diff --git a/main.cpp b/main.cpp
index 911aad0..c071c29 100644
--- a/main.cpp
+++ b/main.cpp
@@ -51,12 +51,6 @@ void logo()
     <<"input <nasal -h> to get help .\n";
 }
 
-void die(const char* stage,const std::string& filename)
-{
-    std::cout<<"["<<stage<<"] in <"<<filename<<">: error(s) occurred,stop.\n";
-    std::exit(1);
-}
-
 void err()
 {
     std::cout
@@ -67,43 +61,37 @@ void err()
 
 void execute(const std::string& file,const uint32_t cmd)
 {
-    nasal_lexer   lexer;
-    nasal_parse   parse;
-    nasal_import  linker;
-    nasal_codegen gen;
+    // front end use the same error module
+    nasal_err     nerr;
+    nasal_lexer   lexer(nerr);
+    nasal_parse   parse(nerr);
+    nasal_import  linker(nerr);
+    nasal_codegen gen(nerr);
+    // back end
     nasal_vm      vm;
 
     // lexer scans file to get tokens
     lexer.scan(file);
-    if(lexer.err())
-        die("lexer",file);
     if(cmd&VM_LEXINFO)
         lexer.print();
     
     // parser gets lexer's token list to compile
     parse.compile(lexer);
-    if(parse.err())
-        die("parse",file);
-
     // linker gets parser's ast and load import files to this ast
     linker.link(parse,file);
-    if(linker.err())
-        die("import",file);
     if(cmd&VM_ASTINFO)
         parse.print();
     
     // code generator gets parser's ast and linker's import file list to generate code
     gen.compile(parse,linker);
-    if(gen.err())
-        die("code",file);
     if(cmd&VM_CODEINFO)
         gen.print();
     
     // run bytecode
     if(cmd&VM_DEBUG)
     {
-        nasal_dbg dbg;
-        dbg.run(gen,linker);
+        nasal_dbg debugger;
+        debugger.run(gen,linker);
     }
     else if(cmd&VM_EXECTIME)
     {
@@ -119,7 +107,7 @@ int main(int argc,const char* argv[])
 {
     if(argc==1)
     {
-        help();
+        logo();
         return 0;
     }
     if(argc==2)
diff --git a/makefile b/makefile
index c65115b..21bb601 100644
--- a/makefile
+++ b/makefile
@@ -1,5 +1,5 @@
 .PHONY=test
-nasal:main.cpp nasal_ast.h nasal_builtin.h nasal_codegen.h nasal_gc.h nasal_import.h nasal_lexer.h nasal_parse.h nasal_vm.h nasal_dbg.h nasal.h
+nasal:main.cpp nasal_ast.h nasal_err.h nasal_builtin.h nasal_codegen.h nasal_gc.h nasal_import.h nasal_lexer.h nasal_parse.h nasal_vm.h nasal_dbg.h nasal.h
 	clang++ -std=c++11 -O3 main.cpp -o nasal -fno-exceptions -ldl -Wshadow -Wall
 test:nasal
 	./nasal test/ascii-art.nas
@@ -11,18 +11,18 @@ test:nasal
 	./nasal test/calc.nas
 	./nasal test/choice.nas
 	./nasal test/class.nas
-	# ./nasal test/exception.nas
+	./nasal -c test/exception.nas
 	./nasal -t test/fib.nas
 	./nasal test/filesystem.nas
 	./nasal test/hexdump.nas
 	./nasal test/json.nas
 	./nasal test/leetcode1319.nas
 	./nasal test/lexer.nas
-	./nasal -c test/life.nas
+	./nasal test/life.nas
 	./nasal -t test/loop.nas
 	./nasal -c test/mandel.nas
 	./nasal -t test/mandelbrot.nas
-	# ./nasal -t test/module_test.nas
+	./nasal -c test/module_test.nas
 	./nasal test/nasal_test.nas
 	./nasal -t test/pi.nas
 	./nasal -t test/prime.nas
diff --git a/nasal.h b/nasal.h
index 402c5a1..5451c88 100644
--- a/nasal.h
+++ b/nasal.h
@@ -127,6 +127,7 @@ std::string rawstr(const std::string& str)
         }
     return ret;
 }
+#include "nasal_err.h"
 #include "nasal_lexer.h"
 #include "nasal_ast.h"
 #include "nasal_parse.h"
diff --git a/nasal_codegen.h b/nasal_codegen.h
index 8a1032b..ab71cf5 100644
--- a/nasal_codegen.h
+++ b/nasal_codegen.h
@@ -193,6 +193,7 @@ class nasal_codegen
 private:
     uint32_t error;
     uint16_t fileindex;
+    nasal_err& nerr;
     const std::string* file;
     std::stack<uint32_t> in_iterloop;
     std::unordered_map<double,int>      num_table;
@@ -205,7 +206,7 @@ private:
     std::vector<std::string>            global; // global : max 4095 values
     std::list<std::vector<std::string>> local;  // local  : max 32768 upvalues 65536 values
     
