MDA Distilled: Principles of Model-Driven Architecture

Merging Mappings for Weaving

A function takes one or more input values and produces an output value. For example, the square root function takes a set of real numbers or integers—say, {25, 16, 9}—and generates a different set of real numbers or integers—in this case, {5, 4, 3}. Such a mapping can be stated as a rule (in other words, what we normally think of as a function), or as an enumeration {25:5, 16:4, 9:3}.

This abstruse factoid applies equally to mapping functions. Specification of mapping functions tends to focus on defining rules, but enumerations, while less sexy, are important when a rule can't easily be specified. It may even be the case that it's impossible to specify a rule. Both rules and correlations (enumerations of pairs of ...