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Answer 1

编辑：我把旧答案放在最后

以下是承诺的更详细的示例：

通常我会从所需语言的示例文件开始：

# example.toy
begin # example of the simple toy language
    x = 23;
    while x > 0 do begin
        x = x - 1;
        print(x*x);
    end;
end;

下一步是在前一个文件中创建一个词法分析器+解析器组合通行证。

这是词法分析器（使用flex -o lexer.c lexer.l生成源代码）。另请注意，词法分析器源取决于解析器源（因为TOKEN_ *常量），因此必须在编译词法分析器源之前运行bison：

%option noyywrap

%{
#include "parser.h"
#include <stdlib.h>
%}

%%

"while" return TOKEN_WHILE;
"begin" return TOKEN_BEGIN;
"end"   return TOKEN_END;
"do"    return TOKEN_DO;
[a-zA-Z_][a-zA-Z0-9_]* {yylval.name = strdup(yytext); return TOKEN_ID;}
[-]?[0-9]+    {yylval.val = atoi(yytext); return TOKEN_NUMBER;}
[()=;]  {return *yytext;}
[*/+-<>] {yylval.op = *yytext; return TOKEN_OPERATOR;}
[ \t\n] {/* suppress the output of the whitespaces from the input file to stdout */}
#.* {/* one-line comment */}

和解析器（用bison -d -o parser.c parser.y编译，-d告诉bison用lexer需要的东西创建parser.h头文件。

%error-verbose /* instruct bison to generate verbose error messages*/
%{
/* enable debugging of the parser: when yydebug is set to 1 before the
 * yyparse call the parser prints a lot of messages about what it does */
#define YYDEBUG 1
%}

%union {
    int val;
    char op;
    char* name;
}

%token TOKEN_BEGIN TOKEN_END TOKEN_WHILE TOKEN_DO TOKEN_ID TOKEN_NUMBER TOKEN_OPERATOR
%start program

%{
/* Forward declarations */
void yyerror(const char* const message);


%}

%%

program: statement';';

block: TOKEN_BEGIN statements TOKEN_END;

statements:
    | statements statement ';'
    | statements block';';

statement: 
      assignment
    | whileStmt
    | block
    | call;

assignment: TOKEN_ID '=' expression;

expression: TOKEN_ID
    | TOKEN_NUMBER
    | expression TOKEN_OPERATOR expression;

whileStmt: TOKEN_WHILE expression TOKEN_DO statement;

call: TOKEN_ID '(' expression ')';

%%

#include <stdlib.h>

void yyerror(const char* const message)
{
    fprintf(stderr, "Parse error:%s\n", message);
    exit(1);
}

int main()
{
    yydebug = 0;
    yyparse();
}

gcc parser.c lexer.c -o toylang-noop之后，toylang-noop < example.toy的调用必须运行且没有任何错误。所以现在解析器本身工作并且可以解析示例脚本。

下一步是创建一个所谓的语法抽象语法树。在这一点上，我首先通过为标记和规则定义不同的类型来扩充解析器，以及为每个解析步骤插入规则。

%error-verbose /* instruct bison to generate verbose error messages*/
%{
#include "astgen.h"
#define YYDEBUG 1

/* Since the parser must return the AST, it must get a parameter where
 * the AST can be stored. The type of the parameter will be void*. */
#define YYPARSE_PARAM astDest
%}

%union {
    int val;
    char op;
    char* name;
    struct AstElement* ast; /* this is the new member to store AST elements */
}

%token TOKEN_BEGIN TOKEN_END TOKEN_WHILE TOKEN_DO
%token<name> TOKEN_ID
%token<val> TOKEN_NUMBER
%token<op> TOKEN_OPERATOR
%type<ast> program block statements statement assignment expression whileStmt call
%start program

%{
/* Forward declarations */
void yyerror(const char* const message);


%}

%%

program: statement';' { (*(struct AstElement**)astDest) = $1; };

block: TOKEN_BEGIN statements TOKEN_END{ $$ = $2; };

statements: {$$=0;}
    | statements statement ';' {$$=makeStatement($1, $2);}
    | statements block';' {$$=makeStatement($1, $2);};

statement: 
      assignment {$$=$1;}
    | whileStmt {$$=$1;}
    | block {$$=$1;}
    | call {$$=$1;}

assignment: TOKEN_ID '=' expression {$$=makeAssignment($1, $3);}

expression: TOKEN_ID {$$=makeExpByName($1);}
    | TOKEN_NUMBER {$$=makeExpByNum($1);}
    | expression TOKEN_OPERATOR expression {$$=makeExp($1, $3, $2);}

whileStmt: TOKEN_WHILE expression TOKEN_DO statement{$$=makeWhile($2, $4);};

call: TOKEN_ID '(' expression ')' {$$=makeCall($1, $3);};

