diff --git a/src/Makefile b/src/Makefile
index 512ae58c..e13e0aa4 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -24,7 +24,7 @@ YACC_TARGET := parser.tab.o
 LEX := flex
 LEX_OPTION :=
 YACC := bison
-YACC_OPTION := -d
+YACC_OPTION := --debug -d
 TARGET := kmc90
 OBJS += $(AST_OBJS)
 OBJS += $(CODEGEN_OBJS)
@@ -45,3 +45,5 @@ $(LEX_INTERMEDIATE): $(LEX_SRC) $(YACC_INTERMEDIATE)
 $(LEX_TARGET): CFLAGS += -Wno-unused-function
 
 .PHONY: build
+
+main.c: $(YACC_INTERMEDIATE)
diff --git a/src/lexer.l b/src/lexer.l
index 5813707d..dbd8a5e1 100644
--- a/src/lexer.l
+++ b/src/lexer.l
@@ -156,5 +156,5 @@ void set_yyin_string(const char *code) {
 }
 
 void set_yyin_file(FILE* fp) {
-  yyin = fp;
+  yyset_in(fp);
 }
diff --git a/src/main.c b/src/main.c
index d5862c48..f94434fa 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1,30 +1,20 @@
 #include <stdio.h>
-#include <llvm-c/Analysis.h>
-#include <llvm-c/BitWriter.h>
-#include <llvm-c/Core.h>
+#include "parser.tab.h"
+#include "sexpr_pool.h"
 #include "utility.h"
 
+extern int yydebug;
+
 int main(int argc, char *argv[]) {
-  LLVMModuleRef module = LLVMModuleCreateWithName("kmc90_module");
-  /* int main() { return 0; } */
-  LLVMTypeRef main_type = LLVMFunctionType(LLVMInt32Type(), NULL, 0, false);
-  LLVMValueRef main_func = LLVMAddFunction(module, "main", main_type);
-  LLVMBasicBlockRef main_entry = LLVMAppendBasicBlock(main_func, "main_entry");
-  LLVMValueRef return_value = LLVMConstInt(LLVMInt32Type(), 0, false);
-  LLVMBuilderRef builder = LLVMCreateBuilder();
-  LLVMPositionBuilderAtEnd(builder, main_entry);
-  LLVMBuildRet(builder, return_value);
-  {
-    char *error = NULL;
-    LLVMVerifyModule(module, LLVMAbortProcessAction, &error);
-    LLVMDisposeMessage(error);
-  }
-  if (LLVMWriteBitcodeToFile(module, "main.bc") != 0) {
-    fputs("failed to write bitcode to file\n", stderr);
+  yydebug = 1;
+  if (1 < argc) {
+    FILE* fp = fopen(argv[1], "r");
+    if (fp) {
+      sexpr_initialize_pool();
+      parse(fp);
+      sexpr_finalize_pool();
+      fclose(fp);
+    }
   }
-  LLVMDisposeBuilder(builder);
-  LLVMDisposeModule(module);
   return 0;
-  UNUSED(argc);
-  UNUSED(argv);
 }
diff --git a/src/parser.y b/src/parser.y
index 52aae5b1..7ffcaf0e 100644
--- a/src/parser.y
+++ b/src/parser.y
@@ -7,6 +7,7 @@
     YYERROR; \
   } while (false)
 
+static YYSTYPE g_ast_root;
 void yyerror(const char *);
 }
 
@@ -14,6 +15,7 @@ void yyerror(const char *);
 int yylex(void);
 void set_yyin_file(FILE* fp);
 void set_yyin_string(const char *code);
+YYSTYPE parse(FILE* fp);
 }
 
 %code requires {
@@ -83,7 +85,7 @@ void set_yyin_string(const char *code);
 %token VOLATILE "volatile"
 %token WHILE "while"
 
-%start translation-unit
+%start entry-point
 
 %%
 
@@ -281,8 +283,31 @@ function-definition
 : declaration-specifiers function-definition-declarator compound-statement
 ;
 
+entry-point
+: translation-unit {
+  g_ast_root = $[translation-unit];
+}
+
 %%
 
 void yyerror(const char* s) {
   fprintf(stderr, "%s\n", s);
 }
+
+YYSTYPE parse(FILE* fp) {
+  set_yyin_file(fp);
+  switch (yyparse()) {
+    case 0:
+      return g_ast_root;
+    case 1:
+      yyerror("invalid input in yyparse");
+      break;
+    case 2:
+      yyerror("memory exhaustion in yyparse");
+      break;
+    default:
+      yyerror("unknown error in yyparse");
+      break;
+  }
+  return NULL;
+}