Hi, This morning I have written an STM implementation as a learning exercise. I thought I'd share it here to get some critique and because it might be useful to others (consider it as being in the public domain). I haven't tested it thoroughly, so there may be edge cases that haven't been thought through. It should be a drop-in replacement (so long as you only use TVars), if anyone has any STM benchmarks already made, I'd love to see how it compares. Here it is: https://gist.github.com/1454995 Thanks, Daniel