%%

#include "astexec.h"
#include <stdlib.h>

void yyerror(const char* const message)
{
    fprintf(stderr, "Parse error:%s\n", message);
    exit(1);
}

int main()
{
    yydebug = 0;
    struct AstElement *a;
    yyparse(&a);
}

如您所见，生成AST的主要部分是创建节点当解析器的某个规则通过时的AST。由于野牛保持了一个当前解析过程本身的堆栈，只需要分配当前解析堆栈元素的状态（这些是$$=foo(bar)行）

目标是内存中的以下结构：

ekStatements
  .count = 2
  .statements
    ekAssignment
      .name = "x"
      .right
        ekNumber
          .val = 23
    ekWhile
      .cond
        ekBinExpression
        .left
          ekId
            .name = "x"
        .right
          ekNumber
            .val=0
        .op = '>'
      .statements
        ekAssignment
          .name = "x"
          .right
            ekBinExpression
              .left
                ekId
                  .name = "x"
              .right
                ekNumber
                  .val = 1
              .op = '-'
        ekCall
          .name = "print"
          .param
            ekBinExpression
              .left
                ekId
                  .name = "x"
              .right
                ekId
                  .name = "x"
              .op = '*'

要获得此图表，需要生成代码，astgen.h：

#ifndef ASTGEN_H
#define ASTGEN_H

struct AstElement
{
    enum {ekId, ekNumber, ekBinExpression, ekAssignment, ekWhile, ekCall, ekStatements, ekLastElement} kind;
    union
    {
        int val;
        char* name;
        struct
        {
            struct AstElement *left, *right;
            char op;
        }expression;
        struct
        {
            char*name;
            struct AstElement* right;
        }assignment;
        struct
        {
            int count;
            struct AstElement** statements;
        }statements;
        struct
        {
            struct AstElement* cond;
            struct AstElement* statements;
        } whileStmt;
        struct
        {
            char* name;
            struct AstElement* param;
        }call;
    } data;
};

struct AstElement* makeAssignment(char*name, struct AstElement* val);
struct AstElement* makeExpByNum(int val);
struct AstElement* makeExpByName(char*name);
struct AstElement* makeExp(struct AstElement* left, struct AstElement* right, char op);
struct AstElement* makeStatement(struct AstElement* dest, struct AstElement* toAppend);
struct AstElement* makeWhile(struct AstElement* cond, struct AstElement* exec);
struct AstElement* makeCall(char* name, struct AstElement* param);
#endif

astgen.c：

#include "astgen.h"
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

static void* checkAlloc(size_t sz)
{
    void* result = calloc(sz, 1);
    if(!result)
    {
        fprintf(stderr, "alloc failed\n");
        exit(1);
    }
}

struct AstElement* makeAssignment( char*name, struct AstElement* val)
{
    struct AstElement* result = checkAlloc(sizeof(*result));
    result->kind = ekAssignment;
    result->data.assignment.name = name;
    result->data.assignment.right = val;
    return result;
}

struct AstElement* makeExpByNum(int val)
{
    struct AstElement* result = checkAlloc(sizeof(*result));
    result->kind = ekNumber;
    result->data.val = val;
    return result;
}

struct AstElement* makeExpByName(char*name)
{
    struct AstElement* result = checkAlloc(sizeof(*result));
    result->kind = ekId;
    result->data.name = name;
    return result;
}

struct AstElement* makeExp(struct AstElement* left, struct AstElement* right, char op)
{
    struct AstElement* result = checkAlloc(sizeof(*result));
    result->kind = ekBinExpression;
    result->data.expression.left = left;
    result->data.expression.right = right;
    result->data.expression.op = op;
    return result;
}

struct AstElement* makeStatement(struct AstElement* result, struct AstElement* toAppend)
{
    if(!result)
    {
        result = checkAlloc(sizeof(*result));
        result->kind = ekStatements;
        result->data.statements.count = 0;
        result->data.statements.statements = 0;
    }
    assert(ekStatements == result->kind);
    result->data.statements.count++;
    result->data.statements.statements = realloc(result->data.statements.statements, result->data.statements.count*sizeof(*result->data.statements.statements));
    result->data.statements.statements[result->data.statements.count-1] = toAppend;
    return result;
}

