Specification of Reactive Hardware/Software Systems - Electronic ...

7.3 The Role of Semantics 233
7.3 The Role of Semantics
We repeatedly mentioned that POOSL language is a formal language 2 . This means that
the syntax (the form) as well as the semantics (the intended meaning) of every language
construct is defined in a formal, mathematical way. Defining the syntax of languages in
a formal way is nowadays quite common. There exists a well-developed and widelyknown
mathematical theory of formal languages which supports the accurate description
of the syntax of languages, and the correct implementation of lexical scanners and
parsers [Ten91]. A similar uniform mathematical theory for language semantics is still
lacking. Nevertheless, there do exist a number of well-developed approaches towards
semantics. Despite of this, only very few languages come equipped with a complete
formal definition of their semantics [Hen90].
With respect to the semantics POOSL, we have chosen for a Plotkin-style structural
operational semantics[Plo77, Plo81, Plo83]. An operational semantics describes how the
language constructs are executed on some abstract machine. The term structural states
that the semantics is defined in terms of the syntactic structure of specifications. The
reasons to develop a semantics, and especially a structural operational semantics, are
diverse:
Language Design
First of all, and most importantly, designing a good (specification) language is a very
complex task. When one attempts to combine different language primitives, unexpected
and counterintuitive interactions arise [Ten91]. Since constructing a formal semantics
requires a thorough understanding of every ’corner’ of the language, these interactions
can be detected as early as possible and different alternatives can be evaluated systematically.
The construction of a semantics further prevents a language from being ’polluted’
by many bizarre language constructs. Therefore, a language and its formal semantics
should be developed simultaneously, thereby using the semantics as a tool which guides
the language design. More about this subject is described in Section 9.7.
Unambiguous Documentation
Next to its use for language design, a semantics, and especially an operational semantics,
can be helpful when the precise meaning of a construct must be known. Since reference
manuals and language standards are, in general, expressed informally, ambiguities may
arise about the precise meaning of some construct. A formal semantics can then be
consulted to resolve these ambiguities. Further, a rigorous semantic description can be
of assistance in creating manuals or standards in the first place.
Behaviour-Preserving Transformations
Behaviour-preserving transformations form an important base of the SHE method. A
transformation takes a specification and transforms it into another one. A behaviourpreserving
transformation is a special kind of transformation whose correctness is known
2 In fact, a more adequate term would be rigorous language. In this thesis we will often use the term
formal instead of term rigorous. In a strict sense, the terms refer to different notions (see for example
[Ten91]).