1. Recursive descent parsing
17-Apr-19

2. The Stack
One easy way to do recursive descent parsing is to have each parse method take the tokens it needs, build a parse tree, and put the parse tree on a global stack
Write a parse method for each nonterminal in the grammar
Each parse method should get the tokens it needs, and only those tokens
Those tokens (usually) go on the stack
Each parse method may call other parse methods, and expect those methods to leave their results on the stack
Each (successful) parse method should leave one result on the stack

3. Example: if statement
<if statement> ::= “if” <condition> <block>
The parse method for an if statement:
Calls the Tokenizer, which returns an “if” token
Makes the “if” into a binary tree, which it puts on the stack
Calls the parser for <condition>, which parses a command and puts it on the stack
Stack now contains: “if” <condition> (“top” is on right)
Calls the parser for <block>, which parses a block and puts it on the stack
Stack now contains: “if” <condition> <block>
Pops the top three things from the stack, assembles them into a tree, and pushes this tree onto the stack

4. Sample Java code
public boolean ifCommand() { if (keyword("if")) { if (condition()) { if (block()) { makeTree(3, 2, 1); return true; } } error("Error in \"if\" statement"); } return false; }

5. Fancier error messages
public boolean ifCommand() { if (keyword("if")) { if (condition()) { if (block()) { makeTree(3, 2, 1); return true; } error("Error in \"if\" block"); } error("Error in \"if\" condition"); } return false; }

6. Alternative code
public boolean ifCommand() { if (keyword("if") && condition() && block()) { makeTree(3, 2, 1); return true; } return false; }
No room for an error condition in this code

7. Alternative code with one message
public boolean ifCommand() { if (keyword("if")) { if (condition()) && (block()) { makeTree(3, 2, 1); return true; } error("Error in \"if\" statement"); } return false; }

8. Parser methods
In the BNF, I have one long definition for <command>
<command> ::= <move> <expression> <eol> | "penup" <eol> | "pendown" <eol> | "color" <colorname> <eol>
In my code, I broke that into multiple methods
<command> ::= <move command> | <penup command> | <pendown command> | <color command>

9. My command() method
public boolean command() { if (move()) return true; if (penup()) return true; if (pendown()) return true;

10. My helper methods
I wrote a number of helper methods for the Parser and for the ParserTest classes
One very useful method is bt, in the ParserTest class
This method lets me build parse trees for use in assertEquals tests
Another is assertStackTop, which is just
private void assertStackTop(BinaryTree bt) { assertEquals(bt, parser.stack.peek()); }
Example:
BinaryTree condition = bt("=", "2", "2"); assertStackTop(bt("if", condition, "list"));

11. The End