struct AstElement* makeWhile(struct AstElement* cond, struct AstElement* exec)
{
    struct AstElement* result = checkAlloc(sizeof(*result));
    result->kind = ekWhile;
    result->data.whileStmt.cond = cond;
    result->data.whileStmt.statements = exec;
    return result;
}

struct AstElement* makeCall(char* name, struct AstElement* param)
{
    struct AstElement* result = checkAlloc(sizeof(*result));
    result->kind = ekCall;
    result->data.call.name = name;
    result->data.call.param = param;
    return result;
}

你可以在这里看到AST元素的生成是一个相当单调的工作。完成该步骤后，程序仍然无效，但AST可以在调试器中查看。

下一步是编写解释器。这是astexec.h：

#ifndef ASTEXEC_H
#define ASTEXEC_H

struct AstElement;
struct ExecEnviron;

/* creates the execution engine */
struct ExecEnviron* createEnv();

/* removes the ExecEnviron */
void freeEnv(struct ExecEnviron* e);

/* executes an AST */
void execAst(struct ExecEnviron* e, struct AstElement* a);

#endif

嗯，这看起来很友好。解释器本身很简单，尽管它是长度。大多数函数仅处理特定类型的AstElement。该 dispatchExpression和dispatchStatement选择了正确的函数功能。 dispatch函数在valExecs中查找目标函数和runExecs数组。

astexec.c：

#include "astexec.h"
#include "astgen.h"
#include <stdlib.h>
#include <assert.h>
#include <stdio.h>

struct ExecEnviron
{
    int x; /* The value of the x variable, a real language would have some name->value lookup table instead */
};

static int execTermExpression(struct ExecEnviron* e, struct AstElement* a);
static int execBinExp(struct ExecEnviron* e, struct AstElement* a);
static void execAssign(struct ExecEnviron* e, struct AstElement* a);
static void execWhile(struct ExecEnviron* e, struct AstElement* a);
static void execCall(struct ExecEnviron* e, struct AstElement* a);
static void execStmt(struct ExecEnviron* e, struct AstElement* a);

/* Lookup Array for AST elements which yields values */
static int(*valExecs[])(struct ExecEnviron* e, struct AstElement* a) =
{
    execTermExpression,
    execTermExpression,
    execBinExp,
    NULL,
    NULL,
    NULL,
    NULL
};

/* lookup array for non-value AST elements */
static void(*runExecs[])(struct ExecEnviron* e, struct AstElement* a) =
{
    NULL, /* ID and numbers are canonical and */
    NULL, /* don't need to be executed */
    NULL, /* a binary expression is not executed */
    execAssign,
    execWhile,
    execCall,
    execStmt,
};

/* Dispatches any value expression */
static int dispatchExpression(struct ExecEnviron* e, struct AstElement* a)
{
    assert(a);
    assert(valExecs[a->kind]);
    return valExecs[a->kind](e, a);
}

static void dispatchStatement(struct ExecEnviron* e, struct AstElement* a)
{
    assert(a);
    assert(runExecs[a->kind]);
    runExecs[a->kind](e, a);
}

static void onlyName(const char* name, const char* reference, const char* kind)
{
    if(strcmp(reference, name))
    {
        fprintf(stderr,
            "This language knows only the %s '%s', not '%s'\n",
            kind, reference, name);
        exit(1);
    }
}

static void onlyX(const char* name)
{
    onlyName(name, "x", "variable");
}

static void onlyPrint(const char* name)
{
    onlyName(name, "print", "function");
}

static int execTermExpression(struct ExecEnviron* e, struct AstElement* a)
{
    /* This function looks ugly because it handles two different kinds of
     * AstElement. I would refactor it to an execNameExp and execVal
     * function to get rid of this two if statements. */
    assert(a);
    if(ekNumber == a->kind)
    {
        return a->data.val;
    }
    else
    {
        if(ekId == a->kind)
        {
            onlyX(a->data.name);
            assert(e);
            return e->x;
        }
    }
    fprintf(stderr, "OOPS: tried to get the value of a non-expression(%d)\n", a->kind);
    exit(1);
}

