskell computational effects are expressed as monads, many people might think that I am talking about the space of all monads. But I am not, and in this post I show two computational effects which are not of the usual Haskell monad kind. They should present a nice programming challenge to Haskell fans.

### Exact real arithmetic in Haskell

- 03 September 2008
- Computation, Guest post, News, Software

*HERA* is an implementation of exact real arithmetic in Haskell using the approach by Andrej Bauer and Iztok Kavkler, see these and these slides. It uses the fast multiple precision floating point library MPFR. Download source, and see documentation and examples of usage at my home page.

[*Note by Andrej:* this is a guest post by Aleš Bizjak, a first-year student of mathematics at my department. I am very proud of the excellent work he did on his summer project.]

### Efficient computation with Dedekind reals

- 24 August 2008
- Computation, Constructive math, Logic, Talks

Two versions of this talk were given at Computability and complexity in analysis 2008 and at Mathematics, Algorithms and Proofs 2008.

Joint work with Paul Taylor.

**Abstract:** Cauchy's construction of reals as sequences of rational approximations is the theoretical basis for a number of implementations of exact real numbers, while Dedekind's construction of reals as cuts has inspired fewer useful computational ideas. Nevertheless, we can see the computational content of Dedekind reals by constructing them within Abstract Stone Duality (ASD), a computationally meaningful calculus for topology. This provides the theoretical background for a novel way of computing with real numbers in the style of logic programming. Real numbers are defined in terms of (lower and upper) Dedekind cuts, while programs are expressed as statements about real numbers in the language of ASD. By adapting Newton's method to interval arithmetic we can make the computations as efficient as those based on Cauchy reals.

**Slides:** slides-map2008.pdf (obsolete version: slides-cca2008.pdf)

**Extended abstract:** abstract-cca2098.pdf

### Representations of uncomputable and uncountable sets

- 06 February 2008
- Computation, Tutorial

Occasionally I hear claims that uncountable and uncomputable sets cannot be represented on computers. More generally, there are all sorts of misguided opinions about representations of data on computers, especially infinite data of mathematical nature. Here is a quick tutorial on the matter whose main point is:

→ continue reading (11 comments)It is

meaninglessto discuss representations of a set by a datatype without also considering operations that we want to perform on the set.

### Seemingly impossible functional programs

- 28 September 2007
- Computation, Guest post, Tutorial

Andrej has invited me to write about certain surprising functional programs. The first program, due to Ulrich Berger (1990), performs exhaustive search over the “Cantor space” of infinite sequences of binary digits. I have included references at the end. A weak form of exhaustive search amounts to checking whether or not a total predicate holds for all elements of the Cantor space. Thus, this amounts to universal quantification over the Cantor space. Can this possibly be done algorithmically, in finite time?

→ continue reading (21 comments)### The Role of the Interval Domain in Modern Exact Real Arithmetic

- 18 September 2007
- Computation, Constructive math, RZ, Talks

With Iztok Kavkler.

**Abstract:** The interval domain was proposed by Dana Scott as a domain-theoretic model for real numbers. It is a successful theoretical idea which also inspired a number of computational models for real numbers. However, current state-of-the-art implementations of real numbers, e.g., Mueller's iRRAM and Lambov's RealLib, do not seem to be based on the interval domain. In fact, their authors have observed that domain-theoretic concepts such as monotonicity of functions hinder efficiency of computation.

I will review the data structures and algorithms that are used in modern implementations of exact real arithmetic. They provide important insights, but some questions remain about what theoretical models support them, and how we can show them to be correct. It turns out that the correctness is not always clear, and that the good old interval domain still has a few tricks to offer.

**Download slides:** domains8-slides.pdf

### Implementing real numbers with RZ

- 12 April 2007
- Computation, Constructive math, Publications, RZ, Talks

With Iztok Kavkler.

**Abstract:** RZ is a tool which translates axiomatizations of mathematical structures to program speciï¬cations using the realizability interpretation of logic. This helps programmers correctly implement data structures for computable mathematics. RZ does not prescribe a particular method of implementation, but allows programmers to write efficient code by hand, or to extract trusted code from formal proofs, if they so desire. We used this methodology to axiomatize real numbers and implemented the speciï¬cation computed by RZ. The axiomatization is the standard domain-theoretic construction of reals as the maximal elements of the interval domain, while the implementation closely follows current state-of-the-art implementations of exact real arithmetic. Our results shows not only that the theory and practice of computable mathematics can coexist, but also that they work together harmoniously.

Presented at Computability and Complexity in Analysis 2007.

**Download paper:** rzreals.pdf

**Download slides:** cca2007-slides.pdf

### Are small sentences of Peano arithmetic decidable?

- 04 November 2006
- Computation, General, Logic

Recently there has been a discussion (here, here, here, and here) on the Foundations of Mathematics mailing list about completeness of Peano arithmetic (PA) with respect to “small” sentences. Harvey Friedman made several conjectures of the following kind: “All true *small* sentences of PA are provable.” He proposed measures of smallness, such as counting the number of distinct variables or restricting the depth of terms. Here are some statistics concerning such statements.

### König's Lemma and the Kleene Tree

- 25 April 2006
- Computation, Constructive math, Publications, Tutorial

For the benefit of the topology seminar audience at the math department of University of Ljubljana, I have written a self-contained explanation of the Kleene tree, which is an interesting object in computability theory. For the benefit of the rest of the planet, I am publishing it here.

→ continue reading (2 comments)### Sometimes all functions are continuous

- 27 March 2006
- Computation, Constructive math, Tutorial

You may have heard at times that there are mathematicians who think that *all* functions are continuous. One way of explaining this is to show that all *computable* functions are continuous. The point not appreciated by many (even experts) is that the truth of this claim depends on what programming language we use.

### Proof hacking

- 16 September 2005
- Computation, Logic, Tutorial