I’m no OOP fan (much less a fan of single-dispatch OOP), but sometimes I miss the implicit lexical scope that single-dispatch provides for methods. Take something as simple as

class Rect {
  int top, left, bottom, right;
  int area () const {
    return (top - bottom) * (right - left);
  }

Most Scheme object systems (see for example the Chicken OOP section) turn the area() method body into something tedious along the lines of

(* (- (slot-ref self 'top) (slot-ref self 'bottom))
   (- (slot-ref self 'right) (slot-ref self 'left)))

or, with objects implemented as closures,

(* (- (self 'top) (self 'bottom) ...) ...)

Until recently, I thought that short of codewalker-based macros, nothing could restore the terseness of single-dispatch methods.

Well, I’ve discovered Blake McBride’s POS (portable object system). With POS, you can write

(define-class Rect (ivars top left bottom right)
  (imeths
    (set-top! (self val) (set! top val)) ...
    (get-area (self) (* (- top bottom) (- right left)))))

POS is a set of pure R5RS macros, and correctly interacts with other syntax-rules macros (e.g. macros can appear within method bodies). The trick is not only to represent the objects as closures, but to also expand method bodies inside the closure:

;; not the actual POS expansion -- just an illustration
(define (make-rect)
  (let ((top #f) (left #f) (bottom #f) (right #f))
    (define self
      (lambda (meth-name . args)
        (case meth-name
          ((set-top!)
           (apply (lambda (self val) (set! top val))
                  (cons self args))) ...
          ((get-area)
           (apply (lambda (self)
                    (* (- top bottom) (- right left))
                  (cons self args)))))))

    self))

This way, methods can access instance variables as simple literals. Each object is a dispatch function that closes over those variables.

POS is very useful, and I plan to add default getters and setters, as well as a way to convert between the closure representation and a-lists. This should help with persistence, among other things.

POS has a couple of extra features (inheritance, access to the parent object, class methods) but really is a light-weight system. The major downside is that methods (and instance variables) can no longer be added dynamically, since it’s impossible to inject code (or data) into a closure.

Update: see the comments for yet another way of simulating an implicit “this” argument.

Most hosting accounts come with cPanel, and by implication Exim and Courier under the hood. Some people access their mail using the cPanel webmail interface (usually via https://example.com:2096), but if you need to send more than the occasional e-mail, you probably want to set up Outlook or Thunderbird to connect to the IMAP server.

Sometimes, the hosting company won’t have a canonical host name and matching SSL certificate for your domain, which will lead to endless security warnings in Thunderbird. If you’ve got shared hosting, there’s not much you can do (short of opening a support ticker and hoping for the best), but if you are a VPS customer, here’s how to fix your problem: first, edit /usr/lib/courier-imap/etc/imapd.cnf (in particular, set the correct hostname in the CN=… line). Then, run Courier’s mkimapdcert. This will generate the file /usr/lib/courier-imap/share/imapd.pem, which combines a key and certificate and is used by the Courier IMAP server. Next, copy and paste the RSA private key (including the delimiter lines) from the PEM file to /etc/exim.key, and similarly the certificate (the second section in the PEM file) to /etc/exim.crt.

When you start Thunderbird, it will complain that it can’t verify the certificate (to avoid this you’d have to pay a Certificate Authority like Verisign or Thawte, but we’re not doing that today). Choose to accept the certificate permanently. Voilà, no more warnings.