How to use Haskell arrows (for beginners)

This post is an introduction to Haskell arrows (Control.Arrow), with no rigour at all. All you need is a basic understanding of Haskell, e.g. functors and monads.

Collapsible trees with CSS

Using CSS make collapsible trees has a certain appeal to it.

Nothing's wrong with ExceptT IO

ExceptT, quoted from the mtl library: Computations which may fail or throw exceptions.