prefix parser.cs

// field _LookAhead

Node Operand()
{
  Node ret;
  if(matches(TokenId.Identifier))
  {
    ret = new Node() 
    {
      Value = _LookAhead.TokenText, 
      // no Children
    };
    match(TokenId.Identifier);
  }
  else
    ret = Expr();
  return ret;
}

Node Expr()
{
  Node ret;
  if(matches(TokenId.Plus, TokenId.Minus, TokenId.Star, TokenId.Slash))
  {
    ret = new Node()
    {
       Value = _LookAhead.TokenText,
    };
    match(TokenId.Plus, TokenId.Minus, ...);
    ret.Children.Add(Operand());
    ret.Children.Add(Operand());
  }
  else if(matches(TokenId.Sqrt))
  {
    ret = new Node()
    {
       Value = _LookAhead.TokenText,
    };
    match(TokenId.Sqrt);
    ret.Children.Add(Operand());
  }
  else
    match(TokenId.Plus, TokenId.Minus, ..., TokenId.Sqrt); // just to get a good error message!
  // NOTE! we won't get here if match we didn't find one of +,-,*,/,r - match will throw.
  return ret;
}

// "matches" should verify that the lookahead is one of the list provided
//    as its argument.  If it is, it should return true.  otherwise false.
// "match" should verify that the lookahead is one of the list provided
//    as its argument.  It should consume the token if it matches, and set
//    the lookahead to the next token from the string.  If it doesn't match
//    it should throw and give a message that the parse failed while expecting
//    one of the tokens passed as the argument (in a normal parser).