here - Stefan-Marr.de

4. Experimentation Platform
Classes and Methods Object fields are defined with a syntax similar to local
variables in methods of other Smalltalk versions. In this case, every object has
a class field, referring to the object’s class.
Note that SOM uses object-based encapsulation. Thus, only the object itself
has access to its fields. This is different from class-based encapsulation as
used for instance in Java [Gosling et al., 2012]. In languages with class-based
encapsulation, all objects of a class can access the private fields of other objects
of the same class.
Methods in Smalltalk have either no arguments, one argument when the
method selector is a special symbol, or they use so-called keyword messages,
which indicate the arguments with colons.
The first method in line 4 is a simple method without arguments. It is an
accessor to the class field and directly returns the value. Note the circumflex
(^), which corresponds to the return keyword in other languages. Further
note that the body text of this dissertation refers to methods in source code
examples by their selector symbol, i. e., the method name with a preceding
hashmark (#). Thus, line 4 defines the method #class.
Line 5 defines a second method #=, which takes the argument other to
implement object equality by using the reference equality message #==. Examples
for binary messages are #= and #==, which take an extra argument in
addition to the receiver. In this particular case #== refers to its argument as
other. Line 6 defines #== to be implemented via a VM primitive, which checks
the receiver and the argument for reference equality. The #ifNil:ifNotNil:
method defined in line 10 is a convenience method implementing a simple
control structure based on blocks. It is a keyword message with two parameters,
here called nilBlock and goBlock.
Blocks are anonymous functions, i. e., lambdas. Depending on the Smalltalk
implementation, they are either full closures or classic restricted blocks as in
Smalltalk-80 [Goldberg and Robson, 1983]. Classic blocks cannot be used once
they left the dynamic extend in which they have been created. Thus, they
cannot be returned from the method in which they were created.
SOM also supports class-side methods and static fields in terms of fields of
the class object. Since classes are objects, the same rules apply and they are
treated identically.
Handling of Globals References to classes are treated as lookups of global
variables. Such global variables can also refer to objects other than classes.
However, assignments to globals are ignored and not directly supported by
92