here - Stefan-Marr.de

4.2. SOM: Simple Object Machine
SOM’s focus on clarity and a minimal set of concepts makes it a good candidate
to express the execution semantics of the OMOP. Essential semantics
can be expressed while the accidental complexity that comes with the extensive
feature set of common mainstream languages is avoided. Compared to
basing the semantics on a calculus such as the impς-calculus of Abadi and
Cardelli [1996], SOM provides a platform that is closer to common language
implementations. It includes common language concepts and their essential
complexity. For instance, this directly allows a discussion of the special case
of VM primitives, which would not be the case with an application of the
impς-calculus, without adding specific extensions to it.
AweSOM 2 is chosen over the other available SOM implementations, because
its Smalltalk implementation is the most concise in the SOM family and
enables the formulation of an executable semantics that is concise enough to
serve for illustration as part of this dissertation.
4.2.1. Language Overview and Smalltalk Specifics
This section gives a brief overview of SOM’s Smalltalk syntax and Smalltalk
concepts used in the source code examples of this dissertation.
SOM’s standard implementation relies on a textual syntax for class definitions.
Lst. 4.1 gives a minimal definition of the class Object. Note that Object
inherits from nil and thus, it is the root class of the class hierarchy. If neither a
superclass nor nil are given, a class will implicitly have Object as superclass.
1 Object = nil ( " defines the class Object , superclass is ‘nil ’"
2 | class | " object fields : each object has a field ‘class ’"
3
4 class = ( ^ class ) " method to return field ‘class ’"
5 = other = ( ^ self == other )
6 == other = primitive " equality test implemented in the VM"
7 asString = ( ^’instance of ’ + ( self class ) )
8 value = ( ^ self )
9 yourself = ( ^ self )
10 ifNil : nilBlock ifNotNil : goBlock = (^ goBlock value )
11
12 " Error recovering "
13 doesNotUnderstand : selector arguments : arguments = (
14 self error : ’ Method not found : ’
15 + class name + ’ >>#’ + selector ) )
Listing 4.1: SOM Language Example: Object class extending nil
2 https://github.com/smarr/SOM/#readme
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