Aug 29, 2003
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- Syntax and Semantics
	- syntax defines structure of language
	- semantics defines meaning of language

- Previously, both
	- used to be described intuitively
	- now, well-defined formal systems available

- How an interpreter works
	- lexical analysis
	- syntax analysis
	- semantic analysis

- What is a language?
	- set of strings over some alphabet
	- legal strings = sentences

- Examples
			    Lex | Syn | Sem
	- Tommy is a bod.   No
	- Tommy boy is a.   Yes   No
	- Boy is a Tommy.   Yes   Yes   No
	- Tommy is a boy.   Yes   Yes   Yes

- What does lexical analysis do?
	- parcel characters into lexemes
	- lexemes fall into "token" categories

- Example: analyzing "int i = 20;"

	  lexemes | tokens
	-------------------------
	- int	  | reserved word	
	- i	  | identfier
	- =	  | special symbol =
	- 20	  | numeric constant
	- ;	  | special symbol ;

- What property did we use?
	- delimiter = white space?
	- a.k.a. free-format language

- Early versions of FORTRAN
	- were fixed format
	- DO 99 I = 1 . 10 is different from
	- DO 99 I = 1 , 10 !

- Grammars define how sentences are constructed
	- e.g., simple grammar for English
		- Tommy is a boy sentences.
	- parse tree and productions

- Lets parse "Time flies like an arrow"
	- 4 different meanings!
	- we say that the grammar is ambiguous!
	- how can we determine intended meaning?
		- need context

- Chomsky hierarchy of languages
	- regular languages
	- context-free languages (CFL)
	- context-sensitive languages (CSL)
	- phase structured languages

- In this course
	- we will study CFLs
	- productions have only one "thing" on the left
		- called "non-terminals"

- An example of a property that is not context-free
	- first letter must be capitalized
	- an example CSL for this!
		- exercise: try redoing it with a CFL!
		  (hint: its possible)

- A Grammar for a Calculator
	- for evaluating expressions