设有如下示意性的 Pascal 源程序，它有三部分，之后给出

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以下是示意性的 Pascal 源程序，它分为三个部分：

program example;

var
  x: integer;

function add(a, b: integer): integer;
begin
  add := a + b;
end;

begin
  x := add(1, 2);
  writeln(x);
end.

第一部分是程序头部，包含了程序名和变量声明。第二部分是函数定义部分，定义了一个名为 `add` 的函数，它接受两个整型参数并返回它们的和。第三部分是程序主体部分，它调用了 `add` 函数，并将结果输出到屏幕上。

相关问题

编写如下 pascal 子集的词法分析程序

以下是一个简单的基于Pascal子集的词法分析程序：

program Lexer;

const
  MAXLEN = 100;

type
  TokenType = (ID, NUM, PLUS, MINUS, MULT, DIV, LPAREN, RPAREN, ASSIGN, SEMI, ERROR, EOF);
  // 定义Token类型

var
  TokenString: string;
  Token: TokenType;
  Line: string;
  LineIndex: integer;

function GetChar: char;
begin
  if LineIndex <= Length(Line) then
  begin
    GetChar := Line[LineIndex];
    LineIndex := LineIndex + 1;
  end
  else
    GetChar := #0;
end;

procedure GetToken;
var
  Ch: char;
begin
  TokenString := '';
  Token := ERROR;
  while (Token = ERROR) and (LineIndex <= Length(Line)) do
  begin
    Ch := GetChar;
    if Ch in ['a'..'z', 'A'..'Z'] then
    begin
      Token := ID;
      while Ch in ['a'..'z', 'A'..'Z', '0'..'9'] do
      begin
        TokenString := TokenString + Ch;
        Ch := GetChar;
      end;
      LineIndex := LineIndex - 1;
    end
    else if Ch in ['0'..'9'] then
    begin
      Token := NUM;
      while Ch in ['0'..'9'] do
      begin
        TokenString := TokenString + Ch;
        Ch := GetChar;
      end;
      LineIndex := LineIndex - 1;
    end
    else if Ch = '+' then
      Token := PLUS
    else if Ch = '-' then
      Token := MINUS
    else if Ch = '*' then
      Token := MULT
    else if Ch = '/' then
      Token := DIV
    else if Ch = '(' then
      Token := LPAREN
    else if Ch = ')' then
      Token := RPAREN
    else if Ch = '=' then
      Token := ASSIGN
    else if Ch = ';' then
      Token := SEMI;
  end;
end;

begin
  Line := 'a := 2 + 3;';
  LineIndex := 1;

  repeat
    GetToken;
    writeln(TokenString);
  until Token = EOF;
end.

以上代码实现了一个简单的词法分析程序，可以识别出变量名、数字、加减乘除、括号、赋值符号和分号。你可以根据需要修改 `Line` 的值来测试不同的输入。

给出pascal语言的文法

Pascal语言的文法比较复杂，这里只给出一个大致的概述：

<program> ::= program <program_name> ";" <block> "."
<program_name> ::= <identifier>
<block> ::= <variable_declaration_part> <function_declaration_part> <compound_statement>
<variable_declaration_part> ::= "var" <variable_declaration> {<variable_declaration>;}
<variable_declaration> ::= <identifier_list> ":" <type>
<identifier_list> ::= <identifier> {"," <identifier>}
<type> ::= <simple_type> | <structured_type>
<simple_type> ::= <scalar_type> | <subrange_type> | <type_identifier>
<scalar_type> ::= "integer" | "real" | "boolean" | "char"
<subrange_type> ::= <constant> ".." <constant>
<type_identifier> ::= <identifier>
<structured_type> ::= <array_type> | <record_type>
<array_type> ::= "array" "[" <index_type> {"," <index_type>} "]" "of" <type>
<index_type> ::= <simple_type>
<record_type> ::= "record" {<variable_declaration>;} "end"
<function_declaration_part> ::= {<function_declaration>;}
<function_declaration> ::= <function_heading> ";" <block> ";" | <procedure_heading> ";" <block> ";"
<function_heading> ::= "function" <identifier> [<formal_parameter_list>] ":" <type>
<formal_parameter_list> ::= "(" [<parameter_list>] ")"
<parameter_list> ::= <parameter> {";" <parameter>}
<parameter> ::= <identifier_list> ":" <type_identifier>
<procedure_heading> ::= "procedure" <identifier> [<formal_parameter_list>]
<compound_statement> ::= "begin" {<statement>;} "end"
<statement> ::= <simple_statement> | <structured_statement>
<simple_statement> ::= <assignment_statement> | <procedure_call_statement> | <exit_statement> | <empty_statement> | <io_statement>
<assignment_statement> ::= <variable> ":=" <expression>
<variable> ::= <identifier> | <indexed_variable> | <field_designator>
<indexed_variable> ::= <variable> "[" <expression> {"," <expression>} "]"
<field_designator> ::= <variable> "." <identifier>
<expression> ::= <simple_expression> [<relational_operator> <simple_expression>]
<simple_expression> ::= [<sign>] <term> {<adding_operator> <term>}
<sign> ::= "+" | "-"
<term> ::= <factor> {<multiplying_operator> <factor>}
<factor> ::= <variable> | <constant> | "(" <expression> ")" | <not_factor>
<not_factor> ::= "not" <factor>
<constant> ::= <integer_constant> | <real_constant> | <string_constant> | <char_constant> | <boolean_constant>
<integer_constant> ::= <digit> {<digit>}
<real_constant> ::= <digit> {<digit>} "." {<digit>} [<exponent>]
<exponent> ::= "e" [<sign>] <digit> {<digit>}
<boolean_constant> ::= "true" | "false"
<string_constant> ::= "'" {<character>} "'"
<char_constant> ::= "'" <character> "'"
<character> ::= <letter> | <digit>
<relational_operator> ::= "=" | "<>" | "<" | "<=" | ">" | ">="
<adding_operator> ::= "+" | "-" | "or"
<multiplying_operator> ::= "*" | "/" | "div" | "mod" | "and"
<structured_statement> ::= <compound_statement> | <conditional_statement> | <repetitive_statement>
<conditional_statement> ::= "if" <expression> "then" <statement> ["else" <statement>]
<repetitive_statement> ::= <while_statement> | <repeat_statement> | <for_statement>
<while_statement> ::= "while" <expression> "do" <statement>
<repeat_statement> ::= "repeat" {<statement>;} "until" <expression>
<for_statement> ::= "for" <control_variable> ":=" <for_list> "do" <statement>
<control_variable> ::= <identifier>
<for_list> ::= <initial_value> ["to" <final_value>] | <initial_value> "downto" <final_value>
<initial_value> ::= <expression>
<final_value> ::= <expression>
<procedure_call_statement> ::= <identifier> [<actual_parameter_list>]
<actual_parameter_list> ::= "(" [<expression_list>] ")"
<expression_list> ::= <expression> {"," <expression>}
<exit_statement> ::= "exit" [<expression>]
<empty_statement> ::= ";"
<io_statement> ::= <write_statement> | <writeln_statement>
<write_statement> ::= "write" <output_parameter> {"," <output_parameter>} | "write" "(" <output_parameter> {"," <output_parameter>} ")"
<writeln_statement> ::= "writeln" [<output_parameter> {"," <output_parameter>}]
<output_parameter> ::= <expression> | <string_constant>

这个文法描述了Pascal语言的语法规则。其中，各个符号的含义可以在Pascal语言的语法定义中找到详细的解释。