diff --git a/cli/internal.h b/cli/internal.h
index c802ddb..2cce2a9 100644
--- a/cli/internal.h
+++ b/cli/internal.h
@@ -18,6 +18,16 @@
 
 #include "modules/common.h"
 
+#if defined(__GNUC__)
+  #pragma GCC diagnostic ignored "-Wint-to-pointer-cast"
+  #pragma GCC diagnostic ignored "-Wunused-parameter"
+#elif defined(__clang__)
+  #pragma clang diagnostic ignored "-Wint-to-pointer-cast"
+  #pragma clang diagnostic ignored "-Wunused-parameter"
+#elif defined(_MSC_VER)
+  #pragma warning(disable:26812)
+#endif
+
 #define CLI_NOTICE                                                            \
   "PocketLang " PK_VERSION_STRING " (https://github.com/ThakeeNathees/pocketlang/)\n" \
   "Copyright (c) 2020 - 2021 ThakeeNathees\n"                                 \
diff --git a/cli/main.c b/cli/main.c
index 09ff9e4..17beff9 100644
--- a/cli/main.c
+++ b/cli/main.c
@@ -21,32 +21,15 @@ void onResultDone(PKVM* vm, PkStringPtr result) {
   }
 }
 
-void errorFunction(PKVM* vm, PkErrorType type, const char* file, int line,
-                   const char* message) {
-
-  VmUserData* ud = (VmUserData*)pkGetUserData(vm);
-  bool repl = (ud) ? ud->repl_mode : false;
-
-  if (type == PK_ERROR_COMPILE) {
-
-    if (repl) fprintf(stderr, "Error: %s\n", message);
-    else fprintf(stderr, "Error: %s\n       at \"%s\":%i\n",
-                 message, file, line);
-
-  } else if (type == PK_ERROR_RUNTIME) {
-    fprintf(stderr, "Error: %s\n", message);
-
-  } else if (type == PK_ERROR_STACKTRACE) {
-    if (repl) fprintf(stderr, "  %s() [line:%i]\n", message, line);
-    else fprintf(stderr, "  %s() [\"%s\":%i]\n", message, file, line);
-  }
+void stderrWrite(PKVM* vm, const char* text) {
+  fprintf(stderr, "%s", text);
 }
 
-void writeFunction(PKVM* vm, const char* text) {
+void stdoutWrite(PKVM* vm, const char* text) {
   fprintf(stdout, "%s", text);
 }
 
-PkStringPtr readFunction(PKVM* vm) {
+PkStringPtr stdinRead(PKVM* vm) {
   PkStringPtr result;
   result.string = read_line(NULL);
   result.on_done = onResultDone;
@@ -110,10 +93,14 @@ PkStringPtr loadScript(PKVM* vm, const char* path) {
 
   // Read source to buffer.
   char* buff = (char*)malloc((size_t)(file_size) + 1);
+
   // Using read instead of file_size is because "\r\n" is read as '\n' in
   // windows.
-  size_t read = fread(buff, sizeof(char), file_size, file);
-  buff[read] = '\0';
+  if (buff != NULL) {
+    size_t read = fread(buff, sizeof(char), file_size, file);
+    buff[read] = '\0';
+  }
+
   fclose(file);
 
   result.string = buff;
@@ -122,16 +109,38 @@ PkStringPtr loadScript(PKVM* vm, const char* path) {
   return result;
 }
 
+// FIXME:
+// Included for isatty(). This should be moved to somewhere. and I'm not sure
+// about the portability of these headers. and isatty() function.
+#ifdef _WIN32
+  #include <Windows.h>
+  #include <io.h>
+#else
+  #include <unistd.h>
+#endif
+
 // Create new pocket VM and set it's configuration.
 static PKVM* intializePocketVM() {
+
   PkConfiguration config = pkNewConfiguration();
-  config.error_fn = errorFunction;
-  config.write_fn = writeFunction;
-  config.read_fn = readFunction;
+  config.stderr_write = stderrWrite;
+  config.stdout_write = stdoutWrite;
+  config.stdin_read = stdinRead;
 
   config.load_script_fn = loadScript;
   config.resolve_path_fn = resolvePath;
 
