Imperative Languages & State

Key Concepts

Mental model is a slightly more abstract view of the Von Neumann architecture

• all about state and state changes
• a program is a series of statements that lead to a final state (probably including output, so changing the state of the external environment)
• variables represent units within the state of the program than can change (vary) as the program runs

Variables

Variables have 6 attributes, which we can represent as a sextuple:

(Name, Address, Value, Type, Lifetime, Scope)

• Name: identifies the variable (Names also apply to functions/procedures, constants, and sometimes user-defined types, so are more generically called identifiers)
  • Rules for valid names are part of the syntax of a language
  • Identifiers should not be mistaken for keywords (some languages allow overlap; most do not)
  • Rules affect readability, writeability
• Type: determines
  • size in memory
  • range of possible values
  • set of valid operations
  (What about abstract types, such as Java interfaces?)
• Lifetime: when is the memory set aside and when is it "freed"? (allocation / deallocation)
  • Examples: parameters, local variables, global variables, class variables
  • Terms: static, stack-dynamic, explicit heap-dynamic, implicit heap-dynamic
  • Examples: what about new AquaFish,    malloc()?
• Scope: where can the variable (or constant or function) be "seen" or used?
  • Examples: parameters, local variables, global variables, class variables
  • Terms: global scope, static scope, dynamic scope, referencing environments

Aliases: when two variable names access the same location in memory (created via pointers, references, C and C++ unions)

Binding

A binding is an association, such as between variable name and a memory location, a symbol and an operation, or a function call and the code that will be executed.

• Static vs. dynamic type binding
  (What does "strong typing" vs "weak typing" mean?)
  (What about type inference?)
• Static variables, stack-dynamic variables, heap-dynamic variables
• Binding times: language design / implementation, compile time, load time, link time, run time

What are the advantages and disadvantages of different language design decisions?