Sep 05, 2003
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- Using attribute grammars
	- example of propagating type information, from book
	- example of making procedure names match in Ada

- Move on to semantics
	- several types 
		- operational
		- denotational
		- axiomatic
		- natural

- Operational semantics
	- also called intensional semantics
	- how would you explain the C construct
		"a ? b : c" to a person from Pascal?
	- Ans: explain it using lower-level primitives
		- this is the basic idea

- More examples of operational semantics
	- switch statement in C
	- while loop in C
	- for loop in C
		- can keep decomposing,	till
		  we get some "basis" set

- Denotational semantics
	- also called extensional semantics
	- only thing that matters is functional	
	  relationship from i/p to o/p
	- details don't matter

- Basic difference
	- two different factorial programs can have
		different operational semantics
		- way in which they are written matters
			(e.g., for-loop versus recursion)
	- but will have same denotational semantics
		- since they compute the same thing

- Axiomatic semantics
	- not really a semantics in the above sense
	- but oriented toward proving properties of programs

- Examples of axiomatic semantics
	- rules for proving odd and even numbers
		N: is_odd when N mod 2 = 1
		   is_even otherwise
	- finding oddness or evenness of expressions

		e1: p1  e2: p2
		--------------
		    e1+e2: p3

		- how would you describe p3 using p1 and p2?

- Another example
	- deriving pre-conditions from postconditions
	- worked out examples from textbook