Lambda the Ultimate

Conor McBride gave an 8-lecture summer course on Dependently typed metaprogramming (in Agda) at the Cambridge University Computer Laboratory:

Dependently typed functional programming languages such as Agda are capable of expressing very precise types for data. When those data themselves encode types, we gain a powerful mechanism for abstracting generic operations over carefully circumscribed universes. This course will begin with a rapid depedently-typed programming primer in Agda, then explore techniques for and consequences of universe constructions. Of central importance are the “pattern functors” which determine the node structure of inductive and coinductive datatypes. We shall consider syntactic presentations of these functors (allowing operations as useful as symbolic differentiation), and relate them to the more uniform abstract notion of “container”. We shall expose the double-life containers lead as “interaction structures” describing systems of effects. Later, we step up to functors over universes, acquiring the power of inductive-recursive definitions, and we use that power to build universes of dependent types.

The lecture notes, code, and video captures are available online.

As with his previous course, the notes contain many(!) mind expanding exploratory exercises, some of which quite challenging.

I'm making this new programming language Ya. I'm not sure I'm right writing to you, yet I just don't know what to do to make people know that new programming language Ya exists. If you know what I should do - please help me. Thank you. Pavel Senatorov, Ya-Lang.com .

Dynamic Region Inference, by David Pereira and John Aycock:

We present a garbage collection scheme based on reference counting and region inference which, unlike the standard reference counting algorithm, handles cycles correctly. In our algorithm, the fundamental operations of region inference are performed dynamically. No assistance is required from the programmer or the compiler, making our algorithm particularly well-suited for use in dynamically-typed languages such as scripting languages. We provide a detailed algorithm and demonstrate how it can be implemented efficiently.

A novel garbage collector that solves reference counting's cycle problems by introducing "regions", which demarcate possibly cyclic subgraphs. These regions are updated by merge and split operations that occur on pointer update and incrementally on region allocation, respectively, ie. adding a pointer to B into aggregate C merges their regions, and trying to allocate a new region first attempts to split some random candidate region by computing the local reference counts via union-find of the region's members.

Obviously dynamic regions don't share contiguous storage like their static counterparts, so "regions" here are purely a logical concept to ensure completeness of reference counting. The implementation adds two words to each object, one for pointing to the object's current region, the other for a "next" pointer for the next object in the region.

The practicality of this approach isn't clear compared to other cycle detection algorithms, and no benchmarks are provided. I haven't found any follow-up work either.