CS 342 Lecture -*- Outline -*-

* Scope rules

** Static (lexical) scoping
		procedures called in environment of def.

--------------
begin
    boolean debugging;

    procedure debug_print_real(x);
    value x; real x;
    begin if debugging then comment ...; end;

    procedure f(a,b,c);
    real a,b,c;
    begin debug_print_real(a); comment ...; end;

    procedure p;
    begin
	boolean debugging;
	debugging := true;
	f(x,y,z)
    end;

    comment ** main program **;
    begin
	boolean debugging;
	debugging := false;
	comment ...;
	p;
    end
end
-------------
	(draw contour diagrams for static and dynamic scope while P is running)

** Dynamic scoping (fluid binding)
		procedures called in environment of caller.

** Analysis
***	Benefits of dynamic scoping
		-Can override bindings dynamically (even accidentally)
		-textually unrelated parts of program can communicate
			without using globals or parameters
		-possible to access parts of envrionment
			that cannot be passed as parameters
			(procedures in some languages)
***	Costs of dynamic scoping
		(see above)
		-more expensive to implement at run-time
		programs are harder to read, because can't tell what names mean
		-static structure of program does not relfect dynamic structure
			violation of structure principle
		-cannot do static type checking (in general)
			x := 3, how do we know that x has type integer?