Search as a Core Feature
One of the main differences between Mailpile and most other Free Software e-mail clients has to do with the approach we take to handling e-mail.
The first generation of e-mail clients focused on the e-mail itself and provided mailboxes or folders as places to store it. Organizing your e-mail meant moving it around, from one mailbox to another. This is how most desktop e-mail clients work today.
Mailpile's approach is different. Inspired by GMail, we decided to make search the central metaphor. Organizing mail in Mailpile then became a matter of labeling messages in such ways that they could easily be searched for; this is how Mailpile's tags work.
Tags are much more flexible and powerful than mailboxes; once the search engine has indexed all your mail it no longer matters in which mailbox or folder the mail is stored since you can access and organize any combination of messages using a mixture of tags and search terms. Searching a well designed index is actually faster (both for the human and the computer) than finding and opening the right mailbox.
So Mailpile is a search engine first and foremost. Most of the other features it has are built on top of that foundation.
What About Mailboxes?
This is all well and good.
But the fact remains that sometimes we need to open a mailbox and look inside; after all, that is where the mail is!
Mailpile has struggled with this from the beginning. Being built around a single search engine meant Mailpile couldn't really do much with the contents of a mailbox until it had all been processed and added to the search index.
This led to usability problems. If Mailpile was given a large number of mailboxes to process it could take quite some time before it got to the one the user was interested in. If the background indexing process had a problem, mail would just never appear. Users coming from traditional mail clients had expectations which Mailpile could not meet. And last but not least, it made troubleshooting very difficult because there were so many layers of code, each introducing potential bugs or delays, that an e-mail had to travel through before it appeared (or failed to appear) in the user interface.
Sometimes you just want to open up a mailbox and look inside, without having to add all the mail to your Mailpile.
The solution I have found to this problem wasn't to stop treating search as a core feature. It was to embrace it and take it a step further: who says there should only be one way to search?
Many IMAP servers offer search features. We should be able to make use of them. Similarly, searching a raw mailbox is relatively straightforward - it may not be elegant or as fast as Mailpile's native index, but running "grep" or the equivalent very often gives useful results. It turns out there are many ways to search mailboxes that haven't been fully processed and added to the main Mailpile index.
So Mailpile now internally supports multiple search indexes. At the moment it only searches one at a time, and some search indexes are not very good at searching yet... but the code is elegant and clean, works well and has interesting potential for the future.
Maybe someday we'll have hybrid search, which searches both remote IMAP servers and the local index. Maybe someday we'll be able to pull in results from notmuch or some alternate index.
But for now, at least this approach makes it easy and quick for Mailpile to look inside raw mailboxes. That alone clears a major roadblock out of the way for a 1.0 release.
I am still mulling over how best to expose this in the user interface. At the very least, it will be the default behaviour when accessing mailboxes from the browsing tool. I may also make it accessible via the per-mailbox tags in the sidebar; doing so is likely to match user expectations better than using Mailpile's internal index. But I'm not entirely sure.
I'll be pushing this up for review once I've finished a few more test.