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6. Evaluation: The OMOP as a Unifying Substrate
ming concepts that can benefit from it, and removing any part would also
result in unsatisfied requirements. The remainder of this sections supports
this claim by discussing the basic interface of the OMOP. The VM-specific aspects
and helper functionality are excluded, because they are not completely
orthogonal to the basic interface (cf. Sec. 5.2).
Ownership Sec. 5.1 argues that a MOP needs a connection between base level,
i. e., application and the meta level, i. e., the language implementation.
The OMOP uses ownership as this meta relation. Other options could
have been instantiation for metaclass-based approaches or a direct connection
between object and metaobject in metaobject-based approaches.
Without such a meta relation, it would not be possible to define more
than one set of policies, i. e., a domain, because it would need to be
applied implicitly by the runtime instead of being based on the meta
relation. Thus, a meta relation is required and the choice of ownership
combines with the requirements of concurrent programming concepts
to represent the notion of owning entities.
Enforcement Status The OMOP needs a mechanism to distinguish between
base-level and meta-level execution. The OMOP represents this difference
by a simple bit. Other representations could allow for more flexibility,
for instance to enable reflective towers [Smith, 1984]. However, in
either case, it requires a representation of the execution levels to distinguish
them. Otherwise every operation would trigger an intercession
handler, which itself would again trigger an intercession handler, leading
to infinite meta regress without base case. The OMOP uses the minimal
required solution by only distinguishing base and meta level.
State Access Reification With #readField: and #write:toField:to: two intercession
handlers are provided by the OMOP to capture reading and
writing of object fields, enabling a customization of all state access. Concurrent
state access is one of the fundamental issues that concurrent
programming tackles by providing means to coordinate the use of state,
i. e., resources. As this chapter demonstrates, customizing state access
policies is required to implement an STM or immutability for instance.
Without it, the OMOP would not be able to support these concepts and
it could not provide the same guarantees for concepts such as actors,
vats, and isolation it gives now. In conclusion, state access reification is
a required mechanism and the OMOP’s capabilities would be reduced
significantly without it.
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