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: / usr / share / ruby / vendor_ruby / puppet / vendor / rgen /
Filename : design_rationale.txt
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=ElementSet vs. Array

Subject:
Use a special array-like class "ElementSet" with the following properties:
* can call methods of elements by . notation
* can use all set and enumerable methods of Array
* enforce constraints regarding type of elements
* auto register/unregister with counterpart of an association

Dependencies:
* Without the constraint and register/unregister functionality of the ElementSet,
  the API of model elements built by MetamodelBuilder has to be different:
  instead of "e.myelements << newel" would be "e.addMyelements(newel)"
  However this can also be an advantage (see Metamodel Many Assoc API)

A1. ElementSet:
+ nice notation for calling methods of elements (.)
+ nice notation for adding/removing elements from a model element
  (e.myelements << newel; e.myelements.delete newel)
- complicated to realize
  if ElementSet inherits from Array:
  	constraints/registration can not be garanteed for all add/remove operations
  	input and output of Array methods must be wrapped into ElementSet objects
  if ElementSet delegates to an Array:
    all (relevant) methods have to be delegated (methods from including
    Enumerable do not automatically return ElementSet objects)
- dot notation for calling methods of elements my lead to errors which are difficult
  to find

A2. Array:
+ a separate operator like >> makes calling methods of elements more explicit
+ very easy to implement
+ easy to understand by users (no "magic" going on)

Decision: (2006-06-08)
A2. Array
Simplicity of implementation and ease of use are more important than a nice notation



= Metamodel Many Assoc API

Subject:
How to implement the API to deal with to-many associations of model elements.
One option is an array like object which is held by the model element for each to-many
association and which is given to the user for modification (external array).
The other option is an internal array which is only accessed via add and remove
methods

Dependencies:
If an external array is used, this array must check the association's constraints
and register/unregister with the other side of the association.
(see ElementSet vs. Array)

A1.External Array
+ nice API (e.myassocs << newel; e. myassocs.delete newel)
+ this is a Rails like API
- a reference to the array might be stored somewhere else in the program and
  accidentially be modified, this would modify the model element it belongs to
  as well as register/unregister with other model elements leading to errors
  which are hard to find
- an external array is complicated to implement (see ElementSet vs. Array)

A2.Internal Array
+ easy to understand for non Ruby/Rails aware users
+ simple implementation

Decision: (2006-06-09)
A2. Internal Array
Simplicity of implementation and ease of use are more important than a nice notation