Class 24: Local Procedure Bindings

Held: Friday, 28 February 2003

Summary: Today we consider techniques for defining local procedures, procedures that are only available to select other procedures.

Related Pages:

• EBoard.
• Reading on Local Procedure Definitions.
• Lab on Local Procedure Definitions.

Due

• Lab Writeup 2 (optional).
• Exam 1.

Assignments

• Homework 3: Web Services.
• Read Pairs and Pair Structures.

Notes:

• Please send me information about three or more books in the following format:

  (list "author-last-name" "author-first-name" "title"
    (list "adjective-1" ... "adjective-5"))
  

• One point of extra credit (overall) for attending the G-Tones Concert Saturday at the Forum and harrasing/cheering Alex. (Sorry, those performing can't get the point.)

Overview:

• Why have local procedures.
• Creating local procedures.
• Lab.

Local Procedure Bindings

• Today's class will focus not on something new, but on a better way to do something old: Define helper procedures.
• We frequently want to define procedures that are only available to certain other procedures (typically to one or two other procedures). We call such procedures local procedures
• Most local procedures can be done with let and let*.
• However, neither let nor let* works for recursive procedures.
• When you want to define a recursive local procedure, use letrec.
• When you want to define only one, you can use let.

letrec

• A letrec expression has the format

  (letrec ((name1 exp1)
           (name2 exp2)
           ...
           (namen expn))
    body)
  

• A letrec is evaluated using the following series of steps.
  • First, enter name1 through namen into the binding table. (Note that no corresponding values are entered.)
  • Next, evaluate exp1 through expn, giving you results result1 through resultn.
  • Finally, update the binding table (associating namei and resulti for each reasonable i.
• Not thate its meaning is fairly similar to that of let, except that the order of entry into the binding table is changed.

Named let

• Named let is somewhat stranger, but is handy for some problems.
• Named let has the format

  (let name 
    ((param1 exp1)
     (param2 exp2)
     ...
     (paramn expn))
    body)
  

• The meaning is as follows:
  • Create a procedure with formal parameters param1 ... paramn and body body.
  • Name that procedure name.
  • Call that procedure with actual parameters exp1 through expn .
• Yes, that's right, we've packaged together the procedure definition and the procedure call.
• In effect, we're just doing

  (letrec ((name (lambda (param1 ...
  paramn
  )
                    body)))
     (name val1 ... valn))
  

An Example: index

• We'll consider as an example the problem of finding the index of a value in a list (the position of that value).
• You can find details in the reading.

Lab

• Start the lab.
• Finish it on your own time.