Recursive descent parsing

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Recursive descent parsing
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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

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Example if statement

• ltif statementgt if ltconditiongt ltblockgt
• 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 ltconditiongt, which parses a
  command and puts it on the stack
• Stack now contains if ltconditiongt (top is
  on right)
• Calls the parser for ltblockgt, which parses a
  block and puts it on the stack
• Stack now contains if ltconditiongt ltblockgt
• Pops the top three things from the stack,
  assembles them into a tree, and pushes this tree
  onto the stack

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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

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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

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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

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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

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Parser methods

• In the BNF, I have one long definition for
  ltcommandgt
• ltcommandgt ltmovegt ltexpressiongt lteolgt
  "penup" lteolgt "pendown" lteolgt
  "color" ltcolornamegt lteolgt
  ...
• In my code, I broke that into multiple methods
• ltcommandgt ltmove commandgt ltpenup
  commandgt ltpendown commandgt
  ltcolor commandgt ...

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My command() method

• public boolean command() if (move())
  return true if (penup()) return true
  if (pendown()) return true ...

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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"))

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The End