[squeak-dev] Re: Re: Unload Traits script (response to edgar)
Trygve Reenskaug
trygver at ifi.uio.no
Sat May 17 11:07:50 UTC 2008
On 17.05.2008 02:03, itsme213 wrote:
> "Trygve Reenskaug" <trygver at ifi.uio.no> wrote in message
>
>
>> This means that the same object can play several roles at
>> the same time by simply being bound to several keys. Conversely, the
>> same role can simultaneously be played by different objects since there
>> can be several contexts on the stack at the same time.
>>
>
> Sounds good.
>
> But can one object play the role Programmer on project 1, and Programmer on
> project 2, at the same time (i.e. the lifetimes of those 2 roles in that
> object overlap) using the same Programmer trait?
>
> Cheers
> - Sophie
>
The short answer is yes, because the two executions of the same trait
can have different bindings from roles to objects. I'm sure there is a
simple way to explain why this is true, but I have ended up with much
more detail than is actually needed.
My BabyIDE hides this detail so that the programmer can focus on the
static code and never think about the stack. Programming shall be
simpler, not harder, when using roles as well as the traditional
classes. But here are the details because I cannot make myself delete
them once I have written them:
It may be misleading to think of traits in isolation here. A role always
occurs in the context of a structure of objects that interact in order
to achieve some common purpose such as a use case. Each participating
object plays a particular role in the interaction. The role represents
the responsibility of the object in the context of the interaction. The
role also names the object so that we can refer to it in our code,
postponing the name->object binding until the interaction is actually
being executed. Finally, it includes the methods that describe how the
object shall behave when playing its role in the interaction.
A role may be played by any object that behaves properly. Therefore, we
do not need to consider the actual class when writing the code for the
role. We merely reference the object by its role name.
I have doctored the parser so that a role name in the code is inlined
into a context dictionary lookup. In the following example, the role
Shape1 is implemented by a trait also called Shape1. The trait defines
one or more methods that describe the behavior of any object that is
playing the role. Here's one such method:
Shape1>>play2
<Roles: #(Arrow23)>
self displayLarge: '2'.
Arrow23 play23.
If I decompile this, I see the context dictionary lookup explicitly:
Shape1>>play2
<Roles: #(#Arrow23)>
self displayLarge: '2'.
(Baby4Collaboration playerForRole: #Arrow23) play23.
playerForRole: is a class method in Baby4Collaboration. It searches down
the stack to find the first context that binds #Arrow23 to an object.
That object must be instance of a class that uses the trait Shape1.
A context is put on the stack at the start of the execution of an
interaction, so it only exists for the duration of this execution. I
assume that project 1 and project 2 in your example above are two
different executions that may overlap.
If we are thinking in terms of single-threaded execution, project 1 will
be put on the stack and the project 1 bindings will be the current
bindings. If we then enter project 2, the project 2 bindings will be
current until the completion of project 2. The project 2 context will be
popped off the stack, and the project 1 bindings will be current until
project 1 is also completed. So both bindings can exist at the same
time, but only one can be active in a single-threaded execution.
If we are thinking in terms of multi-threaded execution, the executions
will have different stacks. So, the project 1 and project 2 contexts
will be on different stacks and the role/trait methods will will bind
role names to different objects since they are executed in different
contexts. And both contexts are active simultaneously.
Cheers
--Trygve
--
Trygve Reenskaug mailto: trygver at ifi.uio.no
Morgedalsvn. 5A http://heim.ifi.uio.no/~trygver
N-0378 Oslo Tel: (+47) 22 49 57 27
Norway
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/squeak-dev/attachments/20080517/59f1df38/attachment.htm
More information about the Squeak-dev
mailing list
|