Chapter 5

1. The yylex() lexical analyzer allocates a leaf and stores it in yylval for each terminal symbol that it returns to yyparse().
2. When a production rule in the grammar is reduced, the semantic action code in the parser allocates an internal node and initializes its children to refer to the leaves and internal nodes corresponding to symbols on the right-hand side of that production rule.
3. yyparse() maintains a value stack that grows and shrinks in lock-step with the parse stack during parsing. Leaves and internal nodes are stored on the value stack until they are inserted as children into a containing internal node.
4. A value stack is fully generic and can contain any type of value. In C, this is done using a union type, which is type-unsafe. In Java, it is done using a parserVal class that contains the tree nodes in a generic way. In Unicon and other dynamic languages, no wrapping or unwrapping is needed.