+// FIXME:
+  if (!!isatty(fileno(stderr))) {
+#ifdef _WIN32
+    DWORD outmode = 0;
+    HANDLE handle = GetStdHandle(STD_ERROR_HANDLE);
+    GetConsoleMode(handle, &outmode);
+    SetConsoleMode(handle, outmode | ENABLE_VIRTUAL_TERMINAL_PROCESSING);
+#endif
+    config.use_ansi_color = true;
+  }
+
   return pkNewVM(&config);
 }
 
diff --git a/docs/wasm/io_api.js b/docs/wasm/io_api.js
index c0d8569..0624627 100644
--- a/docs/wasm/io_api.js
+++ b/docs/wasm/io_api.js
@@ -10,21 +10,13 @@
 mergeInto(LibraryManager.library, {
   /** js_func_name : function() {...} */
 
-  js_errorPrint : function(type, line, message) {
-    var err_text = ''
-    const msg = AsciiToString(message);
-    if (type == 0 /*PK_ERROR_COMPILE*/) {
-      err_text = `[Error at:${line}] ${msg}`;
-    } else if (type == 1 /*PK_ERROR_RUNTIME*/) {
-      err_text = `Error: ${msg}`;
-    } else if (type == 2 /*PK_ERROR_STACKTRACE*/) {
-      err_text = `  [at:${line}] ${msg}`;
-    }
-
+  js_errorPrint : function(message) {
     var out = document.getElementById("code-output");
     // To Indicate error (should be removed before each run request).
     out.classList.add("has-error");
-    out.innerHTML += `<span class="error-line">${err_text}</span>\n`;
+
+    let msg = AsciiToString(message);
+    out.innerHTML += `<span class="error-line">${msg}</span>`;
   },
 
   js_writeFunction : function(message) {
diff --git a/docs/wasm/main.c b/docs/wasm/main.c
index 430a037..f91dc75 100644
--- a/docs/wasm/main.c
+++ b/docs/wasm/main.c
@@ -8,14 +8,13 @@
 #include <string.h>
 
 // IO api functions.
-extern void js_errorPrint(int type, int line, const char* message);
+extern void js_errorPrint(const char* message);
 extern void js_writeFunction(const char* message);
 
-void errorPrint(PKVM* vm, PkErrorType type, const char* file, int line,
-                const char* message) {
+void errorPrint(PKVM* vm, const char* message) {
   // No need to pass the file (since there is only script that'll ever run on
   // the browser.
-  js_errorPrint((int)type, line, message);
+  js_errorPrint(message);
 }
 
 void writeFunction(PKVM* vm, const char* text) {
@@ -26,16 +25,39 @@ EMSCRIPTEN_KEEPALIVE
 int runSource(const char* source) {
 
   PkConfiguration config = pkNewConfiguration();
-  config.error_fn = errorPrint;
-  config.write_fn = writeFunction;
+  config.stderr_write = errorPrint;
+  config.stdout_write = writeFunction;
   config.load_script_fn = NULL;
   config.resolve_path_fn = NULL;
 
   PKVM* vm = pkNewVM(&config);
 
-  PkStringPtr src = { source, NULL, NULL };
-  PkStringPtr module = { "@(TRY)", NULL, NULL };
-  PkResult result = pkInterpretSource(vm, src, module, NULL);
+  // FIXME:
+  // The bellow blocks of code can be simplified with
+  //
+  //    PkResult result = pkInterpretSource(vm, src, path, NULL);
+  //
+  // But the path is printed on the error messages as "@(TRY)"
+  // If we create a module and compile it, then it won't have a
+  // path and the error message is simplified. This behavior needs
+  // to be changed in the error report function.
+
+  PkResult result;
+
+  PkHandle* module = pkNewModule(vm, "@(TRY)");
+  do {
+    PkStringPtr src = { .string = source };
+    result = pkCompileModule(vm, module, src, NULL);
+    if (result != PK_RESULT_SUCCESS) break;
+
+    PkHandle* _main = pkModuleGetMainFunction(vm, module);
+    PkHandle* fiber = pkNewFiber(vm, _main);
+    result = pkRunFiber(vm, fiber, 0, NULL);
+    pkReleaseHandle(vm, _main);
+    pkReleaseHandle(vm, fiber);
+
+  } while (false);
+  pkReleaseHandle(vm, module);
 
   pkFreeVM(vm);
 
diff --git a/src/include/pocketlang.h b/src/include/pocketlang.h
index 10690df..0b27742 100644
--- a/src/include/pocketlang.h
+++ b/src/include/pocketlang.h
@@ -105,7 +105,7 @@ typedef void (*pkErrorFn) (PKVM* vm, PkErrorType type,
                            const char* file, int line,
                            const char* message);
 