-    void die(const std::string,const int);
+    void die(const std::string&,const uint32_t);
     void regist_number(const double);
     void regist_string(const std::string&);
     void find_symbol(const nasal_ast&);
@@ -246,6 +247,7 @@ private:
     void block_gen(const nasal_ast&);
     void ret_gen(const nasal_ast&);
 public:
+    nasal_codegen(nasal_err& e):nerr(e){}
     uint32_t                  err(){return error;}
     void                      compile(const nasal_parse&,const nasal_import&);
     void                      print_op(uint32_t);
@@ -255,10 +257,10 @@ public:
     const std::vector<opcode>&      get_code() const {return code;}
 };
 
-void nasal_codegen::die(const std::string info,const int line)
+void nasal_codegen::die(const std::string& info,const uint32_t line)
 {
-    ++error;
-    std::cout<<"[code] <"<<file[fileindex]<<":"<<line<<"> "<<info<<"\n";
+    nerr.load(file[fileindex]);
+    nerr.err("code",line,info.c_str());
 }
 
 void nasal_codegen::regist_number(const double num)
@@ -1217,6 +1219,7 @@ void nasal_codegen::compile(const nasal_parse& parse,const nasal_import& import)
     gen(op_exit,0,0);
     if(global.size()>=STACK_MAX_DEPTH)
         die("too many global variants: "+std::to_string(global.size())+".",0);
+    nerr.chkerr();
 }
 
 void nasal_codegen::print_op(uint32_t index)
diff --git a/nasal_dbg.h b/nasal_dbg.h
index 51e9670..da6abc5 100644
--- a/nasal_dbg.h
+++ b/nasal_dbg.h
@@ -7,22 +7,19 @@ class nasal_dbg:public nasal_vm
 {
 private:
     bool next_step;
-    uint16_t last_fidx;
     uint16_t bk_fidx;
     uint32_t bk_line;
-    std::vector<std::string> src;
+    file_line src;
 
-    std::vector<std::string> parse(std::string&);
-    uint16_t get_fileindex(std::string&);
+    std::vector<std::string> parse(const std::string&);
+    uint16_t get_fileindex(const std::string&);
     void err();
     void help();
-    void load_src(const std::string&);
     void stepinfo();
     void interact();
 public:
     nasal_dbg():
         next_step(false),
-        last_fidx(0),
         bk_fidx(0),bk_line(0){}
     void run(
         const nasal_codegen&,
@@ -30,7 +27,7 @@ public:
     );
 };
 
-std::vector<std::string> nasal_dbg::parse(std::string& cmd)
+std::vector<std::string> nasal_dbg::parse(const std::string& cmd)
 {
     std::vector<std::string> res;
     std::string tmp="";
@@ -49,7 +46,7 @@ std::vector<std::string> nasal_dbg::parse(std::string& cmd)
     return res;
 }
 
-uint16_t nasal_dbg::get_fileindex(std::string& filename)
+uint16_t nasal_dbg::get_fileindex(const std::string& filename)
 {
     for(uint16_t i=0;i<files_size;++i)
         if(filename==files[i])
@@ -81,30 +78,11 @@ void nasal_dbg::help()
     <<"\tbk,   break     | set break point\n";
 }
 
