A whole value is a quantity used to describe things in a domain. Whole values are not themselves things, but measures of things. As such they do not have an identity of consequence.
For example, we might say a company is worth 50,000,000 dollars. Some thing, the company, its stock, or a proposed investment, is being measured. The thing is a real object while the 50,000,000 dollars is a value, a property of the thing.
In object oriented computing it is commonplace and good practice to model values as objects. Key to this is defining a notion of equality that is independent of object identity. Another key, and the point of the WholeValue pattern, is that the objects model the whole property being measured, not just some part of it. In the example above the whole value is 50,000,000 dollars, not just 50,000,000 and not just "dollars".
WholeValue is a concept introduced to the pattern world by WardCunningham's ChecksPatternLanguage where its partitioning of checking responsibility is featured. For example, if I know a field will contain a whole value or nil, I do not have to be concerned with any conditions outside those modeled by the whole value once I have established that it is not nil. The whole value pattern argues that whole values should be entered in single fields and checked to be well formed in one place (In Fit the TypeAdapter is, or is an interface to, that one place).
|Last edited April 20, 2005
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