CSC151 2007S, Class 48: Variable-Arity Procedures

Admin:
* A favor - Can a few of you try http://language.grinnell.edu/names/
  and let me know by early this afternoon what you think about it?
* Due: HW 16
* Exam 3 distributed.
* EC for Monday's Alumna Scholar talk by Dr. Nesbitt.
* EC for any softball (here) or baseball games (Illinois) that happen 
  to happen this weekend. and for Rugby (here)

Overview:
* Definition of arity.
* Why have variable-arity procedures.
* How to write variable-arity procedures.
* Lab.

/Definition of arity/

* "arity" - a way to say "the number of parameters a procedure has"
* sqrt - arity is 1
* tally - arity is 2 (tally pred? lst)
* + - arity is zero or more (NO MATTER WHAT THE READING SAID, AND I DENY
  WRITING IT OR EDITING IT)
* - - arity is one or more
* "variable arity" - formal term for "or more"

/Why have variable-arity procedures?/

* For some operations, like +, it gets tedious to write a lot of pluses
  (and twice as many parens)
* For some operations, it is necessary
  * We can't write list without variable-arity
* The current display is incredibly annoying, and always gives us errors the first time we try to display multiple values
  * The "hack" to deal with this - put 'em in a list
* Philosophy: If the language designers need it, so do programmers
  * Paden can now write "padens-much-better-display" (disp)

/How to write variable-arity procedures/

* Tradition
  (lambda (params) body) - fixed arity
* First technique
  (lambda params body) - variable arity, 0 or more
    When you call the procedure, params are passed as a list
* Second technique
  (lambda (first . rest) body) - variable arity, 1 or more
* Variants on second
  (lambda (first second . rest) body) - variable arity, 2 or more

/Lab/