Seemingly impossible programs

In case this one went under the radar, at POPL'12, Martín Escardó gave a tutorial on seemingly impossible functional programs:

Programming language semantics is typically applied to
prove compiler correctness and allow (manual or automatic) program
verification. Certain kinds of semantics can also be applied to
discover programs that one wouldn't have otherwise thought of. This is
the case, in particular, for semantics that incorporate topological
ingredients (limits, continuity, openness, compactness). For example,
it turns out that some function types (X -> Y) with X infinite (but
compact) do have decidable equality, contradicting perhaps popular
belief, but certainly not (higher-type) computability theory. More
generally, one can often check infinitely many cases in finite time.

I will show you such programs, run them fast in surprising instances,
and introduce the theory behind their derivation and working. In
particular, I will study a single (very high type) program that (i)
optimally plays sequential games of unbounded length, (ii) implements
the Tychonoff Theorem from topology (and builds finite-time search
functions for infinite sets), (iii) realizes the double-negation shift
from proof theory (and allows us to extract programs from classical
proofs that use the axiom of countable choice). There will be several
examples in the languages Haskell and Agda.

A shorter version (coded in Haskell) appears in Andrej Bauer's blog.

EATCS Award 2014: Gordon Plotkin

Gordon Plotkin is renowned for his groundbreaking contributions to programming language semantics, which have helped to shape the landscape of theoretical computer science, and which have im-pacted upon the design of programming languages and their verification technologies. The in-fluence of his pioneering work on logical frameworks pervades modern proof technologies. In addition, he has made outstanding contributions in machine learning, automated theorem prov-ing, and computer-assisted reasoning. He is still active in research at the topmost level, with his current activities placing him at the forefront of fields as diverse as programming semantics, applied logic, and systems biology.

Well deserved, of course. Congrats!

CFP: Off-the-Beaten-Track (OBT) workshop at POPL 2015

Announcing the 2015 edition of the OBT workshop, to be co-located with POPL 2015, in Mumbai, India. Two-page paper submissions are due November 7, 2014.

From the web page (

Programming language researchers have the principles, tools, algorithms and abstractions to solve all kinds of problems, in all areas of computer science. However, identifying and evaluating new problems, particularly those that lie outside the typical core PL problems we all know and love, can be a significant challenge. This workshop's goal is to identify and discuss problems that do not often show up in our top conferences, but where programming language research can make a substantial impact. We hope fora like this will increase the diversity of problems that are studied by PL researchers and thus increase our community's impact on the world.

While many workshops associated with POPL have become more like mini-conferences themselves, this is an anti-goal for OBT. The workshop will be informal and structured to encourage discussion. We are at least as interested in problems as in solutions.

Domain settings

I am about to make some changes to the name server definitions. Since changes take time to propagate, you may have trouble reaching the site for awhile. If this happens, try using the .com domain instead of the preferred .org domain.

In his blog, Bob Harper, in joint effort with Dave MacQueen and Lars Bergstrom, announces the launch of

The Standard ML Family project provides a home for online versions of various formal definitions of Standard ML, including the "Definition of Standard ML, Revised" (Standard ML 97). The site also supports coordination between different implementations of the Standard ML (SML) programming language by maintaining common resources such as the documentation for the Standard ML Basis Library and standard test suites. The goal is to increase compatibility and resource sharing between Standard ML implementations.

The site includes a history section devoted to the history of ML, and of Standard ML in particular. This section will contain a collection of original source documents relating to the design of the language.

Inferring algebraic effects

Logical methods in computer science just published Matija Pretnar's latest take on algebraic effects and handlers:

We present a complete polymorphic effect inference algorithm for an ML-style language with handlers of not only exceptions, but of any other algebraic effect such as input & output, mutable references and many others. Our main aim is to offer the programmer a useful insight into the effectful behaviour of programs. Handlers help here by cutting down possible effects and the resulting lengthy output that often plagues precise effect systems. Additionally, we present a set of methods that further simplify the displayed types, some even by deliberately hiding inferred information from the programmer.

Pretnar and Bauer's Eff has made previous appearances here on LtU. Apart from the new fangled polymorphic effect system, this paper also contains an Eff tutorial.

LtU's new server

Lambda the Ultimate is now running on a new, faster, more reliable server. The old one is now, uh... pining for the fjords. This should resolve the increasingly frequent outages we've seen recently. Because the old server had started failing, we didn't have time to do as much quality control on the migration as we would have liked. If anyone notices any issues with the site, please comment in this thread. Currently known issues:
  • Non-Latin UTF-8 characters apparently didn't survive the database migration correctly. This is a particular issue if you have a username containing non-Latin characters - you may not be able to log in currently.
  • It's possible that some comments posted later on Monday don't appear on the new site. (Resolved: there were no missing comments)
  • New user signup emails are not yet working.
  • Due to DNS propagation, not everyone will see the new site immediately. (Now resolved)
The struck out issues have been resolved. The remaining issue, with user signup emails, should be resolved in the next few days.

Breaking the Complexity Barrier of Pure Functional Programs with Impure Data Structures

Breaking the Complexity Barrier of Pure Functional Programs with Impure Data Structures by Pieter Wuille and Tom Schrijvers:

Pure functional programming language offer many advantages over impure languages. Unfortunately, the absence of destructive update, imposes a complexity barrier. In imperative languages, there are algorithms and data structures with better complexity. We present our project for combining existing program transformation techniques to transform inefficient pure data structures into impure ones with better complexity. As a consequence, the programmer is not exposed to the impurity and retains the advantages of purity.

This paper is along the same lines a question I asked a couple of years ago. The idea here is to allow programming using immutable interfaces, and then automatically transform it into a more efficient mutable equivalent.

Inside the Wolfram Language

Video of Stephen Wolfram showing off the Wolfram Language and sharing his perspective on the design of the language at Strange Loop conference.

What's in store for the most widely used language by discerning hackers?

Or, in other words, what's the future of Emacs Lisp (and unavoidable HN discussion).

The original message contains some interesting tidbits. I am not sure how the discussion on emacs-devel will develop. But speculating about things such as Guile elisp is, of course, our bailiwick.