This document is meant to shine some light on certain practices within Cuis which we recommend following for various reasons. It tries to help to understand some not so obvious design decisions without linking to the cuis-dev mailinglist.

Message Conventions

This section deals with message sending, the format of certain messages and what message is supposed to be used when.

Replace #respondsTo: with #is:

The message #respondsTo: which is used in code like

  | foo |
  foo := Object new.
  (foo respondsTo: #saveAsModel)
    ifTrue: [ foo saveAsmodel ].

should be replaced by #is: which is derived from messages like #isBag, #isDictionary, #isForm, etc. A changed example would then look like this:

  | foo |
  foo := Object new.
  (foo is: #saveableAsModel)
    ifTrue: [ foo saveAsmodel ].

The reasoning for this is because:

1. #is: looks like the forms #isBag, #isDictionary, etc. which test for the type of the receiving object.

2. #respondsTo: and the #isBag are of the same form, they both receive a symbol and return true or false if the object conforms in some way to the asked question in the argument. So they can be unified into #is: from that perspective.

3. #isBag, etc. require an implementation returning false high enough in the class hierarchy (for example, at Collection or at Object). This those general, abstract classes should be unaware of specific knowledge of those down in the hierarchy. By default, #is: answers false to any unknown value of the parameter.

4. #respondsTo: goes up the class hierarchy when doing lookups, it looks into the selectors of the object, then goes up the class hierarchy until a result is found. At each level, an expensive dictionary lookup is needed. By using #is: the check can be faster, because checks are for identity of symbols.

5. #is: is more flexible. You can use it to ask arbitrary questions to the object. obj is: Class. obj is: #Bag. obj is: 1. and each object can decide of how it handles this based on expectation. #respondsTo: is reserved more for testing if an object can handle a specific message.