On Mar 28, 4:28 pm, David Nolen <dnolen.li...@gmail.com> wrote:
> On Sat, Mar 28, 2009 at 4:40 PM, mikel <mev...@mac.com> wrote:
>
> > So, at minimum, to make a solid port, you need to add a function that
> > can return a sensible type value for any input
>
> Enjoying the thread. Out of curiosity for which Clojure values is the return
> value of the type function undefined?
Try this:
(type (proxy [clojure.lang.IMeta clojure.lang.IRef][]))
java.lang.UnsupportedOperationException: meta (NO_SOURCE_FILE:0)
[Thrown class clojure.lang.Compiler$CompilerException]
No doubt someone is going to point out that the proxy object I created
there is useless; that's true, but beside the point. The point is that
it's straightforward to create some value v for which (type v) is
undefined. In order to make a Clojure-friendly version of CLOS, you
need some concept of object type such that you can define a function
that returns a sensible type for any value.
The way that Dylan handled situations similar to Clojure's proxy
objects was with objects called singletons; a singleton is an object
that represents a specific value as a class. For example, (singleton
5) returns an object that is a class, but is also the integer 5. You
can define methods that specialize on such objects.
For example, below is an excerpt from the UI libraries of Apple's long-
ago Bauhaus project. Notice the first line:
(define-method build-form ((key (singleton label:))
The equivalent syntax in Clojure (if clojure had these mechanisms)
would be:
(define-method build-form [[key (singleton :label)]
(There was no equivalent of defn in Dylan; there was only define-
method, because all values were instances of Dylan classes, and in
that environment a defn is no different from a define-method without
value constraints. Consequently, all functions were polymorphic; all
unsealed functions could be extended by adding new method
definitions.)
(define-method build-form ((key (singleton label:))
remaining-spec
form
current-x
current-y
layout-vector)
(ignore key)
(bind ((label-string (car remaining-spec))
(next (rest remaining-spec)))
(add-feature-to-form
(make <label-text>
string: label-string
top: current-y
left: current-x
style-array: $espy-10-bold-pointer)
next form current-x current-y layout-vector)))
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