RE: Something like an array (list) of a class

[Drew Adams]

> >> > I would like to make some datadriven functionality. Is 
> >> > there a way to make sure a list is filled with a certain
> >> > type of data. I would like to make sure that every element
> >> > of my list contains: a type, a description, a functionname
> >> > and room to store a string. Is this possible?
> >>
> >> ,----
> >> | ELISP> (setq B '(2 "some text" message nil))
> >> | (2 "some text" message nil)
> >> |
> >> | ELISP> (if (and (find-if 'numberp B)
> >> |                 (find-if 'stringp B)
> >> |                 (find-if 'functionp B)
> >> |                 (null (car (last B))))
> >> |            t
> >> |            nil)
> >> | t
> >> `----
> >
> > Partly. This is an 'object' and not a list of objects. But I
> > understand correctly that it is not possible to define real objects?
> > What I would like is something like (C++ code):
> >
> > class Object {
> >     String type;
> >     String description;
> >     String functionname;
> >     String displayString;
> > }
> >
> > std::vector <Object> objectVector;
> >
> > When given a certain type I then need to retreive the 'object' from
> > the list to use.
> 
> There is no class in elisp but you have defstruct (be sure to 
> require 'cl)
> 
> ,----
> | ELISP> (defstruct Object type description functionname 
> |        displaystring)
> |        Object
> | ELISP> (defvar test-obj (make-Object :type 1 :description 
> |         "test" :functionname 'message :displaystring nil))
> | test-obj
> | ELISP> (Object-type test-obj)
> | 1
> | ELISP> (Object-functionname test-obj)
> | message
> | ELISP> (Object-description test-obj)
> | "test"
> `----

Various books and Web sites about Lisp show you how to implement objects in
Lisp, depending on what you mean by "object" (abstract data type? mutable
object? classes? inheritance?...).

In addition to Common Lisp's `defstruct', there is the Common Lisp Object System
(CLOS), but Emacs Lisp support for it is wanting. (From the Elisp manual: "Some
features are too complex or bulky relative to their benefit to Emacs Lisp
programmers. CLOS and Common Lisp streams are fine examples of this group.")

One thing you might also consider (again, depending on what you need/want), is
that Emacs Lisp does provide a type-checking mechanism - in the context of
Customize. This is often overlooked or under-exploited. Customize options can be
complex structures whose parts are well typed, and compile-time and runtime
type-checking are available. In addition, you can define initialization and
set/put methods/triggers.

Whether you want to use options for your data structures in general is another
question. You would be exploiting the type-definition and type-checking features
of Customize without necessarily wanting to create user-visible options. But
this type manipulation is an existing and powerful Emacs-Lisp feature that you
can use in ways other than those originally intended.