Com S 342 meeting -*- Outline -*-

* Expressions in Scheme (and other languages) (1.1)

	Note: This studies Scheme from the perspective of languages
		in general, not just to program in Scheme.

	Students: note both the
		- terminology & perspectives
		- facts

	Keep drawing analogies to other languages (me too!)
		will help think about languages in general

	I don't think it's helpful to give all the definitions,
		which are in the book.
	Maybe give the students a sheet to fill them in, if think more
		important, but I think it's okay to just mention most.

** fundamental expressions (1.1.1)

	use to give categories, and as an overview of Scheme

------------------------------------------
 SYNTACTIC PARTS OF A PROGRAMMING LANGUAGE

		<program>












------------------------------------------

	Q: What are the basic kinds of syntax in a programming language?

		<program>
              /        \      \
	<expr>	       <stmt>	<decl>
         /  \    \        |        |
<literals> <vars> <calls> <while> <defs>


		<literals>
         /       /     \      \
<numerals> <booleans> <chars> <strings>
  |  |        |   |     |         | 
 3.4 7       #t  #f    #\a     "a string"


*** terms relating to variables
------------------------------------------
   TERMINOLOGY RELATING TO VARIABLES

	- x denotes 3
	- x is bound to 3
	- 3 is the binding of x

	- expressed values
	- denoted values
	- keywords vs. identifiers
		vs. reserved words
------------------------------------------
		reserved words always are special in non-quoted contexts,
		keywords only special in some contexts
				(e.g., in Scheme and PL/I)

*** procedure calls

------------------------------------------
	PROCEDURE CALLS

Scheme syntax examples





Scheme: every parenthesis is significant
	no extra, no omitted

Terminology:
  procedure (Scheme)
  function (math)
  application, combination
  operator, operand
  subexpressions
  arguments, actual parameters, actuals,
		parameters
------------------------------------------

...	(+ 3 4)
	(* 5 (+ 3 7))
	(min x (add1 y))

	compare to other languages: FORTRAN, ML, Smalltalk

	Q: What's the translation of 5 + 6 + 7 into Scheme?
	Q: How would sqrt(cos(x + 5)) be translated into Scheme?

	Q: What's the syntax of a procedure call in Scheme?
		<call> ::= (<exp> <exp>*)
	Q: What's a rule for forming the Scheme from the algebraic notation?

	This is an example of

------------------------------------------
		REGULARITY

def: a language feature is *regular* if



examples:
	Scheme procedure call syntax
	FORTRAN numeric syntax

counter-examples:
	a C function can't be declared
		to return an array type
	English verb past tenses
------------------------------------------
... it is governed by rules that have no execptions.

	Q: What's an irregularity in English?

		change -> changed
		chomp -> chomped
		write -> wrote
		sing -> sung

	Q: Are programming languages more regular than natural languages?
	Q: Does that help in learning programming languages?

** definitions, programs, read-eval-print loop (1.1.2)

*** definitions
------------------------------------------
	   DEFINITIONS

Scheme syntax examples

   (define pi
      ; TYPE: number
      3.14159)











Terminology:
	- special form
	- keyword
------------------------------------------
...   (define greeting ; TYPE: string
        "Welcome!")
      (define is-pi?
        ; TYPE: (-> (number) boolean)
        (lambda (n) (= n pi)))
      (define add1
        ; TYPE: (-> (number number) number)
        (lambda (n) (+ n 1)))

	Special forms
		- provide the "magic"
		- don't evaluate their all arguments in random order,
			as do procedures
		- similar to macros

	Q: What is like this in C++? Pascal?

*** programs
------------------------------------------
	  PROGRAMS

Scheme syntax: a series of
	definitions and expressions

Example

   (define pi ; TYPE: number
      3.14159)
   (define greeting ; TYPE: string
      "Welcome!")
   (define is-pi?
     ; TYPE: (-> (number) boolean)
     (lambda (n) (= n pi)))

   (display greeting)
   (newline)
   (is-pi? pi)

Semantics:


Terminology
   - top level (pi vs. n)
------------------------------------------
  ... execute from top (start of file) to end

------------------------------------------
         READ-EVAL-PRINT LOOP

What the interpreter does

                      (exit)
            ---> read  ----> 
                 ^   \
                /     v
             print    eval
                ^-----/
------------------------------------------

	Go to the computer and show how to use
		Show what happens when do display

** if-expressions (1.1.3)

------------------------------------------
         IF-EXPRESSIONS

Transcript of Scheme examples

  > (if #t 3 4)

  > (define x 2)
  > (if (zero? x) (+ x 3) x)

  > (if (< x 0) (- x) x)



------------------------------------------
	...  3
	...  2
	...  2

	Q: So what's the Scheme syntax?
	Q: Is the else-part required? Why would it be?
	Q: How does this differ from an if statement in C or Pascal?
	Q: What's this like in C?  Anything like this in Pascal?
		(like e1 ? e2 : e3 in C)
		nothing in Pascal