-void nasal_dbg::load_src(const std::string& filename)
-{
-    src.clear();
-    std::ifstream fin(filename,std::ios::binary);
-    if(fin.fail())
-    {
-        std::cout<<"[debug] cannot open source code file <"<<filename<<">\n";
-        std::exit(-1);
-    }
-    std::string tmp;
-    while(!fin.eof())
-    {
-        std::getline(fin,tmp);
-        src.push_back(tmp);
-    }
-}
-
 void nasal_dbg::stepinfo()
 {
     uint32_t begin,end;
     uint32_t line=bytecode[pc].line==0?0:bytecode[pc].line-1;
-    if(bytecode[pc].fidx!=last_fidx)
-        load_src(files[bytecode[pc].fidx]);
-    last_fidx=bytecode[pc].fidx;
+    src.load(files[bytecode[pc].fidx]);
     printf("source code:\n");
     begin=(line>>3)==0?0:((line>>3)<<3);
     end=(1+(line>>3))<<3;
@@ -203,7 +181,6 @@ void nasal_dbg::run(
 {
     detail_info=true;
     init(gen.get_strs(),gen.get_nums(),linker.get_file());
-    load_src(files[0]);
     const void* opr_table[]=
     {
         &&nop,     &&intg,     &&intl,   &&loadg,
diff --git a/nasal_err.h b/nasal_err.h
new file mode 100644
index 0000000..87b5da0
--- /dev/null
+++ b/nasal_err.h
@@ -0,0 +1,70 @@
+#ifndef __NASAL_ERR_H__
+#define __NASAL_ERR_H__
+
+#include <iostream>
+#include <fstream>
+#include <cstring>
+
+class file_line
+{
+protected:
+    std::string file;
+    std::vector<std::string> res;
+public:
+    void load(const std::string& f)
+    {
+        if(file==f)
+            return;
+        file=f;
+        res.clear();
+        std::ifstream fin(f,std::ios::binary);
+        if(fin.fail())
+        {
+            std::cout<<"[src] cannot open file <"<<f<<">\n";
+            std::exit(1);
+        }
+        std::string line;
+        while(!fin.eof())
+        {
+            std::getline(fin,line);
+            res.push_back(line);
+        }
+    }
+    void clear()
+    {
+        std::vector<std::string> tmp;
+        res.swap(tmp);
+    }
+    const std::string& operator[](const uint32_t line){return res[line];}
+    const std::string& name(){return file;}
+    size_t size(){return res.size();}
+};
+
+class nasal_err:public file_line
+{
+private:
+    uint32_t error;
+public:
+    void err(const char* stage,const std::string& info)
+    {
+        ++error;
+        std::cout<<"["<<stage<<"] "<<info<<'\n';
+    }
+    void err(const char* stage,uint32_t line,uint32_t column,const std::string& info)
+    {
+        ++error;
+        std::cout<<"["<<stage<<"] "<<file<<":"<<line<<":"<<column<<" "<<info<<"\n"<<res[line-1]<<'\n';
+        for(int i=0;i<(int)column-1;++i)
+            std::cout<<char(" \t"[res[line-1][i]=='\t']);
+        std::cout<<"^\n";
+    }
+    void err(const char* stage,uint32_t line,const std::string& info)
+    {
+        ++error;
+        std::cout<<"["<<stage<<"] "<<file<<":"<<line<<" "<<info<<"\n"<<res[line-1]<<'\n';
+    }
+    void chkerr(){if(error)std::exit(1);}
+    
+};
+
+#endif
\ No newline at end of file
diff --git a/nasal_import.h b/nasal_import.h
index 69e2cb9..0563b9d 100644
--- a/nasal_import.h
+++ b/nasal_import.h
@@ -4,26 +4,19 @@
 class nasal_import
 {
 private:
-    uint32_t                 error;
+    nasal_err&               nerr;
     std::vector<std::string> files;
-    void      die(const std::string&,const char*);
     bool      check_import(const nasal_ast&);
     bool      check_exist(const std::string&);
     void      linker(nasal_ast&,nasal_ast&&);
     nasal_ast file_import(nasal_ast&);
     nasal_ast load(nasal_ast&,uint16_t);
 public:
-    uint32_t  err(){return error;}
-    void      link(nasal_parse&,const std::string&);
+    nasal_import(nasal_err& e):nerr(e){}
+    void link(nasal_parse&,const std::string&);
     const std::vector<std::string>& get_file() const {return files;}
 };
 
