respondTo: is different from a perform based on forging a selector.
respondTo: implies a different control while perform: is just sending a
message.
Some logic of Spec or something else was doing respondTo: and it is an
indication of weak polymorphism.
Either objects are polymorphic or they are not.
Because else you expect a behavior and because of a hidden respondTo: you get a
api not invoked.
This forces you to read all the code and step to find the super cool users of
respondTo:
So I really stand on my claim respondTo: should only be used in last resort.
It covers shitty design.
S.
> On 16 Sep 2020, at 19:28, s...@clipperadams.com wrote:
>
> Stéphane Ducasse <stephane.duca...@inria.fr> wrote:
>
> refrain from using respondTo: I make code difficult to evolve and can
> introduce vicious bugs.
>
> Steph, would you say more about this? It’s something I’ve been wondering
> about.
>
> I was recently reading the Strategy pattern in the Smalltalk Companion to the
> GOF book. On p. 339, they proposed the following "Smalltalk way" as an
> alternative to the common class-per-strategy implementation:
>
> [[[language=smalltalk
>
> Composition>>repair
>
> "Without the strategy pattern, but using perform:."
>
> | selector |
>
> "Construct the name of the method to invoke:"
>
> selector := ('formatWith', formattingStrategy, 'Algorithm') asSymbol.
>
> self perform: selector
>
> ]]]
>
> It then dismissed the approach as "''clever but difficult from a program
> understanding perspective. Even static analysis tools such as code browsers'
> "senders" and "messages" fail on this code.''"
>
> It struck me as perhaps a bit extreme (i.e. too clever indeed) to construct
> the algorithm selector via string concatenation. Maybe "senders" search
> capabilities have gotten more sophisticated since publication, but Pharo
> seems to support symbol arguments, even for e.g. message renames. I thought,
> why not the following:
>
> [[[language=smalltalk
>
> Composition>>repair
>
> "Without the strategy pattern, but using perform:."
>
> self perform: self formattingStrategy
>
> ]]]
>
> Then client code like ==aComposition formattingStrategy:
> #formatWithSimpleAlgorithm== would show up in senders, message renames, etc.
>
> But from what you’re saying, I feel I may be missing something…
>
>
>
--------------------------------------------
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
40, avenue Halley,
Parc Scientifique de la Haute Borne, Bât.A, Park Plaza
Villeneuve d'Ascq 59650
France
