Lambda the Ultimate

Generic programming — Or: write everything once

Ralf Hinze summarizes polytypic programming, with a focus on Generic Haskell. He discusses:

• Generic functions on types
• Generic functions on type constructors
• Generic types

In addition to the usual generic equality and serialization, he mentions fold and map (though not by name), along with a dozen other ideas that use polytypism. He also notes that the Generic Haskell experiment will be continued and integrated into a "standard Haskell compiler".

Systematic search for lambda expressions by Susumu Katayama

This paper presents a system for searching for desired small functional programs by just generating a sequence of type-correct programs in a systematic and exhaustive manner and evaluating them. The main goal of this line of research is to ease functional programming, along with the subgoal to provide an axis to evaluate heuristic approaches to program synthesis such as genetic programming by telling the best performance possible by exhaustive search algorithms. While our previous approach to that goal used combinatory expressions in order to simplify the synthesis process, which led to redundant combinator expressions with complex types, this time we use de Bruijn lambda expressions and enjoy improved results.

Although the algorithm is slow and only works on small expressions, it looks helpful. Perhaps there could be some synthesis with Hoogλe, or whatever the name is after Google makes them change it.

Guess what I stumbled across at my local bookstore?

Previously mentioned on LtU, and now available... When the book was announced, Ehud said:

Authored by two of my favorites, Dan Friedman and Oleg, I have such high expectations, that however great the book is going to be, I am sure to be disappointed...

After working through the first five chapters (and sneaking a look at the implementation at the end), I'm pleased to announce that no one is likely to be disappointed... It's a real tour de force.

As expected, the focus this time is logic programming, in the form of a new set of primitives elegantly implemented around a backtracking monad in Scheme. Of course the format is familiar and comfortable, and of course it's charmingly illustrated by Duane Bibby.

So, get your copy today, and congratulations to the authors on a job well done!

A fairly recent blog that might interest some LtU readers.

It's early days yet, so I am sure the author could use some encouragement and support...

(via the LtU forum)

The presentations aren't online unfortunately, but it's good to know about this meeting.

We present a file server/OS where threading and exceptions are all realized via delimited continuations. We use zipper to navigate within any term. If the term in question is a finite map whose keys are strings and values are either strings or other finite maps, the zipper-based file system looks almost the same as the Unix file system. Unlike the latter, however, we offer: transactional semantics; undo of any file and directory operation; snapshots; statically guaranteed the strongest, repeatable read, isolation mode for clients; built-in traversal facility; and just the right behavior for cyclic directory references.



We can easily change our file server to support NFS or 9P or other distributed file system protocol. We can traverse richer terms than mere finite maps with string keys. In particular, we can use lambda-terms as our file system: one can cd into a lambda-term in bash.



The implementation of ZFS has no unsafe operations, no GHC let alone Unix threads, and no concurrency problems. Our threads cannot even do IO and cannot mutate any global state --- and the type system sees to it. We thus demonstrate how delimited continuations let us statically isolate effects even if the whole program eventually runs in an IO monad.

zfs-talk: Expanded talk with the demo. It was originally presented as an extra demo at the Haskell Workshop 2005

ZFS.tar.gz: The complete source code, with comments. It was tested with GHC 6.4 on FreeBSD and Linux


From: Zipper-based file server/OS


Neat, a referentially transparent filesystem with transactional semantics in 540 lines of Haskell. I can think of some interesting uses for this.

It rapidly became clear that a single language runtime is too limited for general applications ... as a minimum we needed both a linear-flow language and a recursive tree composition language ... while declarative languages are excellent for rapid assembly of XML operations, they are terrible for expressing business logic and logical flow-control ... Our other declarative language is XML Recursion Language (XRL). XRL is like XInclude with services, in which inclusion references fire service invocations into the URI address space in order to recursively compose an XML document. XRL is an elegant and powerful way of building XHTML applications ... The active URI, in combination with the local NetKernel environment, is a functional program - Introducing NetKernel.

Main site; Tour.

It's another XML pipeline (there's a Freshmeat project that lets Coccoon apps run in NetKernel), apparently from HP, which might interest people here.

Visual Haskell is a complete development environment for Haskell software, based on Microsoft's Visual Studio platform. Visual Haskell integrates with the Visual Studio editor to provide interactive features to aid Haskell development, and it enables the construction of projects consisting of multiple Haskell modules, using the Cabal building/packaging infrastructure.

Visual Haskell is a complete system in one download: it includes a full GHC installation with libraries, and various tools including Haddock, Happy, and Alex.

Worth knowing about.

The Essence of Dataflow Programming

The abstract:

We propose a novel, comonadic approach to dataflow (streambased) computation. This is based on the observation that both general and causal stream functions can be characterized as coKleisli arrows of comonads and on the intuition that comonads in general must be a good means to structure context-dependent computation. In particular, we develop a generic comonadic interpreter of languages for context-dependent computation and instantiate it for stream-based computation. We also discuss distributive laws of a comonad over a monad as a means to structure combinations of effectful and context-dependent computation. We apply the latter to analyse clocked dataflow (partial streams based) computation.

If you've ever wondered about dataflow or comonads, this paper is a good read. It begins with short reviews of monads, arrows, and comonads and includes an implementation. One feature that stood out is the idea of a higher-order dataflow language.

André Pang, Don Stewart, Sean Seefried, and Manuel M. T. Chakravarty. Plugging Haskell In (pdf). Proceedings of the ACM SIGPLAN Workshop on Haskell, 2004, pp. 10-21.

Extension languages enable users to expand the functionality of an application without touching its source code. Commonly, these languages are dynamically typed languages, such as Lisp, Python, or domain-specific languages, which support runtime plugins via dynamic loading of components. We show that Haskell can be comfortably used as a statically typed extension language, and that it can support type-safe dynamic loading of plugins using dynamic types.

Two of the authors also have a more recent paper (pdf) describing applications they've built using hs-plugins.