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Metaobject protocols were originally defined as supplemental interfaces to programming languages that give users the ability to incrementally modify the language's behavior and implementation.
Languages that incorporate metaobject protocols blur the distinction between language designer and language user. Traditionally, designers were expected to produce languages with well-defined, fixed behaviors (or semantics). Users were expected to treat these languages as immutable black-box abstractions, and to derive any needed flexibility or power from constructs built on top of them. This sharp division was thought to constitute an appropriate division of labor. Programming language design was viewed as a difficult, highly-specialized art, inappropriate for average users to engage in. It was also often assumed that a language design must be rigid in order to support portable implementations, efficient compilers, and the like.
The metaobject protocol approach, in contrast, is based on the idea that one can and should open languages up, allowing users to adjust the design and implementation to suit their particular needs. In other words, users are encouraged to participate in the language design process. If handled properly, opening up the language design need not compromise program portability or implementation efficiency.
In a language based upon metaobject protocols, the language implementation itself is structured as an object-oriented program. This allows the power of object-oriented programming techniques to be exploited to make the language implementation adjustable and flexible. In effect, the resulting implementation does not represent a single point in the overall space of language designs, but rather an entire region within that space.
More recently, it has become clear that systems other than programming languages can have a metaobject protocol. In fact, many existing systems can be seen as having ad-hoc metaobject protocols. This was one of the observations leading to the concept of Open Implementations.
We have collected a list of MOPs.
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