This meditation is my reaction to reading Brian Ingerson's perl.com article on his new IO::All module, which (in brief) is an API for doing IO.
Now, IO::All is interesting for a number of reasons, but I'm interested only in the design of the API in this node. If you read the article, or download the module and play with it, it's my opinion that you'll find this is a pretty good API. Some parts of it I'm less fanboy of, but overall I think it's great - it hits you as, "why didn't Perl have this from the start?!".
How does one go about designing this kind of thing? In fact, do you design it at all? Ingy put it down to XP practices, at least in part. And there is definitely some "knowing when to break the rules" going on - such as exporting, by default, a function to the caller. This kind of thing is usually frowned upon, but Spiffy.pm is... well, Spiffy.
There is, of course, no real answer to the "how to I get my API right from the start?". It's likely even Ingy didn't get it right on his first attempt, maybe not even his second. And, of course, the light sabre is the tool of the Jedi, which probably impies something. But, what works for people? Let the sands of time erode the rough edges? Decimate the functionality with every major release, to instill fear into the remaining functions? Keep rewriting it until you like it? I guess this is a fixed-chord problem, but your thoughts are most welcome.