CSC302 2006S, Class 12: Scheme Syntax

Admin:
* Homework 1 due at 5:00 p.m.
* No non-reading (+question) homework next week to give you time to read carefully.  Please take that time.
* About the symbols after your name in readings.
* What do you know about Runescape?
* EC: Angeline, Dimitar

Overview:
* Quasiquotation
* Your questions on the syntax
* Preparing to read semantics

/Quasiquotation/

* Quote: Don't evaluate, just give me the following verbatim
  * (quote .....)
  * '...
* Quote works for list structures as well as for symbols
* Problem: Sometimes you want to evaluate some parts of the
  quoted structures
* Silly example: Build an association list of grades
  - Quote free attempt
(define grades0
  (lambda (samg samuelg alexg alexanderg)
    (list
      (list 'sam samg)
      (list 'samuel samuelg)
      (list 'alex alexg)
      (list 'alexander alexanderg))))
  * Temptation: Remove all those inner quotes and lists and just use a quote
(define grades1
  (lambda (samg samuelg alexg alexanderg)
    '((sam samg)
      (samuel samuelg)
      (alex alexg)
      (alexander alexanderg))))
  * Quasiquote permits us to mix the two
(define grades2
  (lambda (samg samuelg alexg alexanderg)
    `((sam ,samg)
      (samuel ,samuelg)
      (alex ,alexg)
      (alexander ,alexanderg))))
    * The back quote means "Generally take verbatim"
    * The comma means "Evaluate this"
* Complicating quasiquote: Nesting!
* Some does not believe in quotes for beginning programmers
'1 - What value?  What type?
  * 1, integer
(car '('1 'b)) - What value?  What type
  * 1, symbol ; NOPE - 1 is an integer
  * (quote 1), symbol - Closer
  * (quote 1), list
    - Shorthand for (quote ((quote 1) (quote b)))

(car '('a 'b)) - What value? What type
  * a, symbol

/Other Syntax Questions/

* What are the optional # signs at the end of numbers?
  E.g. <decimal 10> -> <digit 10>+ #* <suffix>
  * # means "I know there is some digit there, but I don't know what it is"
  * 3300000 vs. 33#####
  * Only available before the suffix - trailing digits
* Why does it only support base 2, 8, 10, 16
  * The are the only commonly-used bases in computing
* There are signed reals, unsigned reals, unsigned integers, where are the signed integers?
  * I have no idea.
* Given that commands are just expressions, why do they distinguish between the two?
  * Related: Why 
    <sequence> => <command>* <expression> 
    rather than <command>+ or <expression>+?
  * Subtle reason: Want to think about how they are used:
    * At this level, commands we evaluate and discard the result, expressions we evaluate and use the result
    * Sequences end with expressions so that they end with a result
  * Example
    (let ((x 5))
      (display x)
      (newline)
      x)
* Why does set! get special treatment (that is, why is it a reserved keyword)?
  * It changes something (nope, but good guess)
  * All of the reserved keywords are "functions" that do not use the normal scheme of evaluation
  * We write
    (set! a 10)
    Not
    (set! 'a 10)