3.3 Visitor design pattern

1. decoupling algorithms from data structures

Recall the current formulation of the immutable list structure using the composite pattern.

Each time we want to compute something new, we apply the interpreter pattern add appropriate methods to IList and implement those methods in MTList and NEList.  This process of extending the capability of the list structure is error-prone at best and cannot be carried out if one does not own the source code for this structure. Any method added to the system can access the private fields of MTList and NEList and modify them at will.  In particular, the code can change _fist and _rest of NEList breaking the invariant immutable property the system is supposed to represent. The system so designed is inherently fragile, cumbersome, rigid, and limited.  We end up with a forever changing IList that includes a multitude of unrelated methods.

These design flaws come of the lack of delineation between the intrinsic and primitive behavior of the structure itself and the more complex behavior needed for a specific application. The failure to decouple primitive and non-primitive operations also causes reusability and extensibility problems. The weakness in bundling a data structure with a predefined set of operations is that it presents a static non-extensible interface to the client that cannot handle unforeseen future requirements. Reusability and extensibility are more than just aesthetic issues; in the real world, they are driven by powerful practical and economic considerations. Computer science students should be conditioned to design code with the knowledge that it will be modified many times. In particular is the need for the ability to add features after the software has been delivered.  Therefore one must seek to decouple the data structures from the algorithms (or operations) that manipulate it.  Before we present an object-oriented approach to address this issue, let's first eat!