Editing Self (programming language) (section)

=== Adding slots ===
Objects in Self can be modified to include additional slots. This can be done using the graphical programming environment, or with the primitive '_AddSlots:'. A '''primitive''' has the same syntax as a normal keyword message, but its name starts with the underscore character. The _AddSlots primitive should be avoided because it is a left over from early implementations. However, we will show it in the example below because it makes the code shorter.

An earlier example was about refactoring a simple class called Vehicle in order to be able to differentiate the behaviour between cars and trucks. In Self one would accomplish this with something like this:

<syntaxhighlight lang="smalltalk">_AddSlots: (| vehicle <- (|parent* = traits clonable|) |).</syntaxhighlight>

Since the receiver of the '_AddSlots:' primitive isn't indicated, it is "self". In the case of expressions typed at the prompt, that is an object called the "lobby". The argument for '_AddSlots:' is the object whose slots will be copied over to the receiver. In this case it is a literal object with exactly one slot. The slot's name is 'vehicle' and its value is another literal object. The "<-" notation implies a second slot called 'vehicle:' which can be used to change the first slot's value.

The "=" indicates a constant slot, so there is no corresponding 'parent:'. The literal object that is the initial value of 'vehicle' includes a single slot so it can understand messages related to cloning. A truly empty object, indicated as (| |) or more simply as (), cannot receive any messages at all.

<syntaxhighlight lang="smalltalk">vehicle _AddSlots: (| name <- 'automobile'|).</syntaxhighlight>

Here the receiver is the previous object, which now will include 'name' and 'name:' slots in addition to 'parent*'.

<syntaxhighlight lang="smalltalk">_AddSlots: (| sportsCar <- vehicle copy |).
sportsCar _AddSlots: (| driveToWork = (''some code, this is a method'') |).</syntaxhighlight>

Though previously 'vehicle' and 'sportsCar' were exactly alike, now the latter includes a new slot with a method that the original doesn't have. Methods can only be included in constant slots.

<syntaxhighlight lang="smalltalk">_AddSlots: (| porsche911 <- sportsCar copy |).
porsche911 name:'Bobs Porsche'.</syntaxhighlight>

The new object 'porsche911' started out exactly like 'sportsCar', but the last message changed the value of its 'name' slot. Note that both still have exactly the same slots even though one of them has a different value.