-void nasal_import::die(const std::string& file,const char* stage)
-{
-    ++error;
-    std::cout<<"[import] in <\""<<file<<"\">: error(s) occurred in "<<stage<<".\n";
-}
-
 bool nasal_import::check_import(const nasal_ast& node)
 {
 /*
@@ -65,8 +58,8 @@ void nasal_import::linker(nasal_ast& root,nasal_ast&& add_root)
 
 nasal_ast nasal_import::file_import(nasal_ast& node)
 {
-    nasal_lexer lex;
-    nasal_parse par;
+    nasal_lexer lex(nerr);
+    nasal_parse par(nerr);
     // get filename and set node to ast_null
     std::string filename=node[1][0].str();
     node.clear();
@@ -77,17 +70,7 @@ nasal_ast nasal_import::file_import(nasal_ast& node)
     
     // start importing...
     lex.scan(filename);
-    if(lex.err())
-    {
-        die(filename,"lexer");
-        return {0,ast_root};
-    }
     par.compile(lex);
-    if(par.err())
-    {
-        die(filename,"parser");
-        return {0,ast_root};
-    }
     nasal_ast tmp=std::move(par.ast());
     // check if tmp has 'import'
     return load(tmp,files.size()-1);
@@ -110,7 +93,6 @@ nasal_ast nasal_import::load(nasal_ast& root,uint16_t fileindex)
 void nasal_import::link(nasal_parse& parse,const std::string& self)
 {
     // initializing
-    error=0;
     files={self};
     // scan root and import files,then generate a new ast and return to import_ast
     // the main file's index is 0
diff --git a/nasal_lexer.h b/nasal_lexer.h
index e9a3e1b..8e59c54 100644
--- a/nasal_lexer.h
+++ b/nasal_lexer.h
@@ -90,30 +90,36 @@ struct
 struct token
 {
     uint32_t line;
+    uint32_t column;
     uint32_t type;
     std::string str;
-    token(uint32_t l=0,uint32_t t=tok_null,std::string s=""){line=l;type=t;str=s;}
+    token(uint32_t l=0,uint32_t c=0,uint32_t t=tok_null,std::string s="")
+    {
+        line=l;
+        column=c;
+        type=t;
+        str=s;
+    }
 };
 
 class nasal_lexer
 {
 private:
-    uint32_t    error;
     uint32_t    line;
+    uint32_t    column;
     uint32_t    ptr;
-    std::string filename;
-    std::string code;
+    nasal_err&  nerr;
     std::string res;
     std::vector<token> tokens;
 
     uint32_t get_type(const std::string&);
-    void die(const char*);
+    void die(const char* info){nerr.err("lexer",line,column,info);};
     void open(const std::string&);
     std::string id_gen();
     std::string num_gen();
     std::string str_gen();
 public:
-    uint32_t err(){return error;}
+    nasal_lexer(nasal_err& e):nerr(e){}
     void scan(const std::string&);
     void print();
     const std::vector<token>& get_tokens() const {return tokens;}
@@ -121,13 +127,11 @@ public:
 
 void nasal_lexer::open(const std::string& file)
 {
-    filename=file;
     std::ifstream fin(file,std::ios::binary);
     if(fin.fail())
-    {
-        ++error;
-        std::cout<<"[lexer] cannot open file <"<<file<<">.\n";
-    }
+        nerr.err("lexer","cannot open file <"+file+">.");
+    else
+        nerr.load(file);
     std::stringstream ss;
     ss<<fin.rdbuf();
     res=ss.str();
@@ -141,21 +145,12 @@ uint32_t nasal_lexer::get_type(const std::string& tk_str)
     return tok_null;
 }
 
-void nasal_lexer::die(const char* info)
-{
-    ++error;
-    std::cout<<"[lexer] "<<filename<<":"<<line<<":"<<code.length()<<"\n"<<code<<'\n';
-    for(int i=0;i<(int)code.size()-1;++i)
-        std::cout<<char(" \t"[code[i]=='\t']);
-    std::cout<<'^'<<info<<'\n';
-}
-
 std::string nasal_lexer::id_gen()
 {
     std::string str="";
     while(ptr<res.size() && (ID(res[ptr])||DIGIT(res[ptr])))
         str+=res[ptr++];
-    code+=str;
+    column+=str.length();
     return str;
 }
 
