I started a quixotic quest to work through Chris Okasaki's Purely Functional Data Structures, implementing all the examples and exercises in Objective-C. This makes little sense, but if you're curious, I wrote a bit on my blog about why I started tilting at that particular windmill.
In the interim, Apple released Swift. So, a change of plans. I'm going to work through the book in Swift instead. I worked through most of chapter 2 in Objective-C before shelving that. I'll likely pull out a blog post comparing the Swift and Objective-C versions of that chapter. In the meantime, take a look at the chapter2 branch here to see progress.
I haven't decided exactly how to organize this code yet, so stuff may move around. My apologies if that makes anything hard to follow. I'll try to lean heavily on version control to track work by chapter while organizing the code as if it might be used practically. I'm going to try to push branches to github for each chapter while letting master track my overall progress through the book.
I dusted this off again after a year or so away. I've finished the work for chapter2 and merged that to master. Look for "NOTE" in the comments to see things that I thought were interesting.