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5.1. Open Implementations and Metaobject Protocols
execution. This means it can change state, adapt program structures, or trap
certain operations to refine their behavior. The reified program structures are
referred to as metaobjects. In order to explain MOPs, Kiczales et al. [1991]
state that in general, a protocol is formed by “a set of object types and operations
on them, which can support not just a single behavior, but a space or region of behaviors”.
Therefore, they conclude that a MOP enables the encoding of individual
decisions about language behavior via the operations of metaobjects.
Categories of MOPs Tanter [2009, p. 12] distinguishes metaobject protocols
(MOPs) for object-oriented reflection by the correspondence of the meta relation
to other aspects of the system. He identifies a number of common ideas:
metaclass-based models, metaobject-based models, group-based models, and
message-reification-based models.
Metaclass-based Metaclass-based models such as in CLOS, Perl’s Moose,
or Smalltalk enable for instance the customization of method dispatch or
object fields semantics. The metaclass of a class therefore describes the semantics
of this class, i. e., the metaclass’ instance. The meta relation in this
case is the instantiation relationship between a class and its metaclass. This
technique has been used, e. g., to implement persistent objects, which are automatically
mapped to a database [Paepcke, 1993], or even to parallelize programs
[Rodriguez Jr., 1991].
Metaobject-based Metaobject-based models decouple meta interfaces from
the class hierarchy. One example for a language using this model is the
prototype-based language 3-KRS [Maes, 1987]. Since 3-KRS does not have the
notion of classes, it is designed with a one-to-one relation between a base-level
object and a metaobject. However, the model can be applied to class-based languages,
for instance as is the case in Albedo, a Smalltalk system [Ressia et al.,
2010]. The independence from the class hierarchy enables modifications that
are orthogonal to the class hierarchy and results in a model with greater flexibility.
Instead of having a one-to-one mapping, other variations of the model are
possible as well. One common example is proxy-based MOPs, for instance
as the one proposed for the next version of ECMAScript [Van Cutsem and
Miller, 2010]. 3 Metaobjects are defined in the form of proxy objects that reify
3 Direct Proxies, Tom Van Cutsem, access date: 4 July 2012
http://wiki.ecmascript.org/doku.php?id=harmony:direct_proxies
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