-// A function callback to write [text] to stdout.
+// Function callback to write [text] to stdout or stderr.
 typedef void (*pkWriteFn) (PKVM* vm, const char* text);
 
 // A function callback to read a line from stdin. The returned string shouldn't
@@ -159,14 +159,6 @@ enum PkVarType {
   PK_INSTANCE,
 };
 
-// Type of the error message that pocketlang will provide with the pkErrorFn
-// callback.
-enum PkErrorType {
-  PK_ERROR_COMPILE = 0, // Compile time errors.
-  PK_ERROR_RUNTIME,     // Runtime error message.
-  PK_ERROR_STACKTRACE,  // One entry of a runtime error stack.
-};
-
 // Result that pocketlang will return after a compilation or running a script
 // or a function or evaluating an expression.
 enum PkResult {
@@ -203,13 +195,16 @@ struct PkConfiguration {
   // pointer is NULL it defaults to the VM's realloc(), free() wrappers.
   pkReallocFn realloc_fn;
 
-  pkErrorFn error_fn;
-  pkWriteFn write_fn;
-  pkReadFn read_fn;
+  pkWriteFn stderr_write;
+  pkWriteFn stdout_write;
+  pkReadFn stdin_read;
 
   pkResolvePathFn resolve_path_fn;
   pkLoadScriptFn load_script_fn;
 
+  // If true stderr calls will use ansi color codes.
+  bool use_ansi_color;
+
   // User defined data associated with VM.
   void* user_data;
 };
diff --git a/src/pk_compiler.c b/src/pk_compiler.c
index 4b38e0f..2feca34 100644
--- a/src/pk_compiler.c
+++ b/src/pk_compiler.c
@@ -297,12 +297,8 @@ typedef struct sForwardName {
   // The function where the name is used, and the instruction is belongs to.
   Fn* func;
 
-  // The name string's pointer in the source.
-  const char* name;
-  int length;
-
-  // Line number of the name used (required for error message).
-  int line;
+  // Name token that was lexed for this name.
+  Token tkname;
 
 } ForwardName;
 
@@ -412,9 +408,15 @@ typedef struct sParser {
 
   bool repl_mode;
   bool parsing_class;
-  bool has_errors;
   bool need_more_lines; //< True if we need more lines in REPL mode.
 
+  // [has_errors] is for all kinds of errors, If it's set we don't terminate
+  // the compilation since we can cascade more errors by continuing. But
+  // [has_syntax_error] will set to true if we encounter one and this will
+  // terminatie the compilation.
+  bool has_syntax_error;
+  bool has_errors;
+
 } Parser;
 
 struct Compiler {
@@ -509,7 +511,10 @@ static void parserInit(Parser* parser, PKVM* vm, Compiler* compiler,
   parser->current_char = parser->source;
   parser->current_line = 1;
 
+  parser->previous.type = TK_ERROR;
+  parser->current.type = TK_ERROR;
   parser->next.type = TK_ERROR;
+
   parser->next.start = NULL;
   parser->next.length = 0;
   parser->next.line = 1;
@@ -524,6 +529,7 @@ static void parserInit(Parser* parser, PKVM* vm, Compiler* compiler,
   parser->repl_mode = !!(compiler->options && compiler->options->repl_mode);
   parser->parsing_class = false;
   parser->has_errors = false;
+  parser->has_syntax_error = false;
   parser->need_more_lines = false;
 }
 
@@ -543,7 +549,14 @@ static void compilerInit(Compiler* compiler, PKVM* vm, const char* source,
   compiler->new_local = false;
   compiler->is_last_call = false;
 
-  parserInit(&compiler->parser, vm, compiler, source, module->path->data);
+  const char* source_path = "@??";
+  if (module->path != NULL) {
+    source_path = module->path->data;
+  } else if (options->repl_mode) {
+    source_path = "@REPL";
+  }
+
+  parserInit(&compiler->parser, vm, compiler, source, source_path);
 
   // Cache the required built functions.
   compiler->bifn_list_join = findBuiltinFunction(vm, "list_join", 9);
@@ -555,16 +568,14 @@ static void compilerInit(Compiler* compiler, PKVM* vm, const char* source,
 /*****************************************************************************/
 
 // Internal error report function for lexing and parsing.
-static void reportError(Parser* parser, const char* file, int line,
+static void reportError(Parser* parser, Token tk,
                         const char* fmt, va_list args) {
 
-  // On REPL mode only the first error is reported.
-  if (parser->repl_mode && parser->has_errors) {
-    return;
-  }
-
   parser->has_errors = true;
 