@@ -168,7 +163,7 @@ std::string nasal_lexer::num_gen()
         ptr+=2;
         while(ptr<res.size() && HEX(res[ptr]))
             str+=res[ptr++];
-        code+=str;
+        column+=str.length();
         if(str.length()<3)// "0x"
             die("invalid number.");
         return str;
@@ -180,7 +175,7 @@ std::string nasal_lexer::num_gen()
         ptr+=2;
         while(ptr<res.size() && OCT(res[ptr]))
             str+=res[ptr++];
-        code+=str;
+        column+=str.length();
         if(str.length()<3)// "0o"
             die("invalid number.");
         return str;
@@ -198,7 +193,7 @@ std::string nasal_lexer::num_gen()
         // "xxxx." is not a correct number
         if(str.back()=='.')
         {
-            code+=str;
+            column+=str.length();
             die("invalid number.");
             return "0";
         }
@@ -213,12 +208,12 @@ std::string nasal_lexer::num_gen()
         // "xxxe(-|+)" is not a correct number
         if(str.back()=='e' || str.back()=='E' || str.back()=='-' || str.back()=='+')
         {
-            code+=str;
+            column+=str.length();
             die("invalid number.");
             return "0";
         }
     }
-    code+=str;
+    column+=str.length();
     return str;
 }
 
@@ -226,18 +221,19 @@ std::string nasal_lexer::str_gen()
 {
     std::string str="";
     char begin=res[ptr];
-    code+=begin;
+    ++column;
     while(++ptr<res.size() && res[ptr]!=begin)
     {
-        code+=res[ptr];
+        ++column;
         if(res[ptr]=='\n')
         {
-            code="";
+            column=0;
             ++line;
         }
         if(res[ptr]=='\\' && ptr+1<res.size())
         {
-            code+=res[++ptr];
+            ++column;
+            ++ptr;
             switch(res[ptr])
             {
                 case '0': str+='\0';    break;
@@ -264,7 +260,7 @@ std::string nasal_lexer::str_gen()
         die("get EOF when generating string.");
         return str;
     }
-    code+=res[ptr-1];
+    ++column;
     if(begin=='`' && str.length()!=1)
         die("\'`\' is used for string that includes one character.");
     return str;
@@ -273,7 +269,8 @@ std::string nasal_lexer::str_gen()
 void nasal_lexer::scan(const std::string& file)
 {
     line=1;
-    error=ptr=0;
+    column=0;
+    ptr=0;
     open(file);
 
     std::string str;
@@ -282,11 +279,11 @@ void nasal_lexer::scan(const std::string& file)
         while(ptr<res.size() && (res[ptr]==' ' || res[ptr]=='\n' || res[ptr]=='\t' || res[ptr]=='\r' || res[ptr]<0))
         {
             // these characters will be ignored, and '\n' will cause ++line
-            code+=res[ptr];
+            ++column;
             if(res[ptr++]=='\n')
             {
                 ++line;
-                code="";
+                column=0;
             }
         }
         if(ptr>=res.size()) break;
@@ -294,20 +291,20 @@ void nasal_lexer::scan(const std::string& file)
         {
             str=id_gen();
             uint32_t type=get_type(str);
-            tokens.push_back({line,type?type:tok_id,str});
+            tokens.push_back({line,column,type?type:tok_id,str});
         }
         else if(DIGIT(res[ptr]))
-            tokens.push_back({line,tok_num,num_gen()});
+            tokens.push_back({line,column,tok_num,num_gen()});
         else if(STR(res[ptr]))
-            tokens.push_back({line,tok_str,str_gen()});
+            tokens.push_back({line,column,tok_str,str_gen()});
         else if(SINGLE_OPERATOR(res[ptr]))
         {
             str=res[ptr];
-            code+=res[ptr];
+            ++column;
             uint32_t type=get_type(str);
             if(!type)
                 die("invalid operator.");
-            tokens.push_back({line,type,str});
+            tokens.push_back({line,column,type,str});
             ++ptr;
         }
         else if(res[ptr]=='.')
@@ -316,8 +313,8 @@ void nasal_lexer::scan(const std::string& file)
             if(ptr+2<res.size() && res[ptr+1]=='.' && res[ptr+2]=='.')
                 str+="..";
             ptr+=str.length();
-            code+=str;
-            tokens.push_back({line,get_type(str),str});
+            column+=str.length();
+            tokens.push_back({line,column,get_type(str),str});
         }
         else if(CALC_OPERATOR(res[ptr]))
         {
@@ -325,19 +322,21 @@ void nasal_lexer::scan(const std::string& file)
             str=res[ptr++];
             if(ptr<res.size() && res[ptr]=='=')
                 str+=res[ptr++];
-            code+=str;
-            tokens.push_back({line,get_type(str),str});
+            column+=str.length();
+            tokens.push_back({line,column,get_type(str),str});
         }
         else if(NOTE(res[ptr]))// avoid note, after this process ptr will point to a '\n', so next loop line counter+1
             while(++ptr<res.size() && res[ptr]!='\n');
         else
         {
-            code+=res[ptr++];
+            ++column;
+            ++ptr;
             die("unknown character.");
         }
     }
-    tokens.push_back({line,tok_eof,"eof"});
-    code=res="";
+    tokens.push_back({line,column,tok_eof,"eof"});
+    res="";
+    nerr.chkerr();
 }
 
