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Please explain self

I understand if you have Class A with methods p and q that you can call q from p doing:

self q

But there are some cases where a method doesn’t exist but self is still used. I found a tutorial on creating a Reddit type app and it had a .mcz which I loaded.

Here is an example from Class StLoggedInComponent and method refreshReport:

self report rows: (self session user tasks select: self filterBlock).

There is a method ‘report’ so I understand that, but there is no method named ‘session’ so what is the use of self?

And Class StDatabase has method ‘updateTask:’ has:

self subclassResponsibility.

But no method called ‘subclassResponsibility’ exists in the Class.

Also, Class StLoggedInComponents has method ‘testTasks’ with this:

^ self class testTasks

What is ‘self class’?

Thank you.

Davide Grandi-3

Re: Please explain self

Objects are instances of a class.
Classes are objects, so they have methods.
'self class' returns the class of self.

If a class doesn't have a methods ...
... the search continues in the superclass.
(every class has a superclass ... apart from Object).

If you download the Blue Book - "Smalltalk-80 - The language and it's implementation",
it's free on the net, you'll find an AWSOME explanation and diagram from page 270 on, until ...
... the final map : classes and metaclass.

but you've to read it these pages, before. There's a 'metaclass' concept to acquire.

Bon voyage !

Davide

On 12/12/2020 20:20, g_patrickb--- via Pharo-users wrote:

I understand if you have Class A with methods p and q that you can call q from p doing:

self q

But there are some cases where a method doesn’t exist but self is still used. I found a tutorial on creating a Reddit type app and it had a .mcz which I loaded.

Here is an example from Class StLoggedInComponent and method refreshReport:

self report rows: (self session user tasks select: self filterBlock).

There is a method ‘report’ so I understand that, but there is no method named ‘session’ so what is the use of self?

And Class StDatabase has method ‘updateTask:’ has:

self subclassResponsibility.

But no method called ‘subclassResponsibility’ exists in the Class.

Also, Class StLoggedInComponents has method ‘testTasks’ with this:

^ self class testTasks

What is ‘self class’?

Thank you.

-- 
Ing. Davide Grandi
linkedin : http://linkedin.com/in/davidegrandi

Richard Sargent

Re: Please explain self

Administrator

I'll add a small bit to Davide's excellent answer, by way of connecting some of the dots. In particular, the browser makes a class look like one thing and we talk about instance side and class side.

The reality is that the browser presents either the class' behaviours or the metaclass' behaviours, depending on whether we have chosen to see "the instance side" or "the class side". Admittedly, the names we use for the sides add to the possible confusion.

On Sat, Dec 12, 2020, 12:20 Davide Grandi <[hidden email]> wrote:

Objects are instances of a class.
Classes are objects, so they have methods.
'self class' returns the class of self.

If a class doesn't have a methods ...
... the search continues in the superclass.
(every class has a superclass ... apart from Object).

If you download the Blue Book - "Smalltalk-80 - The language and it's implementation",
it's free on the net, you'll find an AWSOME explanation and diagram from page 270 on, until ...
... the final map : classes and metaclass.

but you've to read it these pages, before. There's a 'metaclass' concept to acquire.

Bon voyage !

Davide

On 12/12/2020 20:20, g_patrickb--- via Pharo-users wrote:

I understand if you have Class A with methods p and q that you can call q from p doing:

self q

But there are some cases where a method doesn’t exist but self is still used. I found a tutorial on creating a Reddit type app and it had a .mcz which I loaded.

Here is an example from Class StLoggedInComponent and method refreshReport:

self report rows: (self session user tasks select: self filterBlock).

There is a method ‘report’ so I understand that, but there is no method named ‘session’ so what is the use of self?

And Class StDatabase has method ‘updateTask:’ has:

self subclassResponsibility.

But no method called ‘subclassResponsibility’ exists in the Class.

Also, Class StLoggedInComponents has method ‘testTasks’ with this:

^ self class testTasks

What is ‘self class’?

Thank you.
-- 
Ing. Davide Grandi
linkedin : http://linkedin.com/in/davidegrandi

mjldpppiomdnfbme.png (42K) Download Attachment

Richard O'Keefe

Re: Please explain self

In reply to this post by Pharo Smalltalk Users mailing list

#subclassResponsibility is defined in Object.

A good way to find this out is to

 - select 'subclassResponsibility' with the mouse

 - type Control-M

Control-M asks for a list of the iMplementors of the method.

Control-N asks for a list of the seNders of the message.

This particular method just raises an exception.

It is how you define what some languages call an "abstract method";

a method whose *interface* is defined in the current class

but whose *implementation* is deferred to subclasses.

I can't help you with #session except to suggest that it is

probably defined in some superclass.  In the browser button bar

between the upper and lower panes you may see

(*) Flat ( ) Hier

Clicking on Hier will change the class pane (2nd from the left,

upper level) to an indented list, making it easy to see what

the superclass chain is.

On Sun, 13 Dec 2020 at 08:21, g_patrickb--- via Pharo-users <[hidden email]> wrote:

I understand if you have Class A with methods p and q that you can call q from p doing:
self q
But there are some cases where a method doesn’t exist but self is still used. I found a tutorial on creating a Reddit type app and it had a .mcz which I loaded.
Here is an example from Class StLoggedInComponent and method refreshReport:
self report rows: (self session user tasks select: self filterBlock).
There is a method ‘report’ so I understand that, but there is no method named ‘session’ so what is the use of self?
And Class StDatabase has method ‘updateTask:’ has:
self subclassResponsibility.
But no method called ‘subclassResponsibility’ exists in the Class.
Also, Class StLoggedInComponents has method ‘testTasks’ with this:
^ self class testTasks
What is ‘self class’?
Thank you.

Stéphane Ducasse

Re: Please explain self

In reply to this post by Pharo Smalltalk Users mailing list

Hello

All this is explained in the Mooc (with mutliple videos and gradually)

and Pharo by Example chapter.

Pay attention you do not need to understand the metaclass core to program in Pharo.

This is just for fun and a bonus. This is why in general I do not show this diagram directly.

Check the mooc videos we designed based on more than 10 years of experience

teaching Pharo to people.

On 12 Dec 2020, at 20:20, g_patrickb--- via Pharo-users <[hidden email]> wrote:

I understand if you have Class A with methods p and q that you can call q from p doing:
self q
But there are some cases where a method doesn’t exist but self is still used. I found a tutorial on creating a Reddit type app and it had a .mcz which I loaded.
Here is an example from Class StLoggedInComponent and method refreshReport:
self report rows: (self session user tasks select: self filterBlock).
There is a method ‘report’ so I understand that, but there is no method named ‘session’ so what is the use of self?
And Class StDatabase has method ‘updateTask:’ has:
self subclassResponsibility.
But no method called ‘subclassResponsibility’ exists in the Class.
Also, Class StLoggedInComponents has method ‘testTasks’ with this:
^ self class testTasks
What is ‘self class’?
Thank you.

--------------------------------------------

Stéphane Ducasse

http://stephane.ducasse.free.fr / http://www.pharo.org

03 59 35 87 52

Assistant: Aurore Dalle

FAX 03 59 57 78 50

TEL 03 59 35 86 16

S. Ducasse - Inria

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