+  PKVM* vm = parser->vm;
+  if (vm->config.stderr_write == NULL) return;
+
   // If the source is incomplete we're not printing an error message,
   // instead return PK_RESULT_UNEXPECTED_EOF to the host.
   if (parser->need_more_lines) {
@@ -572,51 +583,47 @@ static void reportError(Parser* parser, const char* file, int line,
     return;
   }
 
-  if (parser->vm->config.error_fn == NULL) return;
+  reportCompileTimeError(vm, parser->file_path, tk.line, parser->source,
+                         tk.start, tk.length, fmt, args);
 
-  // TODO: fix the buffer size. A non terminated large string could cause this
-  // crash.
-
-  char message[ERROR_MESSAGE_SIZE];
-  int length = vsnprintf(message, sizeof(message), fmt, args);
-  __ASSERT(length >= 0, "Error message buffer failed at vsnprintf().");
-  parser->vm->config.error_fn(parser->vm, PK_ERROR_COMPILE,
-                              file, line, message);
-}
-
-// Error caused at the middle of lexing (and TK_ERROR will be lexed instead).
-static void lexError(Parser* parser, const char* fmt, ...) {
-  va_list args;
-  va_start(args, fmt);
-  reportError(parser, parser->file_path, parser->current_line, fmt, args);
-  va_end(args);
 }
 
 // Error caused when parsing. The associated token assumed to be last consumed
 // which is [parser->previous].
-static void parseError(Compiler* compiler, const char* fmt, ...) {
+static void syntaxError(Compiler* compiler, Token tk, const char* fmt, ...) {
+  Parser* parser = &compiler->parser;
 
-  Token* token = &(compiler->parser.previous);
+  // Only one syntax error is reported.
+  if (parser->has_syntax_error) return;
 
-  // Lex errors would reported earlier by lexError and lexed a TK_ERROR token.
-  if (token->type == TK_ERROR) return;
+  parser->has_syntax_error = true;
+  va_list args;
+  va_start(args, fmt);
+  reportError(parser, tk, fmt, args);
+  va_end(args);
+}
+
+static void semanticError(Compiler* compiler, Token tk, const char* fmt, ...) {
+  Parser* parser = &compiler->parser;
+
+  // If the parser has synax errors, semantic errors are not reported.
+  if (parser->has_syntax_error) return;
 
   va_list args;
   va_start(args, fmt);
-  reportError(&(compiler->parser), compiler->parser.file_path,
-              token->line, fmt, args);
+  reportError(parser, tk, fmt, args);
   va_end(args);
 }
 
 // Error caused when trying to resolve forward names (maybe more in the
 // future), Which will be called once after compiling the module and thus we
 // need to pass the line number the error originated from.
-static void resolveError(Compiler* compiler, int line, const char* fmt, ...) {
+static void resolveError(Compiler* compiler, Token tk, const char* fmt, ...) {
+  Parser* parser = &compiler->parser;
+
   va_list args;
   va_start(args, fmt);
-  reportError(&(compiler->parser),
-              compiler->parser.file_path,
-              line, fmt, args);
+  reportError(parser, tk, fmt, args);
   va_end(args);
 }
 
@@ -625,8 +632,8 @@ static void resolveError(Compiler* compiler, int line, const char* fmt, ...) {
 static void checkMaxConstantsReached(Compiler* compiler, int index) {
   ASSERT(index >= 0, OOPS);
   if (index >= MAX_CONSTANTS) {
-    parseError(compiler, "A module should contain at most %d "
-      "unique constants.", MAX_CONSTANTS);
+    semanticError(compiler, compiler->parser.previous,
+        "A module should contain at most %d unique constants.", MAX_CONSTANTS);
   }
 }
 
@@ -636,6 +643,7 @@ static void checkMaxConstantsReached(Compiler* compiler, int index) {
 
 // Forward declaration of lexer methods.
 
