1. Prénom Nom Formal Semantics Introduction Rolf Ingold Département d'Informatique Université de Fribourg http://moodle.unifr.ch > Formal Semantics [SP 08]

2. © Prof. Rolf Ingold Chapter content  Role of formal semantics  Difference between syntax and semantics  Difference between mathematics and programming  Referential Transparency  Difference between language and metalanguage

3. © Prof. Rolf Ingold Formal Semantics  Formal semantics serves several purposes:  Help in understanding programming languages  Support for language design and specification  Useful for standardization  Support for compiler development and testing  Support for software specification  Support for program verification and validation  Limitations of formal semantics  High complexity, hard to learn and to master  Inappropriate for most existing languages

4. © Prof. Rolf Ingold Syntax and semantics  What is the difference between syntax and semantics ?  Syntax describes the structure of programs.  It defines well-formed programs.  Semantics characterizes the behavior of programs.  Semantics relies on syntax.  In this course, we distinguish  Static semantics, describing static constraints that are not taken into account by syntax,  Dynamic semantics describing the effect of programs at runtime.

5. © Prof. Rolf Ingold Mathematics vs. programming  The problem with imperative features  Most programming languages are imperative and rely on instructions with side effects.  Side effects are hard to describe.  In mathematics, the notation is asserting the equality between two objects.  In programming languages, is a command that assigns the value of e to a variable x.  No propriety can be directly inferred.  Pure functional languages avoid this difficulty but their practical interest is limited.

6. © Prof. Rolf Ingold Referential Transparency  The following propriety is referred to as referential transparency. if and are true, then is true  Standard mathematical notations are referentially transparent.

7. © Prof. Rolf Ingold Referential Transparency  But programming languages are generally not referentially transparent.  Considering the program (Pascal style) the following properties are obtained var y : Integer; function f(var x: Integer): Integer begin y:=y+1; f:=y+x end; y:=0; f(1) + f(1) = 3 but y:=0; 2 * f(1) = 2 y:=0; z:=0; f(y) = 2 but y:=0; z:=0; f(z) = 1

8. © Prof. Rolf Ingold Referential Transparency  The objective of the course is to describe constructs of programming languages, which are referentially opaque, by a formalism being referentially transparent.

9. © Prof. Rolf Ingold Language and Metalanguage  The programming languages to be studied will be referred to as the languages.  The formalism used to describe properties of languages will be called metalanguage.  Such a distinction avoids circularity of descriptions.