Sep 03, 2003
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- More on context free grammars
	- and ambiguity
	- dangling else problem

- Another example of ambiguity
	- (<t>)-<t> in C
		- can mean two different things
			- typecasting
			- subtracting

- Constructs and capabilities
	- regular expressions can capture
		- rules for a valid identifier in C
	- cfgs can capture
		- rules for a valid mathematical 
		  expression in C
	- neither can capture
		- that a variable must be declared
		  before its usage

- Solution
	- use more powerful grammars
	- alt: use attribute grammars
		- cfgs, decorated with "rules"

- Attribute grammars
	- due to Knuth
	- example for variable declaration rule
		- int a;
		  a=2;
		  b=3;
		  int b;
	- example grammar
		<compstatement> -> <statement> <compstatement>
		<compstatement> -> epsilon
		<statement> -> <declaration>
		<statement> -> <assignment>
		<declaration> -> <type><var>
		<assignment> -> <var> = <constant>
		<type> -> ....
		<var> -> ....
		<constant> -> ....

- More examples
	- procedures in Ada
		- "end name" must match "beginning name"
		- how would you decorate the cfg?