I was struggling to understand Rich Hickey's notion of transducers in Clojure that he announced in a blog post and posted some sample code for in response to a discussion thread. The discussion there was very confusing to me, and I decided I could not really understand what was going on until I actually reimplemented the ideas in a statically typed language.
Fortunately, on another discussion thread, he posted actually running Haskell code, which helped me tremendously, since most of the discussion I had seen so far was either very vague or very abstract.
So I refactored his code to fit just exactly what he did, not something far more general, and posted my refactored code in response. I hope this will help more people understand exactly what they do, using types.
Note that the critical component of this work involves using rank-2 types. Transducers cannot be expressed in a weaker type system that does not support higher-rank types.
I hope that the introduction of transducers will create a lot of interest among those who are not already familiar with higher-rank types.
I wrote a blog post about this code.