static int execBinExp(struct ExecEnviron* e, struct AstElement* a)
{
    assert(ekBinExpression == a->kind);
    const int left = dispatchExpression(e, a->data.expression.left);
    const int right = dispatchExpression(e, a->data.expression.right);
    switch(a->data.expression.op)
    {
        case '+':
            return left + right;
        case '-':
            return left - right;
        case '*':
            return left * right;
        case '<':
            return left < right;
        case '>':
            return left > right;
        default:
            fprintf(stderr,  "OOPS: Unknown operator:%c\n", a->data.expression.op);
            exit(1);
    }
    /* no return here, since every switch case returns some value (or bails out) */
}

static void execAssign(struct ExecEnviron* e, struct AstElement* a)
{
    assert(a);
    assert(ekAssignment == a->kind);
    onlyX(a->data.assignment.name);
    assert(e);
    struct AstElement* r = a->data.assignment.right;
    e->x = dispatchExpression(e, r);
}

static void execWhile(struct ExecEnviron* e, struct AstElement* a)
{
    assert(a);
    assert(ekWhile == a->kind);
    struct AstElement* const c = a->data.whileStmt.cond;
    struct AstElement* const s = a->data.whileStmt.statements;
    assert(c);
    assert(s);
    while(dispatchExpression(e, c))
    {
        dispatchStatement(e, s);
    }
}

static void execCall(struct ExecEnviron* e, struct AstElement* a)
{
    assert(a);
    assert(ekCall == a->kind);
    onlyPrint(a->data.call.name);
    printf("%d\n", dispatchExpression(e, a->data.call.param));
}

static void execStmt(struct ExecEnviron* e, struct AstElement* a)
{
    assert(a);
    assert(ekStatements == a->kind);
    int i;
    for(i=0; i<a->data.statements.count; i++)
    {
        dispatchStatement(e, a->data.statements.statements[i]);
    }
}

void execAst(struct ExecEnviron* e, struct AstElement* a)
{
    dispatchStatement(e, a);
}

struct ExecEnviron* createEnv()
{
    assert(ekLastElement == (sizeof(valExecs)/sizeof(*valExecs)));
    assert(ekLastElement == (sizeof(runExecs)/sizeof(*runExecs)));
    return calloc(1, sizeof(struct ExecEnviron));
}

void freeEnv(struct ExecEnviron* e)
{
    free(e);
}

现在解释器已经完成，并且在更新main函数之后可以运行示例：

#include <assert.h>

int main()
{
    yydebug = 0;
    struct AstElement *a = 0;
    yyparse(&a);
    /* Q&D WARNING: in production code this assert must be replaced by
     * real error handling. */
    assert(a);
    struct ExecEnviron* e = createEnv();
    execAst(e, a);
    freeEnv(e);
    /* TODO: destroy the AST */
}

现在这种语言的翻译工作正常。请注意，此解释器中存在一些限制：

它只有一个变量和一个函数
- 且只有一个函数参数
只有值类型int
很难添加goto支持，因为对于每个AST元素，解释器调用解释函数。 Goto可以通过将某些内容隐藏到execStmt函数中来实现，但是要在不同的块或级别之间跳转，必须显着更改执行机制（这是因为不能在解释器中的不同堆栈帧之间跳转）。例如，AST可以转换为字节代码，这个字节代码由vm解释。
我需要查找的其他一些内容：）

您需要为您的语言定义语法。像这样的事情（词法分析器和解析器都不完整）：

/* foo.y */
%token ID IF ELSE OR AND /* First list all terminal symbols of the language */
%%

statements: /* allow empty statements */ | stm | statements ';' stm;

stm: ifStatement
   | NAME
   | NAME expList
   | label;

expList: expression | expList expression;

label: ':' NAME { /* code to store the label */ };

ifStatement: IF expression statements
           | IF expression statements ELSE statements;

expression: ID                          { /* Code to handle the found ID */ }
          | expression AND expression   { /* Code to con cat two expression with and */ }
          | expression OR expression
          | '(' expression ')';

然后使用bison -d foo.y -o foo.c编译此文件。 -d开关指示bison生成包含解析器使用的所有标记的标头。现在你创建你的词法分析器

/* bar.l */
%{
#include "foo.h"
%}

%%

IF   return IF;
ELSE return ELSE;
OR   return OR;
AND  return AND;
[A-Z]+  { /*store yylval somewhere to access it in the parser*/ return ID; }

在此之后你完成了词法分析器和解析器，并且“只”需要为你的语言编写语义动作。

我如何在Flex / bison中实现If语句

1 个答案: