[an error occurred while processing this directive]

<center><h2>cs150: Notes 21</h2></center>

<h4>Assignments Due</h4>

<ul>
<li> <b>Wednesday, 14 March</b>: <a href="ps/ps5/">Problem Set 5</a>
<li> <b>Before Monday, 19 March</b>: Read rest of GEB part I (Chapters 2-4 and 6-9, in
addition to Chapters 1 and 5 you have already read). 
</ul>

<center><h3>Evaluation Rules (with state)</h3></center>

<b>Evaluation Rule 2 (with state): Names.</b>  A name expression
evaluates to the value associated with the name.  To find the value
associated with a name, look for the name in the frame associated with
the evaluation environment.  If it contains a place with the name, the
value of the name expression is the value in that place.  If it does not
contain a place with the name, the value of the name expression is the
value of the name expression evaluated in the parent environment if the
current evaluation environment has a parent.  Otherwise, the name
expression evaluates to an error (the name is not defined).
<p>

<b>Definition Rule (with state).</b> A definition creates a new place
named after the definition name in the frame associated with the
evaluation environment.  The value in the place is value of the
expression.  If there is already a place with the name in the current
frame, the definition replaces the old place with the new place and value.
<p>

<b>Application Rule 2 (with state): Constructed Procedures.</b> To apply
a constructed procedure:
<ol>
<li> Construct a new environment, whose parent is the environment to
which the environment pointer of the applied procedure points.
<li> Create a place in the frame of the new environment for each
parameter containing the value of the corresponding operand expression.
<li> Evaluate the body of the procedure in the newly created
environment.  The resulting value is the value of the application.
</ol>


<pre>
(define double 
   (lambda (x) (+ x x)))



(define nest 
  (lambda (x)
    (lambda (x)
      (+ x x))))

((nest 3) 4)
</pre>

<pre>
(define (evaluate-name name env)
   (if (null? env) (erro r)
       (if (frame-contains name (get-frame env))
               (lookup name (get-frame env))
           (evaluate-name name 
             (parent-environment 
                (get-frame env))))))
</pre>

[an error occurred while processing this directive]