Semantics

• Semantics


  Meaning, which can be subtle and open to interpretation
• Ways to specify semantics

  • Language reference manual
  • Defining translator
  • Formal definition
• Language reference manual


  • Most common way to specify semantics
  • Suffer from lack of precision inherent in natural language descriptions
  • May have omissions and ambiguities
• Defining translator


  • Advantage is that questions about a language can be answered by experiment
• Formal definition


  • Precise, but complex and abstract, and require study to understand
  • Different formal methods available, with choice depending on intended use
• Attributes

  Properties or characteristics associated with a name
• Binding

  The process of associating an attribute with a name
• Semantic functions

  Functions that define the meaning of language constructs
• Static binding

  Binding that occurs prior to execution
• Dynamic binding

  Binding that occurs during execution
• Static attribute


  Attribute that is bound statically
• Dynamic attribute

  Attribute that is bound dynamically
• Declarations


  Principal method for establishing bindings
• Definitions


  Declarations that bind all potential attributes
• Declarations

  Declarations that only partially specify attributes
• Block

  Language construct consisting of a sequence of declarations followed by a sequence of statements
• Local declarations

  Declarations associated with a specific block
• Non-local declarations

  Declarations in surrounding blocks or global declarations
• Scope


  Region of the program over which a binding is maintained
• Lexical scope

  Scope rule where scope follows the structure of the blocks as they appear in the written code
• Symbol table

  Data structure that supports insertion, lookup, and deletion of names with associated attributes, representing the bindings in declarations
• Scope analysis


  Process of adding declarations to the symbol table on entry into a block and removing them on exit
• Static environment

  Environment constructed at load time
• Dynamic environment

  Environment constructed at execution time
• Variable


  Object whose stored value can change during execution
• Assignment by sharing


  Assignment where locations are copied instead of values
• Assignment by cloning


  Assignment where a new location is allocated, the value is copied, and the variable is bound to the new location
• Constant


  Language entity with a fixed value for the duration of its existence in a program
• Static constant


  Constant whose value can be computed prior to execution
• Dynamic constant


  Constant whose value can only be computed during execution
• In C, load-time constants cannot be defined because of a separate rule restricting initial values of global variables to a narrow subset of compile-time expressions
• In any programming language, it is the use of names or identifiers, to denote language entities or constructs

  Fundamental abstraction mechanism
• In addition to names, a description of the semantics of most programming languages includes the concepts of location and value

  TRUE
• Values

  Any storable quantities, such as the integers, the reals, or even array
• Locations

  Places values can be stored. Like addresses in the memory
• The meaning of a name is determined by the properties or ______

  attributes
• In some languages, constructs that cause values to be bound to names are in fact called bindings rather than declarations
• ML Code Examples


  let val x = 2; val y = 3 in x + y
• Bindings can be determined by a declaration either implicitly or explicitly
• The notion of blocks containing declarations began with Algol60, and all Algol descendants exhibit this block structure in various ways