+static Token makeErrToken(Parser* parser);
 static char peekChar(Parser* parser);
 static char peekNextChar(Parser* parser);
 static char eatChar(Parser* parser);
@@ -643,7 +651,9 @@ static void setNextValueToken(Parser* parser, TokenType type, Var value);
 static void setNextToken(Parser* parser, TokenType type);
 static bool matchChar(Parser* parser, char c);
 
-static void eatString(Parser* parser, bool single_quote) {
+static void eatString(Compiler* compiler, bool single_quote) {
+  Parser* parser = &compiler->parser;
+
   pkByteBuffer buff;
   pkByteBufferInit(&buff);
 
@@ -658,7 +668,8 @@ static void eatString(Parser* parser, bool single_quote) {
     if (c == quote) break;
 
     if (c == '\0') {
-      lexError(parser, "Non terminated string.");
+      syntaxError(compiler, makeErrToken(parser), "Non terminated string.");
+      return;
 
       // Null byte is required by TK_EOF.
       parser->current_char--;
@@ -678,7 +689,9 @@ static void eatString(Parser* parser, bool single_quote) {
 
         } else { // Name Interpolation.
           if (!utilIsName(c)) {
-            lexError(parser, "Expected '{' or identifier after '$'.");
+            syntaxError(compiler, makeErrToken(parser),
+                        "Expected '{' or identifier after '$'.");
+            return;
 
           } else { // Name interpolation (ie. "Hello $name!").
 
@@ -695,8 +708,9 @@ static void eatString(Parser* parser, bool single_quote) {
         }
 
       } else {
-        lexError(parser, "Maximum interpolation level reached (can only "
-                 "interpolate upto depth %d).", MAX_STR_INTERP_DEPTH);
+        semanticError(compiler, makeErrToken(parser),
+                     "Maximum interpolation level reached (can only "
+                     "interpolate upto depth %d).", MAX_STR_INTERP_DEPTH);
       }
       break;
     }
@@ -714,8 +728,9 @@ static void eatString(Parser* parser, bool single_quote) {
         case '$':  pkByteBufferWrite(&buff, parser->vm, '$'); break;
 
         default:
-          lexError(parser, "Error: invalid escape character");
-          break;
+          syntaxError(compiler, makeErrToken(parser),
+                      "Invalid escape character.");
+          return;
       }
     } else {
       pkByteBufferWrite(&buff, parser->vm, c);
@@ -776,7 +791,8 @@ static void eatName(Parser* parser) {
 }
 
 // Complete lexing a number literal.
-static void eatNumber(Parser* parser) {
+static void eatNumber(Compiler* compiler) {
+  Parser* parser = &compiler->parser;
 
 #define IS_HEX_CHAR(c)            \
   (('0' <= (c) && (c) <= '9')  || \
@@ -794,7 +810,8 @@ static void eatNumber(Parser* parser) {
     uint64_t bin = 0;
     c = peekChar(parser);
     if (!IS_BIN_CHAR(c)) {
-      lexError(parser, "Invalid binary literal.");
+      syntaxError(compiler, makeErrToken(parser), "Invalid binary literal.");
+      return;
 
     } else {
       do {
@@ -806,7 +823,8 @@ static void eatNumber(Parser* parser) {
         // Check the length of the binary literal.
         int length = (int)(parser->current_char - parser->token_start);
         if (length > STR_BIN_BUFF_SIZE - 2) { // -2: '-\0' 0b is in both side.
-          lexError(parser, "Binary literal is too long.");
+          semanticError(compiler, makeErrToken(parser),
+                        "Binary literal is too long.");
           break;
         }
 
@@ -825,7 +843,8 @@ static void eatNumber(Parser* parser) {
 
     // The first digit should be either hex digit.
     if (!IS_HEX_CHAR(c)) {
-      lexError(parser, "Invalid hex literal.");
+      syntaxError(compiler, makeErrToken(parser), "Invalid hex literal.");
+      return;
 
     } else {
       do {
@@ -837,7 +856,8 @@ static void eatNumber(Parser* parser) {
         // Check the length of the binary literal.
         int length = (int)(parser->current_char - parser->token_start);
         if (length > STR_HEX_BUFF_SIZE - 2) { // -2: '-\0' 0x is in both side.
-          lexError(parser, "Hex literal is too long.");
+          semanticError(compiler, makeErrToken(parser),
+                        "Hex literal is too long.");
           break;
         }
 
@@ -872,7 +892,8 @@ static void eatNumber(Parser* parser) {
       }
 
       if (!utilIsDigit(peekChar(parser))) {
-        lexError(parser, "Invalid number literal.");
+        syntaxError(compiler, makeErrToken(parser), "Invalid number literal.");
+        return;
 
       } else { // Eat the exponent.
         while (utilIsDigit(peekChar(parser))) eatChar(parser);
@@ -884,7 +905,8 @@ static void eatNumber(Parser* parser) {
     if (errno == ERANGE) {
       const char* start = parser->token_start;
       int len = (int)(parser->current_char - start);
-      lexError(parser, "Number literal is too large (%.*s).", len, start);
+      semanticError(compiler, makeErrToken(parser),
+                    "Number literal is too large (%.*s).", len, start);
       value = VAR_NUM(0);
     }
   }
@@ -923,6 +945,16 @@ static void setNextTwoCharToken(Parser* parser, char c, TokenType one,
   }
 }
 
