The message s-expression (Mu4e 1.12.5 user manual)

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E.4 The message s-expression ¶

As a word of warning, the details of the s-expression are internal to the mu4e - mu communications, and are subject to change between versions.

A typical message s-expression looks something like the following:

(:docid 32461
 :from ((:name "Nikola Tesla" :email "niko@example.com"))
 :to ((:name "Thomas Edison" :email "tom@example.com"))
 :cc ((:name "Rupert The Monkey" :email "rupert@example.com"))
 :subject "RE: what about the 50K?"
 :date (20369 17624 0)
 :size 4337
 :message-id "C8233AB82D81EE81AF0114E4E74@123213.mail.example.com"
 :path  "/home/tom/Maildir/INBOX/cur/133443243973_1.10027.atlas:2,S"
 :maildir "/INBOX"
 :priority normal
 :flags (seen attach)
 ....
")

This s-expression forms a property list (plist), and we can get values from it using plist-get; for example (plist-get msg :subject) would get you the message subject. However, it’s better to use the function mu4e-message-field to shield you from some of the implementation details that are subject to change; and see the other convenience functions in mu4e-message.el.

Some notes on the format:

The address fields are lists of plists of the form (:name <name> :email <email>), where name can be nil.
The date is in format Emacs uses (for example in current-time).²⁰

Example: ping-pong

E.4.1 Example: ping-pong ¶

As an example of the communication between mu4e and mu, let’s look at the ping-pong-sequence. When mu4e starts, it sends a command ping to the mu server backend, to learn about its version. mu server then responds with a pong s-expression to provide this information (this is implemented in mu-cmd-server.c).

We start this sequence when mu4e is invoked (when the program is started). It calls mu4e--server-ping, and registers a (lambda) function for mu4e-server-pong-func, to handle the response.

-> (ping)
<-<prefix>(:pong "mu" :props (:version "x.x.x" :doccount 78545))

When we receive such a pong (in mu4e-server.el), the lambda function we registered is called, and it compares the version we got from the pong with the version we expected, and raises an error if they differ.

Footnotes

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Emacs 32-bit integers have only 29 bits available for the actual number; the other bits are use by Emacs for internal purposes. Therefore, we need to split time_t in two numbers.