Re: Preventing type extensions

[Brian Drummond <brian_drummond@btconnect.com>]

On Sat, 25 Sep 2010 09:10:58 +0200, Pascal Obry <pascal@obry.net> wrote:

>Ludovic,
>
>> I do not remember anything in Smalltalk resembling class-wide types;
>> could you please send me an URL? I'm curious.
>
>My Smalltalk is maybe rusty but I remember about Metaclass. A side of
>the class definition where a variable  is a class variable shared by
>every instance and where a method  is a class method shared also by
>every instance. This looks like class-wide variable/method applied to
>some kind of class-wide type (the metaclass).
>
>Now, as I said my Smalltalk is rusty and I may have misremembered something.
>
>Pascal.

My understanding (from Linn Lingo which had Smalltalk-like semantics but a more
block-structured syntax) was that class variables and class methods actually
applied to the class itself (or derived classes), rather than instances of those
classes.

Thus a Bounded_String class would have a class variable describing (and perhaps
allowing you to manipulate) the upper bound of the string size - this would be
used by the constructor for all Bounded_Strings.

Class methods relate to the class - e.g. to get/set the class variable above, or
describing properties of the class (e.g. "Integer max" = 2^31 - 1)

Every class has two methods which list the "instance methods" defined for an
instance of the class - one listing the methods defined for the class itself,
another listing all methods in the hierarchy above it, back to Object.

(possibly TMI : in Lingo it actually returns the selectors - tokens - rather
than the methods themselves; finding the methods from there is trivial)

"Integer instanceSelectors" (or even "2 class instanceSelectors") returns
+ - * / asString printedOn: asFloat 
and others.
"Integer inheritedInstanceSelectors" returns the same plus inherited methods
(obviously including "class", to support "2 class" above)

There are similar methods to return the class methods: "classSelectors" and
"inheritedClassSelectors". Their typical implementation is to return
"instanceSelectors" on the Metaclass object (a class variable) belonging to the
class itself.

Thus Metaclass does not seem to be related to class-wide types, but a mechanism
for implementing Class as a first-class object.


Ludovic is correct on dynamic despatch. It need not be slow; (there are
optimisations) but it does hide the flow of control (or abstract over it!)


- Brian