+// Returns an error token from the current position for reporting error.
+static Token makeErrToken(Parser* parser) {
+  Token tk;
+  tk.type = TK_ERROR;
+  tk.start = parser->token_start;
+  tk.length = (int)(parser->current_char - parser->token_start);
+  tk.line = parser->current_line;
+  return tk;
+}
+
 // Initialize the next token as the type.
 static void setNextToken(Parser* parser, TokenType type) {
   Token* next = &parser->next;
@@ -939,7 +971,9 @@ static void setNextValueToken(Parser* parser, TokenType type, Var value) {
 }
 
 // Lex the next token and set it as the next token.
-static void lexToken(Parser* parser) {
+static void lexToken(Compiler* compiler) {
+  Parser* parser = &compiler->parser;
+
   parser->previous = parser->current;
   parser->current = parser->next;
 
@@ -957,7 +991,7 @@ static void lexToken(Parser* parser) {
     if (parser->si_name_end != NULL) {
       if (parser->current_char == parser->si_name_end) {
         parser->si_name_end = NULL;
-        eatString(parser, parser->si_name_quote == '\'');
+        eatString(compiler, parser->si_name_quote == '\'');
         return;
       } else {
         ASSERT(parser->current_char < parser->si_name_end, OOPS);
@@ -987,7 +1021,7 @@ static void lexToken(Parser* parser) {
 
             char quote = parser->si_quote[parser->si_depth - 1];
             parser->si_depth--; //< Exit the depth.
-            eatString(parser, quote == '\'');
+            eatString(compiler, quote == '\'');
             return;
 
           } else { // Decrease the open brace at the current depth.
@@ -1043,7 +1077,8 @@ static void lexToken(Parser* parser) {
           setNextToken(parser, TK_DOTDOT); // '..'
         } else if (utilIsDigit(peekChar(parser))) {
           eatChar(parser);   // Consume the decimal point.
-          eatNumber(parser); // Consume the rest of the number
+          eatNumber(compiler); // Consume the rest of the number
+          if (parser->has_syntax_error) return;
         } else {
           setNextToken(parser, TK_DOT);    // '.'
         }
@@ -1103,33 +1138,37 @@ static void lexToken(Parser* parser) {
         setNextTwoCharToken(parser, '=', TK_FSLASH, TK_DIVEQ);
         return;
 
-      case '"': eatString(parser, false); return;
+      case '"': eatString(compiler, false); return;
 
-      case '\'': eatString(parser, true); return;
+      case '\'': eatString(compiler, true); return;
 
       default: {
 
         if (utilIsDigit(c)) {
-          eatNumber(parser);
+          eatNumber(compiler);
+          if (parser->has_syntax_error) return;
 
         } else if (utilIsName(c)) {
           eatName(parser);
 
         } else {
-          if (c >= 32 && c <= 126) {
-            lexError(parser, "Invalid character '%c'", c);
-          } else {
-            lexError(parser, "Invalid byte 0x%x", (uint8_t)c);
-          }
           setNextToken(parser, TK_ERROR);
+
+          if (c >= 32 && c <= 126) {
+            syntaxError(compiler, parser->next,
+                        "Invalid character '%c'", c);
+          } else {
+            syntaxError(compiler, parser->next,
+                        "Invalid byte 0x%x", (uint8_t)c);
+          }
         }
         return;
       }
     }
   }
 
+  parser->token_start = parser->current_char;
   setNextToken(parser, TK_EOF);
-  parser->next.start = parser->current_char;
 }
 
 /*****************************************************************************/
@@ -1145,7 +1184,10 @@ static TokenType peek(Compiler* compiler) {
 // and return true otherwise return false.
 static bool match(Compiler* compiler, TokenType expected) {
   if (peek(compiler) != expected) return false;
-  lexToken(&(compiler->parser));
+
+  lexToken(compiler);
+  if (compiler->parser.has_syntax_error) return false;
+
   return true;
 }
 
@@ -1154,15 +1196,13 @@ static bool match(Compiler* compiler, TokenType expected) {
 static void consume(Compiler* compiler, TokenType expected,
                     const char* err_msg) {
 
