CS 342 Lecture -*- Outline -*-

* core interpreter structure

** how to read the code
	generally organized around kinds of data (ADTs)
	run the program, put in print statements, see what happens
		it's just a program
	flip back and forth between appendix and description
		description is the comments
	look for errors!
		check boundary cases especially
		check for initializations
	look for invariant properties of data structures
	write in comments
	what abstract concept corresponds to each type of data?

** syntax-directed intrpretation
	read ()-balanced input into userinput array.
	parse it
	evaluate it

***	reading
		read until get some non blanks, a whole line,
		and a parenthesis balanced string.

***	parsing is "recursive descent" (top-down)

	<input> ::= <expression> | <fundef>
		so 2 cases: function def's recognized by (define ...
				handled by parseDef

	<fundef> ::= (define <function> <arglist> <expression>)
		so after finding (define ,
		parseDef looks for <function>, then <arglist>,
			then <expression>, then ).

		note: formal vs. actual parameter

	<expression> has several alternatives, so have several cases.
		parseExp (bottom of 488) checks for applications: (...
			or values or names, etc.

	example:  follow parse of (+ x 2)



	result of parse is a syntax tree (or function def tree)
		draw expression tree for (+ x 2) see page 11
			(apexp: + ((varexp: y) (valexp: 3)))

***	evaluation of expressions
****		data structures
*****		   environments
			def: environment = a mapping from variables to values
			def: x is bound to 3, 3 is the binding of x
			rep: pair of lists of same length (invariant)
				names and values
			used for: global variables, parameters
			changed by: set, function call, see eval (p. 13)
*****	 	   syntax trees
			see page 9
			explain variant records

			data structure  grammar		code
			variant		A ::= B|C	if then ..B.. else ..C
			record		A ::= B C	parseB; parseC

*****		   function defs
			name, formals, body (exp)

****	some questions about the interpreter
	what does / mean?
	what happens to syntax tress after they are evaluated (no g.c.?)
	what happens if one redefines a function
	what does while return?
	what changes could we make to set?