To summarize my general reaction to several of the other threads, I think that the particular code layout you use -- breaking the usual expectations of indentation for nested code blocks -- is getting in the way of the "elegance" you're trying to offer.

It may seem elegant to you, but because it's very different, you're making the learning curve a lot steeper than it needs to be.

I think the same is true of syntax. I think the move to 'readable' and 'writeable' and so on is a step in the right direction, but those names don't naturally communicate that they set a one-time event callback. I know you were looking to move away from 'on_' or 'when_' prefixes, but I really wonder if a more expressive name would make it easier for people to learn.

For many people in a work context, I suspect that maintainability will be an important consideration. So making IO::Lambda more expressive and easier to learn may seem less 'elegant' but ultimately may make it more useful to others.