**재귀 하향 파서**(recursive descent parser)는 상호 순환 절차(procedure)의 집합(또는 비재귀 적인 동등한 구문)으로 이루어진 하향식 구문 분석이다. 한 절차는 문법의 한 생성 규칙을 처리한다. 따라서 프로그램의 구조는 문법을 반영한 모양이 된다.

예제 파서
-----

```
 program = block "." .

 block =
     ["const" ident "=" number {"," ident "=" number} ";"]
     ["var" ident {"," ident} ";"]
     {"procedure" ident ";" block ";"} statement .

 statement =
     ident ":=" expression
     | "call" ident
     | "begin" statement {";" statement } "end"
     | "if" condition "then" statement
     | "while" condition "do" statement .

 condition =
     "odd" expression
     | expression ("="|"#"|"<"|"<="|">"|">=") expression .

 expression = ["+"|"-"] term {("+"|"-") term} .

 term = factor {("\*"|"/") factor} .

 factor =
     ident
     | number
     | "(" expression ")" .

```

### C 구현

```
typedef enum {ident, number, lparen, rparen, times, slash, plus,
 minus, eql, neq, lss, leq, gtr, geq, callsym, beginsym, semicolon,
 endsym, ifsym, whilesym, becomes, thensym, dosym, constsym, comma,
 varsym, procsym, period, oddsym} Symbol;

Symbol sym;
void nextsym(void);
void error(const char msg[]);

int accept(Symbol s) {
 if (sym == s) {
 nextsym();
 return 1;
 }
 return 0;
}

int expect(Symbol s) {
 if (accept(s))
 return 1;
 error("expect: unexpected symbol");
 return 0;
}

void factor(void) {
 if (accept(ident)) {
 ;
 } else if (accept(number)) {
 ;
 } else if (accept(lparen)) {
 expression();
 expect(rparen);
 } else {
 error("factor: syntax error");
 nextsym();
 }
}

void term(void) {
 factor();
 while (sym == times || sym == slash) {
 nextsym();
 factor();
 }
}

void expression(void) {
 if (sym == plus || sym == minus)
 nextsym();
 term();
 while (sym == plus || sym == minus) {
 nextsym();
 term();
 }
}

void condition(void) {
 if (accept(oddsym)) {
 expression();
 } else {
 expression();
 if (sym == eql || sym == neq || sym == lss || sym == leq || sym == gtr || sym == geq) {
 nextsym();
 expression();
 } else {
 error("condition: invalid operator");
 nextsym();
 }
 }
}

void statement(void) {
 if (accept(ident)) {
 expect(becomes);
 expression();
 } else if (accept(callsym)) {
 expect(ident);
 } else if (accept(beginsym)) {
 do {
 statement();
 } while (accept(semicolon));
 expect(endsym);
 } else if (accept(ifsym)) {
 condition();
 expect(thensym);
 statement();
 } else if (accept(whilesym)) {
 condition();
 expect(dosym);
 statement();
 } else {
 error("statement: syntax error");
 nextsym();
 }
}

void block(void) {
 if (accept(constsym)) {
 do {
 expect(ident);
 expect(eql);
 expect(number);
 } while (accept(comma));
 expect(semicolon);
 }
 if (accept(varsym)) {
 do {
 expect(ident);
 } while (accept(comma));
 expect(semicolon);
 }
 while (accept(procsym)) {
 expect(ident);
 expect(semicolon);
 block();
 expect(semicolon);
 }
 statement();
}

void program(void) {
 nextsym();
 block();
 expect(period);
}

```

### 파스칼 구현

```
// Modern Pascal implementation (www.ModernPascal.com)
// C example ported to Pascal by Ozz Nixon

{$S-}

type
   SYMBOL = (ident, number, lparen, rparen, times, slash, plus,
    minus, eql, neq, lss, leq, gtr, geq, callsym, beginsym, semicolon,
    endsym, ifsym, whilesym, becomes, thensym, dosym, constsym, comma,
    varsym, procsym, period, oddsym);

var
   sym:SYMBOL;

procedure expression; forward;

procedure nextsym;
begin
// you implement //
end;

procedure error(const msg:String);
begin
// you implement //
end;

function accept(S:Symbol):boolean;
begin
   if sym=s then begin
      nextsym();
      Result:=True;
   end
   else Result:=False;
end;

function expect(S:Symbol):boolean;
begin
   Result:=accept(s);
   if not result then error('Expect: unexpected symbol');
end;

procedure factor;
begin
   if (accept(ident)) then begin
//
   end
   else if (accept(number)) then begin
//
   end
   else if (accept(lparen)) then begin
      expression();
      expect(rparen);
   end
   else begin
      error('factor: syntax error');
      nextsym();
   end;
end;

procedure term;
begin
   factor();
   while (sym=times) or (sym=slash) do begin
      nextsym();
      factor();
   end;
end;

procedure expression;
begin
   if (sym=plus) or (sym=minus) then nextsym();
   term();
   while (sym=plus) or (sym=minus) do begin
      nextsym();
      term();
   end;
end;

procedure condition;
begin
   if (accept(oddsym)) then begin
      expression();
   end
   else begin
      expression();
      if (sym=eql) or (sym=neq) or (sym=lss) or (sym=leq) or (sym=gtr) or (sym=geq) then begin
         nextsym();
         expression();
      end
      else begin
         error('condition: invalid operator');
         nextsym();
      end;
   end;
end;

procedure statement;
begin
   if (accept(ident)) then begin
      expect(becomes);
      expression();
   end
   else if (accept(callsym)) then begin
      expect(ident);
   end
   else if (accept(beginsym)) then begin
      repeat
         statement();
      until not (accept(semicolon));
      expect(endsym);
   end
   else if (accept(ifsym)) then begin
      condition();
      expect(thensym);
      statement();
   end
   else if (accept(whilesym)) then begin
      condition();
      expect(dosym);
      statement();
   end
   else begin
      error('statement: syntax error');
      nextsym();
   end;
end;

procedure block;
begin
   if (accept(constsym)) then begin
      repeat
         expect(ident);
         expect(eql);
         expect(number);
      until not (accept(comma));
      expect(semicolon);
   end;
   if (accept(varsym)) then begin
      repeat
         expect(ident);
      until not (accept(comma));
      expect(semicolon);
   end;
   while (accept(procsym)) do begin
      expect(ident);
      expect(semicolon);
      block();
      expect(semicolon);
   end;
   statement();
end;

begin
   nextsym();
   block();
   expect(period);
end;

```