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5. An Ownership-based MOP for Expressing Concurrency Abstractions
5.1. Open Implementations and Metaobject Protocols
Considering the requirements identified in Sec. 3.4 and that the contemporary
multi-language VMs are object-oriented programming environments (cf.
Sec. 3.1.1.2), reflective programming techniques provide the natural foundation
for a program to interface with a VM, i. e., its runtime environment. Thus,
this section briefly reviews the notion of open implementations and metaobject
protocols, because they are commonly used to provide properties similar to the
ones required for a unifying substrate for concurrent programming concepts.
The notion of open implementations described by Kiczales [1996] provides
a general design strategy for increasing the control that client code has over
a software component it is using. The general motivation is that client code
often needs to adapt or work around the concrete software component it is
using, to change semantics or performance characteristics to the context it is
used in. Thus, open implementations are designed to facilitate the adaptation
of implementation strategies [Kiczales et al., 1997]. This notion can also be
used to enable adaptive language guarantees and semantics based on a meta
interface. Meta interfaces for this purpose are commonly known as metaobject
protocols:
Metaobject protocols are interfaces to the language that give users the
ability to incrementally modify the language’s behavior and implementation,
as well as the ability to write programs within the language.
[Kiczales et al., 1991, p. 1]
Introduction Today, metaobject protocols (MOPs) can be found in a number
of languages, for instance in the Common Lisp Object System (CLOS),
Smalltalk-80 [Goldberg and Robson, 1983], Groovy, 1 and Perl’s Moose object
system. 2 While not all of them offer the full functionality of the MOP discussed
by Kiczales et al. [1991], they provide capabilities to reflect on the
executing program and adapt the language’s behavior.
To start from the beginning, important foundations for MOPs are the notions
of reflection and reification [Friedman and Wand, 1984]. Reflection builds
on introspection and intercession. Introspection is the notion of having a program
querying itself for information. This information is then reified in terms
of program structures which can be processed. Based on these reified program
structures, intercession enables the program to interfere with its own
1 http://groovy.codehaus.org/
2 http://search.cpan.org/~flora/Class-MOP/
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