CS150 Notes 19 (7 October 2005)

Upcoming Assignments

• Before 11:59pm Sunday: If you don't want to be assigned a partner for PS5, send me email with your choosen partner (or that you want to work alone) and your answer to Question 4.
• Monday, 17 October: PS5
• Before Friday, 23 October: Read rest of GEB part I (Chapters 2-4 and 6-9, in addition to Chapters 1 and 5 you have already read). See the study guide questions on Notes 18. I recommend reading a chapter every few days rather than trying to read it all just before October 23.

Places, Frames and Envrionments

• Names identify places. (define x exp) and (lambda (x) exp) both create places named x.
• Frames hold names. All names must be inside a frame.
• An environment is a pointer to a frame.
• All frames except the outermost frame have pointers to a parent frame.
• The global environment is a pointer to the outermost frame. The outermost frame contains all the Scheme primitives.

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

1. Construct a new frame, enclosed in the environment of this procedure (that is, the parent of this frame is the frame this procedure's environment points to)
2. Make places in that frame with the names of each parameter
3. Put the values of the parameters in those places
4. Evaluate the body in the new environment

How do we find the value associated with a name in an environment?