42 The Implementation Perspective

Choice of Implementation

The choice of implementation determines how easy it is to understand and process a set of relationships. For example, the second sentence of this chapter is a natural language implementation of a set of relationships in the Simpson family:

A subject-predicate-object syntax makes the relationships more explicit:

In the following example of a potential XML implementation syntax, we emphasize class inclusion relationships by using elements as containers, and the relationships among the members of the family are expressed explicitly through references, using XML’s ID and IDREF attribute types:^[1]

None of the models we have presented so far in this chapter represents the complexities of modern families that involve multiple marriages and children from more than one marriage, but they are sufficient for our limited demonstration purposes.

Syntax and Grammar

The syntax and grammar of a language consist of the rules that determine which combinations of its words are allowed and are thus grammatical or well-formed. Natural languages have substantial similarities by having nouns, verbs, adjectives and other parts of speech, but they differ greatly in how they arrange them to create sentences. Conformance to the rules for arranging these parts makes a sentence syntactically compliant but does not mean that an expression is semantically comprehensible; the classic example is Chomsky’s anomalous sentence:

Any meaning this sentence has is odd, difficult to visualize, and outside of readily accessible experience, but anyone who knows the English language can recognize that it follows its syntactic rules, as opposed to this sentence, which breaks them and seems completely meaningless:

Requirements for Implementation Syntax

The most basic requirement for implementation syntax is that it can represent all the expressions that it needs to express. For the examples in this chapter, we have used an informal combination of English words and symbols (arrows and parentheses) that you could understand easily, but simple language is incapable of expressing most of what we readily say in English. But this benefit of natural language only accrues to people, and the more restrictive and formal syntax is easier to understand for computers.

A second consideration is that the implementation can be understood and used by its intended users. We can usually express a relationship in different languages while preserving its meaning, just as we can usually implement the same computing functionality in different programming languages. From a semantic perspective these three expressions are equivalent:

However, whether these expressions are equivalent for someone reading them depends on which languages they understand.

An analogous situation occurs with the implementation of web pages. HTML was invented as a language for encoding how web pages look in a browser, and most of the tags in HTML represent the simple structure of an analogous print document. Representing paragraphs, list items and numbered headings with <P> and <LI> and <Hn> makes using HTML so easy that school children can create web pages. However, the “web for eyes” implemented using HTML is of less efficient or practical for computers that want to treat content as product catalogs, orders, invoices, payments, and other business transactions and information that can be analyzed and processed. This “web for computers” is best implemented using domain-specific vocabularies in XML.