Static Properties of Variables

The Cond special form:

performs a multiway branch based on series of test
expressions.

(cond 
  [test1  consequent1]
  [test2  consequent2]
  [test3  consequent3]
  ...
  ...
  [testn  consequentn]
  [else   alternative])

OR

(cond 
  (test1  consequent1)
  (test2  consequent2)
  (test3  consequent3)
  ...
  ...
  (testn  consequentn)
  (else   alternative))

else is a keyword.  

Test expressions are evaluated in the order in which
they occur until one returns true. The associated
consequent expression is then evaluated.
If none of the test expressions are true, the
alternative expression is evaluated.

cond expressions are equivalent to nested if
expressions. 

(if test1
    consequent1
    (if test2
        consequent2
        ....
        (if testn
	    consequentn
            alternative) ... ))




Scope and Lexical Address:

Each declaration of a variable has a region of text
where that declaration is effective

In scheme, the region of effectiveness of a formal
parameter is the body of the lambda expression.

The SCOPE of a variable declaration is the text within
which references to the variable refer to the
declaration. 

> (lambda (x x) ... ) is ILLEGAL




In most languages, a variables region and scope can be
determined statically.

Scheme/C/C++

Lexically/Statically scoped languages == Block
structured languages

> (define x
     (lambda (x) 
        (map (lambda (x) (+ x 1)) x)))


Inner declaration creates hole is the scope of the
outer one. Region != scope

The inner declaration shadows outer declarations




The declaration of a variable v has a scope that
includes all references to v that OCCUR FREE in the
region associated with v. 

Those references to v that OCCUR BOUND in the region
associated with v's declaration are shadowed by inner
declarations. 

----
Simple algorithm for finding which declaration 
is associated with a variable reference

Search outward for declaration from variable reference
region. First one found is the associated declaration.
If none found even at topmost (global) level then ERROR.


(lambda (x)
  (lambda (y) 
    ((lambda (x)
       (x y))
     x)))

(lambda (z)
  ((lambda (a b c)
     (a (lambda (a) (+ a c)) b))
   (lambda (f x)
     (f (z x)))))

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Wed: Finish reading chapter 2.