CS 342 Lecture -*- Outline -*-

* Procedure parameters
**	problem: ensuring procedure parameter has proper access to variables
----------
program proc_parameters;
	function twice(function f(n:integer): integer,
				x: integer): integer;
	begin
		twice := f(f(x))
	end;

	procedure tm(m: integer);
		function by_m(i: integer): integer;
		begin
			by_m := i*m
		end;
	begin
		writeln(twice(by_m,3))
	end;

begin
	tm(4)
end.
----------
	*draw stack as this executes, "m" illustrates the problem.

** Solution: closures
	pass static link (or display) along with procedure parameters.
***	closure: pair of static link (or display) and code address
		(name derivation: procedure said to be closed if
			it does not refer to global variables.
			Closure is thus process of putting globals "into" proc)

***	What static link is passed?
		in general, if def of actual is at static distance SD from call
			-traverse SD static links to find SL for closure
 		How to call formal procedure
		1. set static link from formal.EP
		2. jump indirect through formal.IP

***	What display must be passed?
		-simplest if pass entire display,
		 save and restore whole display when call procedure parameter

** What happens if allow returns of functions?
----------
function curried_add(m: integer): function(i: integer): integer;
	function f(i: integer): integer;
	begin f := i+m; end;
begin curried_add := f end	{illegal}
----------
	returning a closure will cause a dangling reference,
		-AR for curried_add will be popped
		-can allocate AR on the heap: called *full closure*