Names, Scopes, and Bindings

1
Names, Scopes, and Bindings

• Chen Ding
• MIT 6.035, September 2007
• PLP Book, Chapters 3 and 9

2
High-level Names

• Assembly level
• memory address and memory content
• Language and program level
• variables
• types
• procedures

3
Binding Time

• Language design time
• primitive types, control flows
• Language implementation time
• machine dependent parameters
• Programming time
• user defined functions and data
• Compile time
• the layout of statically defined data
• Link time
• intermodule references
• Load time
• physical memory allocation

4
Object lifetime

• The life of an object
• creation of the object
• creation of the binding
• references
• deactivation and activation
• destruction of bindings
• destruction of objects

5
Allocation Types

• Static
• fixed size, live throughout the execution
• global variables, strings, tables for debugging,
  run-time type checking, exception handling, etc
• often read-only

6
Stack and Heap

• Stack-based allocation
• frame or activation record
• parameters, local vars, temporaries
• Heap-based allocation
• dynamically sized objects
• heap management
• garbage collection

7
Static Scope

• Single, global scope
• early Basic
• common blocks in Fortran
• Nested subroutines
• Algol style closest nested scope rule
• check the innermost scope and then successively
  surrounding scopes

8
Pascal Example

• procedure P1(A ...)
• var X real
• procedure P2(B ...)
• var X integer
• ...
• procedure P3(C ...)
• ....

9
Modules

• Info. hiding through explicit import/export
• fewer name conflicts, more robust system and
  better error reporting
• A module is a collection of objects
• objects inside are visible inside but not visible
  outside unless exported
• Clusters in Clu, Modula, packages in Ada,
  namespaces in C

10
Module-as-Type

• Object-oriented programming
• a class can be viewed as a module augmented with
  inheritance
• encapsulation
• the this pointer
• my_stack.push(x), instead of
• push(my_stack, x)
• inheritance

11
Dynamic Scope

• int a
• void first() a1
• void second() int a first()
• void test()
• a 2
• if (b) seond()
• else first()
• print(a)

12
Reference Environment

• First-class values
• can be passed as a parameter, returned from a
  subroutine, or assigned into a variable
• subroutines in functional languages
• Second-class values
• can be passed as a parameter
• subroutines in imperative languages
• Third-class values
• e.g. labels

13
Dynamic Binding

• Alias
• common blocks, equivalence statements
• pass-by-reference in C
• Overloading
• subroutine names, operators
• false polymorphism
• Polymorphism
• same name, different actions
• generic routines

14
Example Polymorphism

• class Animal void MakeNoise() ...
• class Dog inherits Animal void MakeNoise()..
• class Cat inherits Animal void MakeNoise()...
• void Sing(Animal a) a.MakeNoise()

15
Implementing Scope

• Symbol tables
• name-to-object bindings
• variables and types (classes)
• forward references
• The scope stack
• search down from top to bottom

16
Summary

• Binding times
• Allocation schemes
• Modules and classes
• Reference environment
• Scope rules
• Dynamic binding
• Symbol tables