Okay I think I get free monads now. They seem pretty awesome, essentially letting you plug in an interpreter for the function you're going to run. I can see high-level how this would appear to be a great generic DI option--you set up an "interpreter" to handle the statements in your function however you want: for reals, for test, for reals with logging, etc. And automagically everything gets executed exactly how you want with no additional cruft.

What makes me cautious about the concept though is that e.g. `do_x_and_y()` would be interpreted differently than `do_x(); do_y()`, even if they were fundamentally the same. While "so what?" is a perfectly valid response, that little tidbit just makes me feel like, while FM's are a very cool abstraction for something, it's not really ideal for DI. It's just something meant for a different level. The article "The Wrong Abstraction" comes to mind.