:id "pre" :style "font-size: 70%" url))
  
  (define-service (foo)
     (
        ( :jscript (service-resource foo "foo.scm"))
        (
         ( :onclick ~(with-hop ($(service () (new-url))) update)
              "New barchart")
           (
 :style "margin-top: 2ex"
              img
              pre))))


The server modules imports the client module named ++client++ defined as:

  (module foo-client
     $(import foo-server)
     (export (update)))

  (define (update src)
     (innerHTML-set! $pre src)
     (set! $img.src src))


These modules assume a "module access file" the ++.afile++ defined as:

  (
    (server "foo.hop")
    (client "foo.scm")
  )


== Server Side core language ==

This section presents the HOP constructions that are only applicable to
server-side programming.


=== Classes and Objects === 

The server side HOP object model is inspired by the
[[http://en.wikipedia.org/wiki/Common_Lisp_Object_System|Common Lisp Object System]].  
Contrary to languages such as SmallTalk or Java, in
that model, //methods// are not attached to classes but to //generic
functions//. This enables code to add new functionalities to
pre-existing data structures (objects). The HOP declination uses
single inheritance and single dispatch. That is, a class may inherit
from exactly one class and only the first argument of method explored
to resolve the late binding.

~~ Server side classes are defined in the module declaration. A class may either
be //exported// in which case all the weblet might create objects of class,
declare subclasses, generic functions, and methods.

~~ For the sake of the example, here is a simple source code that defines 
two classes:


  (module example
     (static (class point x y))
     (export (class point3d::point z))
     (export (generic to-string ::object)))

  (define-generic (to-string obj)
     (with-output-to-string (lambda () (display obj))))

  (define-method (to-string obj::point)
     (with-access::point obj (x y)
        (format "point:~a-~a" x y)))

  (define-method (to-string obj::point3d)
     (with-access::point3d obj (z)
        (format "~a-~a" (call-next-method) z)))



The form ++define-generic++ defines a new generic function. The form
++define-method++ adds a method to a generic function. The generic function
must be visible by the module declaring the method. The form ++with-access++
opens an object up. Objects are creates with the ++instantiate++ form. Here
is an expression creating an instance of the ++point3d++:


  (instantiate::point3d
     (z 10)
     (x 0)
     (y 5))



Class fields that are provided with a default value are not required  by
++instantiate++.

== Character set ==

The character set (charset) used for denoting strings of characters varies
depending on the execution context and the configuration parameters.
On the client-side, the charset used is the one of the hosting platform.
Since HOP client-side expressions are designed to be strictly compatible
with JavaScript, these expressions adopt the JavScript charset which is
UCS-2. 

~~ The communications between the client and the server can be changed but
by default UTF-8 is used.

~~ The charset of the server depends on two parameters: ++hop-locale++
and ++hop-charset++. The first one specifies the charset of the native
environment hosting the server (i.e., the charset of the operating that
is running the server). The latter specifies how HOP internally represents
character strings. Hence, if ++hop-locale++ differs from ++hop-charset++
a translation mechanism is involved when expressions are read. Several
functions (++charset-convert++, ++iso-latin->utf8++, or 
++ucs2-string->utf8-string++) can be used for converting strings from
one charset into another.