Briefly, you cannot and (for a different reason) you should not.
The IsbnVerifierTest class defines the API; the method must be
isValidIsbn: aString
and it MUST be on the instance side.
The reason why you shouldn't is that an empty string should NOT
be a special case.�� This is a problem where the design style
described by Dijkstra in "A Discipline of Programming"
and further fleshed out by Reynolds and Gries in related books
pays off.�� You want to inspect all the characters of the string
once each in a single left-to-right loop.

On Wed, 2 Sep 2020 at 18:19, Roelof Wobben via Pharo-users <pharo-users@lists.pharo.org> wrote:
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