 void nasal_lexer::print()
diff --git a/nasal_parse.h b/nasal_parse.h
index 859d743..00ddb69 100644
--- a/nasal_parse.h
+++ b/nasal_parse.h
@@ -42,11 +42,11 @@ class nasal_parse
 #define is_call(type) ((type)==tok_lcurve || (type)==tok_lbracket || (type)==tok_dot)
 private:
     uint32_t ptr;
-    uint32_t error;
     uint32_t in_func; // count function block
     uint32_t in_loop; // count loop block
     const token* tokens;// ref from nasal_lexer
     nasal_ast root;
+    nasal_err& nerr;
 
     void die(uint32_t,const std::string&);
     void match(uint32_t type,const char* info=nullptr);
@@ -98,7 +98,7 @@ private:
     nasal_ast break_expr();
     nasal_ast ret_expr();
 public:
-    uint32_t   err(){return error;}
+    nasal_parse(nasal_err& e):nerr(e){}
     void       print(){root.print(0);}
     void       compile(const nasal_lexer&);
     nasal_ast& ast(){return root;}
@@ -107,7 +107,7 @@ public:
 void nasal_parse::compile(const nasal_lexer& lexer)
 {
     tokens=lexer.get_tokens().data();
-    ptr=in_func=in_loop=error=0;
+    ptr=in_func=in_loop=0;
 
     root={1,ast_root};
     while(tokens[ptr].type!=tok_eof)
@@ -119,11 +119,12 @@ void nasal_parse::compile(const nasal_lexer& lexer)
         else if(need_semi_check(root.child().back()) && tokens[ptr].type!=tok_eof)
             die(error_line,"expected \";\"");
     }
+    nerr.chkerr();
 }
 void nasal_parse::die(uint32_t line,const std::string& info)
 {
-    ++error;
-    std::cout<<"[parse] line "<<line<<": "<<info<<".\n";
+    int col=(int)tokens[ptr].column-(int)tokens[ptr].str.length();
+    nerr.err("parse",line,col<0?0:col,info);
 }
 void nasal_parse::match(uint32_t type,const char* info)
 {
diff --git a/test/calc.nas b/test/calc.nas
index 39edc3c..89e506a 100644
--- a/test/calc.nas
+++ b/test/calc.nas
@@ -1,6 +1,6 @@
 import("lib.nas");
-var filename=["main.cpp","nasal_ast.h","nasal_builtin.h","nasal_codegen.h","nasal_gc.h","nasal_import.h","nasal_lexer.h","nasal_parse.h","nasal_vm.h","nasal_dbg.h","nasal.h"];
-var space=["       ","    ","","","     "," ","  ","  ","     ","    ","        "];
+var filename=["main.cpp","nasal_err.h","nasal_ast.h","nasal_builtin.h","nasal_codegen.h","nasal_gc.h","nasal_import.h","nasal_lexer.h","nasal_parse.h","nasal_vm.h","nasal_dbg.h","nasal.h"];
+var space=["       ","    ","    ","","","     "," ","  ","  ","     ","    ","        "];
 var enter_cnt=func(s)
 {
     var (cnt,len,enter)=(0,size(s),'\n'[0]);
diff --git a/test/life.nas b/test/life.nas
index a01740a..ee6fc6a 100644
--- a/test/life.nas
+++ b/test/life.nas
@@ -30,7 +30,10 @@ var prt=func()
             s~=elem~' ';
         s~='\n';
     }
-    system("cls");
+    if(os.platform()=="windows")
+        system("cls");
+    else
+        system("clear");
     print(s);
 }