UML241 — UML 2 Super/Interactions/ LOOP construct specifications

This issue is submitted on behalf of Craig Larman (see signature at the end fo this e-mail):

Current: The LOOP keyword, used in sequence diagrams in a frame, has the syntax LOOP, LOOP( repeat ), or LOOP( min, max)

Problem: Iteration over a collection of objects, sending the same “doX” message to each member is a very very common idiom, so it would be nice if the UML had an easy notation to support this (and otherwise, how will you reverse engineer to code to the right diagram case, or vice versa? It is currently not clear…). AAt present, it is not clear how to say “for i = 1 to collection.size”, nor is there support to allow this ‘i’ loop variable to be reference in a lifeline *selector*

Solution:
1. change the LOOP syntax to (in its fullest form), LOOP( <varName>, <start>, <end>, <increment>) e.g., LOOP( i, 1, 10, 1), or LOOP( i, 10, 1, -1). Increment should default to +1.
a. Also <start> and <end> may be expressions referring to participants in the interaction, such as end = lineItems.size where lineItems is a collection of SalesLineItem objects. Note that there is already syntax for a “max” (similar to <end>), and one aspect of this change is making (or ensuring) it can be an expression involving lifeline participants, not just a constant.

2. allow a lifeline contained within the frame to have its selector refer to the LOOP var. e.g., “ lineItems[ i ] : SalesLineItem” to indicate selecting one SalesLineItem from the lineItems collection, given the current value of ‘i’. Note that a selector such as “lineItem[ i ]” is already allowed in the spec (and there are examples in the spec). this request is for tying the ‘i’ variable to the LOOP construct.

Variant Solutions:
Perhaps the upper-bound can be handled in the LOOP guard instead. E.g., [ i < lineItems.size ]. However, in this case, i still need a way to indicate the incrementing of ‘i’, and the ability to legally refer to “i” in the selector “lineItems[ i ].

i attach a picture to illustrate.