-  lexToken(&(compiler->parser));
-  if (compiler->parser.previous.type != expected) {
-    parseError(compiler, "%s", err_msg);
+  lexToken(compiler);
+  if (compiler->parser.has_syntax_error) return;
 
-    // If the next token is expected discard the current to minimize
-    // cascaded errors and continue parsing.
-    if (peek(compiler) == expected) {
-      lexToken(&(compiler->parser));
-    }
+  Token *prev = &compiler->parser.previous;
+  if (prev->type != expected) {
+    syntaxError(compiler, *prev, "%s", err_msg);
+    return;
   }
 }
 
@@ -1172,8 +1212,10 @@ static bool matchLine(Compiler* compiler) {
   bool consumed = false;
 
   if (peek(compiler) == TK_LINE) {
-    while (peek(compiler) == TK_LINE)
-      lexToken(&(compiler->parser));
+    while (peek(compiler) == TK_LINE) {
+      lexToken(compiler);
+      if (compiler->parser.has_syntax_error) return false;
+    }
     consumed = true;
   }
 
@@ -1213,7 +1255,9 @@ static bool matchEndStatement(Compiler* compiler) {
 // Consume semi collon, multiple new lines or peek 'end' keyword.
 static void consumeEndStatement(Compiler* compiler) {
   if (!matchEndStatement(compiler)) {
-    parseError(compiler, "Expected statement end with '\\n' or ';'.");
+    syntaxError(compiler, compiler->parser.current,
+                "Expected statement end with '\\n' or ';'.");
+    return;
   }
 }
 
@@ -1234,7 +1278,8 @@ static void consumeStartBlock(Compiler* compiler, TokenType delimiter) {
     const char* msg;
     if (delimiter == TK_DO) msg = "Expected enter block with newline or 'do'.";
     else msg = "Expected enter block with newline or 'then'.";
-    parseError(compiler, msg);
+    syntaxError(compiler, compiler->parser.previous, msg);
+    return;
   }
 }
 
@@ -1313,8 +1358,8 @@ static int addUpvalue(Compiler* compiler, Func* func,
   }
 
   if (func->ptr->upvalue_count == MAX_UPVALUES) {
-    parseError(compiler, "A function cannot capture more thatn %d upvalues.",
-               MAX_UPVALUES);
+    semanticError(compiler, compiler->parser.previous,
+            "A function cannot capture more thatn %d upvalues.", MAX_UPVALUES);
     return -1;
   }
 
@@ -1455,7 +1500,7 @@ static int compilerAddConstant(Compiler* compiler, Var value);
 static int compilerAddVariable(Compiler* compiler, const char* name,
                                uint32_t length, int line);
 static void compilerAddForward(Compiler* compiler, int instruction, Fn* fn,
-                               const char* name, int length, int line);
+                               Token* tkname);
 static void compilerChangeStack(Compiler* compiler, int num);
 
 // Forward declaration of grammar functions.
@@ -1587,8 +1632,12 @@ static void emitPushName(Compiler* compiler, NameDefnType type, int index) {
   ASSERT(index >= 0, OOPS);
 
   switch (type) {
-    case NAME_NOT_DEFINED:
+    case NAME_NOT_DEFINED: {
+      if (compiler->parser.has_errors) {
+        return;
+      }
       UNREACHABLE();
+    }
 
     case NAME_LOCAL_VAR:
       if (index < 9) { //< 0..8 locals have single opcode.
@@ -1629,8 +1678,10 @@ static void emitStoreName(Compiler* compiler, NameDefnType type, int index) {
   switch (type) {
     case NAME_NOT_DEFINED:
     case NAME_BUILTIN_FN:
-    case NAME_BUILTIN_TY:
+    case NAME_BUILTIN_TY: {
+      if (compiler->parser.has_errors) return;
       UNREACHABLE();
+    }
 
     case NAME_LOCAL_VAR:
       if (index < 9) { //< 0..8 locals have single opcode.
@@ -1722,9 +1773,11 @@ static void exprFunction(Compiler* compiler) {
 
 static void exprName(Compiler* compiler) {
 
-  const char* start = compiler->parser.previous.start;
-  int length = compiler->parser.previous.length;
-  int line = compiler->parser.previous.line;
+  Token tkname = compiler->parser.previous;
+
+  const char* start = tkname.start;
+  int length = tkname.length;
+  int line = tkname.line;
   NameSearchResult result = compilerSearchName(compiler, start, length);
 
   if (compiler->l_value && matchAssignment(compiler)) {
@@ -1767,8 +1820,14 @@ static void exprName(Compiler* compiler) {
       compileExpression(compiler);
 
