CS 541 Lecture -*- Outline -*-

* State and Mutation (Schmidt Ch. 5)

** Mutation
	changing the value of an object
	idea: reference object (cell, container, location)
		refers to some other object, reference may be altered

***	the language FL!
		(ref E)	makes a new reference (refers to result of E)
		(^ E) returns the value of the reference E (dereference)
		(:= E1 E2) alters the reference E1 to refer to E2's value
		(begin E1 E2) evaluates E1 and then E2, returns E2 (sequential)

***	semantics
		(go over definition and look at)
		semantic domains: Proc, Reference, Value, Outcome,
				Store, Storable
			notes: stores map locations to storable-values * Tr
				the boolean tells whether that location is used

		valuation function signatures: valP, valE
		semantic clauses: P, new constructs
			what is the calling mechanism?

***	Analysis
****	    Problems
		makes programs more sequential
		makes programs harder to reason about
			note that can't find value of expression without store
			no referential transparency (f(E) <> f(E))

****	    What good is this?
		can communicate with arbitrary other parts of program
		can change small part of large data structure without making
			a copy, and without explicitly passing as argument
			e.g., databases, simulations, ...
		corresponds to von Neuman machines so efficient on them...

****	    What changes could be made?
		break store into pieces (arrays)
			(ref I E)
			(^ I E)
			makes it easier to tell that two things are different
		preventing side effects during expression evaluation
		use while loops instead of recursion (see SL)
		OR separate functions from procedures (as in Euclid)

** State
***	Imperative language (SL)
		idea: program state is given by pc + memory
			use "if" and "while" for delayed evaluation

***	Semantics
		look at semantics...
			the pc is maintained in the semantics...

** Combining procedures with imperatives (BS) like Pascal
	idea: program state is given by environment + pc + memory