Visitor18

The solution to the [[expression problem]] - e.g. to write an extensible [[visitor]] - presented at the [[visitor]] page (and also [[Chapter 18]] of [[TheAPIBook]]) is another realization of the general principle to separate [[ClientAPI]] from [[ProviderAPI]]. Client part of the [[visitor]] can be enriched by adding new ''dispatch'' methods (into '''final''' class) with each new version. The interface part of [[visitor]] is immutable, a fixed point, which stays the same for those who implement it. Each version defines its own unique interface (according to the list of expression types it supports). The internals of the [[API]] then bridge the ''dispatch'' methods to appropriate interface ''visit'' methods.

The solution to the [[expression problem]] - e.g. to write an extensible [[visitor]] - presented at the [[visitor]] page (and also [[Chapter 18]] of [[TheAPIBook]]) is another realization of the general principle to separate [[ClientAPI]] from [[ProviderAPI]]. Client part of the [[visitor]] can be enriched by adding new ''dispatch'' methods (into '''final''' class) with each new version. The interface part of [[visitor]] is immutable, a fixed point, which stays the same for those who implement it. Each version defines its own unique interface (according to the list of expression types it supports). The internals of the [[API]] then bridge the ''dispatch'' methods to appropriate interface ''visit'' methods.