Lambda the Ultimate

This paper illustrates the relevance of distributive laws for the solution of recursive equations, and shows that one approach for obtaining coinductive solutions of equations via infinite terms is in fact a special case of a more general approach using an extended form of coinduction via distributive laws...

Turi and Plotkin first investigated such a situation where one functor G describes the syntax of a programming language and the other functor F the behaviour of programs (terms) in that language. Having a distributive law GF => FG means that the behaviour on terms is well-defined, and leads to results like: (coalgebraic) bisimilarity is an (algebraic) congruence. Hence distributive laws capture where "algebra meets coalgebra".

Interesting stuff.

Be warned: This paper is highly technical, and requires familiarity with the fundamental notions of category theory.

twill is a reimplementation of PBP, the "Python Browser Poseur"... The primary use for twill (as with PBP) is to do automated testing of Web applications via a straightforward declarative language. In addition to basic Web crawling, I wanted to be able to extend the language via Python, and I also wanted to be able to record things with maxq. Hence, twill.