In type theory, the existential quantifier, $\exists$ is the dual to the more familiar universal quantifier, $\forall$ (which expresses parametric polymorphism). Believe it or not, these existentially quantified types can be useful in day-to-day programming in languages like Haskell where you wish to perform some kind of “information hiding”. As a simple example, consider this program:

This is a pretty obscure problem, but I’m going to put a post up about it on the off chance I can help someone else out. My regular reader (hi dad!) will probably find this of no interest and should give it a miss :-)

The Haskell community has built up a great resource: the Hackage Haskell package database, where we recently hit the 500-package mark!

I was given the opportunity to speak on the topic of compiler plugins for GHC at AngloHaskell 2008 last weekend, and the slides and audio are now available here for those interested.

Are we six weeks into it already? It’s flown by. What did I get up to this week?