here - Stefan-Marr.de

4.2. SOM: Simple Object Machine
1 SOMUniverse = ( " ... "
2 bootstrapFrameWithArguments : args = (
3 ( interpreter pushNewFrameWithMethod : self bootstrapMethod )
4 push : ( self globalAt : # system );
5 push : args ;
6 yourself " convenience method to return self " ) )
Listing 4.2: Cascaded Message Sends
the language. Instead, the binding of global variables can only be changed by
the explicit use of a VM primitive. Thus, supposedly constant globals such as
true, false, and class references cannot simply be assigned. While they might
appear to be keywords or special literals, similar to how they are treated in
languages such as C++ or Java, in SOM, they are treated like any other global
and will be looked up in the globals dictionary. Following the Smalltalk spirit
of everything is an object, true, false, and nil are objects as well. More precisely,
they are the sole instances of their corresponding class. Consequently,
they can be adapted and customized when necessary.
Cascaded Message Sends Some code examples use Smalltalk’s cascaded message
sends for conciseness, which is a language feature that is uncommon in
other languages. The example in Lst. 4.2 defines a method that uses a complex
expression in line 3 that yields a frame object. This frame object still needs to
be populated with initial data, i. e., the method pushes a number of objects
onto the frame. For conciseness, instead of assigning the frame object to a
temporary variable, the method uses cascaded message sends. Thus, the resulting
frame object of the expression becomes the receiver of the first regular
message send in line 4, which pushes the #system global, and then indicated
by the semicolons, also becomes the receiver of the two following message
sends in lines 5 and 6. Note the last message #yourself in Lst. 4.2, it is a
convenience method defined in Object. It is often used in cascaded message
sends to return self. In this case it returns the frame object that was the result
of the initial expression on line 3.
Dynamic Array Creation Some of the code examples in this dissertation
use a notation for dynamically created arrays that is currently not part of the
SOM implementation. However, it is commonly used in Squeak and Pharo
Smalltalk to instantiate arrays in concise one-liners. The syntax uses curly
braces to delimit a list of Smalltalk statements. The result value of each of
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