Makefile

all: y.tab.c lex.yy.c
	cc -DYYERROR_VERBOSE -DYYDEBUG -o Parse y.tab.c lex.yy.c

y.tab.c:
	yacc -dv parser.y
lex.yy.c: y.tab.c
	lex parser.l

parser.l

%{
#include <stdio.h>
#include <stdlib.h>
#include "y.tab.h"

int yywrap(void) { return 1; }
%}
digit [0-9]
ops [-+*/=]
symbol [a-z]

%%
{ops} { 
    switch (yytext[0]) {
    case '+':
        return PLUS;
    case '-':
        return MINUS;
    case '*':
        return MUL;
    case '/':
        return DIV;
    case '=':
        return EQ;
    }
}

{digit}+ { 
    yylval.int_value = atoi(yytext);
    return NUM;
}

"print" {
    return PRINT;
}

\; { return EOL; }

\( { return PAREN_L; }
\) { return PAREN_R; }

{symbol}+ {
    char* varname = malloc(yyleng);
    strlcpy(varname, yytext, yyleng+1);
    yylval.var.name = varname;
    return VAR;
}
%%

parser.y

%{
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define YYDEBUG 1

void addvar(char* name, int val);
int search(char* name);
int yylex(void);
int yyerror(char const *str);
void hoge();
struct _unit {};
struct _unit* unit;
%}
%union {
    int   int_value;
    struct _unit* unit; 
    struct {
        char* name;
        int value;
    } var;
}
%token <int_value> NUM
%token <var> VAR
%right EQ
%left PLUS MINUS
%left MUL DIV
%token PRINT EOL PAREN_L PAREN_R

%type <int_value> expression number
%type <unit> assignment print_statement program
%type <var> variable
%%
program : statement EOL { $$ = unit; }
        | program statement EOL { $$ = unit; }

statement: print_statement | assignment

assignment: variable EQ expression { 
                addvar($1.name, $3);
                $$ = unit; }
                ;
print_statement : PRINT expression { printf("%d", $2); $$ = unit; }
                ;
expression :    expression PLUS expression { $$ = $1 + $3; }
        |       expression MINUS expression { $$ = $1 - $3; }
        |       expression MUL expression { $$ = $1 * $3; }
        |       expression DIV expression { $$ = $1 / $3; }
        |       variable { $$ = search($1.name); } 
        |       number
        |       PAREN_L expression PAREN_R  { $$ = $2; }
                ;
variable: VAR { $$ = $1; }
                ;
number: NUM
                ;
%%
typedef struct _varlist {
    char* name;
    int value;
    struct _varlist* next;
} varlist;

varlist* head;
varlist* last;

int search(char* name) { // 環境から名前を探す
    varlist* tmp = head->next;
    while (tmp != NULL) {
        if (strcmp(name, tmp->name) == 0) { return tmp->value; }
        else { tmp = tmp->next; }
    }

    printf("undefined variable: %s", name);
    exit(1);
}

void addvar(char* name, int val) { //環境に変数を追加する
    if (head == NULL) {
        head = malloc(sizeof(varlist));
        last = head; 
    }
    varlist* tmp = malloc(sizeof(varlist));
    tmp->name = name;
    tmp->value = val;
    last->next = tmp;
    last = tmp;
}

int yyerror(char const *str)
{
    extern char *yytext;
    printf("err, %s\n", str);
    return 0;
}

int main(void)
{
    yyparse();
}