CSc 520 - Principles of Programming Languages
24 : Names, Scope, Bindings -- Dynamic Scope

Christian Collberg

Department of Computer Science

University of Arizona

1 Static vs. Dynamic Scope

• Pascal is lexically scoped. We can look (textually, or at compile-time) at a procedure and determine to which object an identifier refers.
• Some languages (Snobol, APL, Perl, some dialects of LISP) are dynamically scoped. The binding between an identifier and the object it refers to is not decided until run-time.

2 Dynamic Scope

• The current binding for an identifier is the one last seen during execution and whose scope has yet to be destroyed.
• Consider the example on the next slide.

  static scope:

  the program prints 1.

  dynamic scope:

  the program prints 2.

• Static scope rules match the use of an identifier with the closest lexically enclosing declaration.
• Dynamic scope rules choose the most recent active declaration at runtime.

3 Dynamic Scope...

1#1

4 Dynamic Scope -- Problems

2#2

• Dynamic scope makes it is easy to accidentally redefine a variable.

5 Dynamic Scope -- Advantages

3#3

• We often have to pass around state so that deeply nested procedures can make use of it. DEBUG-flags is a common example.

6 Dynamic Scope -- Advantages...

4#4

• We can, of course, use global variables.

7 Dynamic Scope -- Advantages...

5#5

• Dynamic scope makes it is easy customize the behavior of procedures.

8 Readings and References

• Read Scott, pp. 115, 131-135
• Dynamic Variables, David R. Hanson and Todd A. Proebsting, PLDI 2001. www.microsoft.com/~drh/pubs/dynamic.pdf.