Giving Golang a second look for writing a mailreader (in 2012)

In 2012, starting again with golang.

• The email address parsing functionality is a little tied into the email system; you apparently have to make a fake header.
• go doc is pretty cool, but looking at the actual library source is even better in most cases.
• Type equivalence is by name, so even if a mail.Header is just a map[string][]string, you still can't mutate it. But you can construct a new one.
• for x := range somearray makes x range over the indices of somearray, not its values. You want _, x :=.
• How do you find the length of an array? Oh, Pythonwise, len(array).

• Okay, so suppose I want to do the equivalent of this Python:

  def main(outbox, inbox):
      important_senders = recipients(outbox)
  

  I guess I need to be able to open the outbox file and accumulate stuff in a map. Hmm, isn't there a thing about named return values? How does that work? Oh, (name type, name type, name type) after the argument list.

• How do I open a file? os.Open.

• How do I print messages to stderr and return failure from main? Looks like log.Fatal does that, and even log.Printf prints to stderr. (Presumably fmt.Printf doesn't.)
• Now that I have the file open, how do I get messages from it? Looks like the file is already an io.Reader, so I'm good.
• Hmm, now it seems like I'm suffering from email parsing problems, from apparently trying to parse a header starting in the middle of a subject line. I guess I need to read that message body.
• But hmm, io.Reader.Read(p) reads up to len(p) bytes. How do I make a slice (?) that I can pass in there? (Looks like make([]byte, 16).) Oh, there's an ioutil.ReadAll? That solves the problem. But now I still have no addresses! ... Also related to problems like these: bufio.NewReader(os.Stdin) and r.ReadString("\n").
• Oh, hmm, this is just message parsing, not mailbox parsing. I still have to do mailbox parsing myself, and provide an io.Reader interface? Well, there's an io.LimitReader in the library.
• Cool, the I/O 2011 talk mentions an SDL binding for Go.
• Looks like the email message parsing chokes on this:

  To: Kragen Sitaker <kragen>