Everything Joachim Tuchel wrote about this is fine EXCEPT that

this is an Exercism task, where Roelof has to implement an API

that he is given and is not at liberty to do it right.

Thanks to the way that IsbnVerifierTest is structured,

- the IsbnVerifier class has NO use for ANY variables of ANY

�� kind: no instance variables, no class variables, and no

�� class instance variables.

- the IsbnVerifier class has only ONE method that is called,

�� and that is on the INSTANCE side.

That is, it's a simple pure function disguised as an object.

A good design might have an ISBN class, and would put

#isValidISBN10: and #isValidISBN13: on the class side.

But to solve the Exercism exercises, you HAVE to implement

the API that the test class is expecting, not something

better.�� If it passes the tests, it is by definition right.

If it doesn't, it is by definition wrong, even if it is a

far better design.�� So

��- one class, IsbnVerifier

��- with no variables of any kind

��- with one and only one method, #isValidIsbn,

���� on the instance side.

It all gets so much clearer when you realise that the Exercism

tasks are NOT designed to teach Object-Oriented Design.�� The

problems have to serve many programming languages, some of

which, like bash and x86 assembly code, are not object-oriented.

On Wed, 2 Sep 2020 at 18:42, jtuchel@objektfabrik.de <jtuchel@objektfabrik.de> wrote:

Roelof,

I don't think so.

A Class cannot access instance variables. Why? because a Class doesn't know which of its instances to ask for it.

Ask yourself: who is "self" in a Class method? Is it the Class or is it an individual instance of the Class?

(Hint: in a Class, #self is the Class, while in an instance, #self is the instance) .

"Who" do you want to validate the ISBN number? An individual instance of your Class (like Book)? Or is validating an ISBNNumber a Task that is independent of an individual item? If it is independent, then why would you want to use semething specific in that method?

You need to understand the differences between Class and Instance variables. I suggest reading Chapter 4 of "The Art and Science of Smalltalk" or better: read the whole book. It's easy to read and sparks quite a few A-HA's per hour.

HTH

Joachim

Am 02.09.20 um 08:18 schrieb Roelof Wobben:

Hello,

I have now a challenge where I have to validate a ISBN number.

Can I do something like this on the class side :

(string isEmpty)
�� ifTrrue: [ ^ false]
�� ifFalse:�� [ digits:= something.
�� controlDigit := something.
�� self validateISBNNumber]

where on the validateISBNNumber I use the instance variables digits and controlDigit to validate the ISBN number on the instance side.

Roelof
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