AST, Lexer и PABC.NET

Type Token = record
TokenType: String;
Value: Real;
constructor Create(T: String; V: Real); begin
Self.TokenType := T;
Self.Value := V;
end;
end;
 
Var
i:=1; 
Input: string;
Tokens: List<Token> = new List<Token>;
  
procedure tokenizeNumber(a: integer); begin
Var number: string;
While Input[i].isDigit or Input[i].Equals('.') do begin number += Input[i]; inc(i); if i >= Input.Length then break; end;
try Var tok : Token = new Token('NUMBER', number.ToReal);
Tokens.add(tok); 
except on System.FormatException do print('Invalid token ' + number);
end;
end;
 
procedure tokenizeOperator(a: integer); begin
Var tok : Token = new Token('PLUS', 0);
case Input[a] of 
'+': tok.TokenType:= 'PLUS';
'-': tok.TokenType:= 'MINUS';
'*': tok.TokenType:= 'STAR';
'/': tok.TokenType:= 'SLASH';
'(': tok.TokenType:= 'OPAREN';
')': tok.TokenType:= 'CPAREN';
end;
Tokens.add(tok);
inc(i);
end;
 
procedure Lexer; begin
  Input += ' ';
  While i <= Input.Length do Case Input[i] of
  '0'..'9': tokenizeNumber(i);
  '+','-','*','/','(',')': tokenizeOperator(i);
  ' ': inc(i);
  end;
end;
 
//------------------------------------------------------------------------------
function Additive: double; forward;
function Multiplicative: double; forward;
function Unary: double; forward;
function Primary: double; forward;
 
function Expression: double; begin
i:=0;
Expression:= Additive;
end;
  
  function Additive: double; begin
  Var res:= Multiplicative;
  While true do begin
  if Tokens[i].TokenType = 'PLUS' then begin inc(i); res:= res + multiplicative end
  else if Tokens[i].TokenType = 'MINUS' then begin inc(i); res:= res - multiplicative end
  else break;
  end;
  Additive:= res;
  end;
  
    function Multiplicative: double; begin
    Var res:= Unary;
    While true do begin
    if Tokens[i].TokenType = 'STAR' then begin inc(i); res:= res * multiplicative end
    else if Tokens[i].TokenType = 'SLASH' then begin inc(i); res:= res / multiplicative end
    else break;
    end;
    Multiplicative:= res;
    end;
    
      function Unary: double; begin
      if Tokens[i].TokenType = 'MINUS' then begin inc(i); Unary:= Primary * -1; exit; end;
      if Tokens[i].TokenType = 'PLUS' then begin inc(i); Unary:= Primary * 1; exit; end;
      Unary:= Primary;
      end;
      
        function Primary: double; begin
        if Tokens[i].TokenType = 'NUMBER' then Primary:= Tokens[i].Value;
        Println('Primary ',Tokens[i].Value);
        if i < Tokens.Count-1 then inc(i);
        end;
//------------------------------------------------------------------------------
        
begin
Readln(Input);
Lexer;
Println;
for i:=0 to Tokens.Count-1 do Println(Tokens[i]);
Writeln(Expression);
end.