* 3.2 the environment model of evaluation

substitution has failed with the new power of assignment.

the substitution model:
  to apply a compound procedure to its arguments, eval the body of the
  procedure with the formal params replaced by the corresponding
  argument.

a variable is no longer a name for a value. now it must designate a
"place" where its value lies.

these new structures are called /environments/.

an environment is a sequence of /frames/.

each frame is a table of /bindings/.	( a frame has at most one
					    binding per variable)

each frame has a pointer to its /enclosed environment/
					(unless it is the /global/
					    environment ) 

a variable's value 
  with respect to its environment is
    the value given by 
      the binding of the variable 
        in the first frame
          in the environment 
            that contains a binding 
              for that variable.
  if no frame in the sequence 
    specifies a binding
      for the variable
        then the variable is
          /unbound/

a binding overiding a different value in a higer frame is /shadowed/ 
  by the other binding

** 3.2.1 the rules for evaluation
to evaluate a combination:
  1. evaluate the subexpressions of the combinataion
  2. apply the value of the operator subexpression to the values of
     the operand subexpressions

in the environment model of evaluation, a procedure is a pair
consisting of:
  - some code
  - a pointer to an environment

to apply a procedure to arguments, create a new environment containing
a frame that binds the parameters to the values of the arguments

the environment model of procedure application can be given by two
rules:

  - a procedure object is applied to a set of arguments by
    constructing a frame, binding the formal parameters of the
    procedure to the arguments of the call and then evaluating the
    body of the procedure in the context of the new environment
    constructed. the new frame has as its enclosing environment the
    environment part of the procedure object being applied
  - a procedure is created by evaluating a lambda expression relative
    to a given environment. the resulting procedure object is a pair
    consisting of the text of the lambda expression and a pointer to
    the environment in which the procedure was created
  - define creates a binding in the current frame, assigning the value
    to the symbol
  - (set! variable value) locates the binding of the variable in the
    environment and changes that binding to a new value

** 3.2.2 applying simple procedures

#+BEGIN_SRC scheme
  (define (square x)
    (* x x))
  (define (sum-of-squares x y)
    (+ (square x) (square y)))
  (define (f a)
    (sum-of-squares (+ a 1) (* a 2)))
  (f 5)
#+END_SRC

** 3.2.3 frames as the repository of local state

#+BEGIN_SRC scheme
  (define (make-withdraw balance)
    (lambda (amount)
      (if (>= balance amount)
	  (begin (set! balance (- balance amount))
		 balance)
	  "insufficient funds")))
  (define W1 (make-withdraw 100))
  (W1 75)
#+END_SRC

** 3.2.4 internal definitions

#+BEGIN_SRC scheme
  (define (sqrt x)
    (define (good-enuf? guess)
      (< (abs (- (square guess) x)) 0.001))
    (define (improve guess)
      (average guess (/ x guess)))
    (define (sqrt-iter guess)
      (if (good-enuf? guess)
	  guess
	  (sqrt-iter (improve guess))))
    (sqrt-iter 1.0))
#+END_SRC

with internal definitions:
  - names of procedures do not interfere with others in the bigger
    environment
  - local procedures can access enclosing arguments via free vars