Lecture -*- Outline -*-

* lexical matters in Haskell (Thompson 3.7, Davie 2.14, appendix C.2-3)

** important and unusual lexical conventions
------------------------------------------
IMPORTANT AND UNUSUAL LEXICAL CONVENTIONS

- Case matters

     identifiers: fact, x y
     constructors: Rational, Stack

- Layout (white space) matters:
   (if omit braces and semicolons,
   following "where", "let", "do", and "of")
  
  Example:

	let x = 3
	    y = 4
	in x + y

  ==>

	let {x = 3
	    ;y = 4
	}in x + y

------------------------------------------
	simplified layout rules:
             first token determines indentation level => {
             same indentation as first token => ;
             less indentation => }

	Watch out, you can get syntax errors from bad indentation!

     see sections 2.7 and 10.3 of the Haskell report for details

** identifiers and operators (Thompson 1.7, Davie 2.14.2)
------------------------------------------
           IDENTIFIERS (NOT INFIX)

can use letters, digits, primes and underscores (_)

CASE MATTERS

varids:

    start with a-z or unicode lowercase letter

    examples
	pythag, fac, x, y', x3'n_, aGoodId


conids:

    start with A-Z or unicode title case letter

    Examples:
       Stack, Rational, Typ'_3

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

------------------------------------------
		OPERATORS 

Examples:
  +, -, !!, ++, ==, /=
  :, :#

Notes:

  1. Drawn from: !#$%&*+./<=>?@\^|-~:
     and unicode symbols and punctuation

  2. An operator is a constructor
      if it starts with :

  3. All operators are infix 
     except unary negation (-)

  4. Any identifer can be made into
     an infix operator using backquotes

		3 `div` 4

  5. An infix operator can be used as
     an identifier when put in parentheses

               (+) 3 4
------------------------------------------

        Because operator strings are maximal length strings of
        symbols, instead of 2*-3 you have to write 2*(-3) or 2 * -3.