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7. Implementation Approaches
code can be executed in enforced mode. Other blocks will not be transformed
as part of a the unenforced method version.
Switching back to unenforced execution is done based on an attribute, similarly
to the approach discussed for SOM and used by Renggli and Nierstrasz.
However, in Squeak and Pharo, the syntax for method definitions is
slightly different from the SOM syntax used here. Therefore, instead of using
an attribute, the method body of an unenforced method needs to contain
the pragma. Methods marked with this pragma are not transformed,
and thus, do not enforce the semantics of the OMOP. However, to
be able to switch back to enforced execution, blocks that receive the #enter:
message are still transformed correctly.
Handling of Primitives Since the AST-OMOP does not change the VM, it
needs another approach to handle primitives. The approach of Renggli and
Nierstrasz to solve the issue was again chosen. All Squeak/Pharo methods
that rely on primitives are marked with a pragma: . The
AST-OMOP adds the pragma to each of these
methods. The method identified by the selector is then used as the enforced
version of the primitive method. This replacement method simply redirects
the execution of the primitive to the corresponding intercession handler in
the domain.
Integration with Smalltalk’s Interactive Environment The image-based development
tools of Squeak and Pharo require minor adaptations to accommodate
the AST-OMOP. As a cosmetic change, the class browser does not
show the enforced methods, because they are generated automatically and
do not need to be changed manually. Suppressing them in the browser avoids
problems and clutter in the IDE.
The generation of transformed methods is performed on demand. To avoid
the memory overhead of keeping all methods in two versions, method transformation
is only triggered when an existing method is changed, which had
an enforced version, or when an enforced version could not be found and
#doesNotUnderstand: got triggered.
To capture changes of existing methods, additions, or removals, the system
offers corresponding events on which the AST-OMOP registers the appropriate
handlers to generate transformed methods when necessary.
Problematic for this approach are the semantics of class hierarchies and
method lookup. Thus, it is not sufficient to generate only the missing method,
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