     } else { // name += / -= / *= ... = (expr);
+
       if (result.type == NAME_NOT_DEFINED) {
-        parseError(compiler, "Name '%.*s' is not defined.", length, start);
+
+        // TODO:
+        // Add to forward names. Create result.type as NAME_FORWARD
+        // and use emitPushName, emitStoreName for here and bellow.
+        semanticError(compiler, tkname,
+                      "Name '%.*s' is not defined.", length, start);
       }
 
       // Push the named value.
@@ -1806,11 +1865,12 @@ static void exprName(Compiler* compiler) {
     // a local depth.
     if (result.type == NAME_NOT_DEFINED) {
       if (compiler->scope_depth == DEPTH_GLOBAL) {
-        parseError(compiler, "Name '%.*s' is not defined.", length, start);
+        semanticError(compiler, tkname,
+                      "Name '%.*s' is not defined.", length, start);
       } else {
         emitOpcode(compiler, OP_PUSH_GLOBAL);
         int index = emitByte(compiler, 0xff);
-        compilerAddForward(compiler, index, _FN, start, length, line);
+        compilerAddForward(compiler, index, _FN, &tkname);
       }
     } else {
       emitPushName(compiler, result.type, result.index);
@@ -2066,19 +2126,24 @@ static void exprSelf(Compiler* compiler) {
   // inside a method.
 
   if (!compiler->parser.parsing_class) {
-    parseError(compiler, "Invalid use of 'self'.");
+    semanticError(compiler, compiler->parser.previous,
+                  "Invalid use of 'self'.");
   } else {
     // FIXME:
-    parseError(compiler, "TODO: Closures cannot capture 'self' for now.");
+    semanticError(compiler, compiler->parser.previous,
+                  "TODO: Closures cannot capture 'self' for now.");
   }
 }
 
 static void parsePrecedence(Compiler* compiler, Precedence precedence) {
-  lexToken(&(compiler->parser));
+  lexToken(compiler);
+  if (compiler->parser.has_syntax_error) return;
+
   GrammarFn prefix = getRule(compiler->parser.previous.type)->prefix;
 
   if (prefix == NULL) {
-    parseError(compiler, "Expected an expression.");
+    syntaxError(compiler, compiler->parser.previous,
+                "Expected an expression.");
     return;
   }
 
@@ -2092,7 +2157,8 @@ static void parsePrecedence(Compiler* compiler, Precedence precedence) {
   compiler->is_last_call = false;
 
   while (getRule(compiler->parser.current.type)->precedence >= precedence) {
-    lexToken(&(compiler->parser));
+    lexToken(compiler);
+    if (compiler->parser.has_syntax_error) return;
 
     TokenType op = compiler->parser.previous.type;
     GrammarFn infix = getRule(op)->infix;
@@ -2131,8 +2197,8 @@ static int compilerAddVariable(Compiler* compiler, const char* name,
     }
   }
   if (max_vars_reached) {
-    parseError(compiler, "A module should contain at most %d %s.",
-               MAX_VARIABLES, var_type);
+    semanticError(compiler, compiler->parser.previous,
+            "A module should contain at most %d %s.", MAX_VARIABLES, var_type);
     return -1;
   }
 
@@ -2156,10 +2222,10 @@ static int compilerAddVariable(Compiler* compiler, const char* name,
 }
 
 static void compilerAddForward(Compiler* compiler, int instruction, Fn* fn,
-                               const char* name, int length, int line) {
+                               Token* tkname) {
   if (compiler->parser.forwards_count == MAX_FORWARD_NAMES) {
-    parseError(compiler, "A module should contain at most %d implicit forward "
-               "function declarations.", MAX_FORWARD_NAMES);
+    semanticError(compiler, *tkname, "A module should contain at most %d "
+                 "implicit forward function declarations.", MAX_FORWARD_NAMES);
     return;
   }
 
@@ -2167,9 +2233,7 @@ static void compilerAddForward(Compiler* compiler, int instruction, Fn* fn,
                            compiler->parser.forwards_count++];
   forward->instruction = instruction;
   forward->func = fn;
-  forward->name = name;
-  forward->length = length;
-  forward->line = line;
+  forward->tkname = *tkname;
 }
 
 // Add a literal constant to module literals and return it's index.
@@ -2376,18 +2440,21 @@ static int compileClass(Compiler* compiler) {
   vmPushTempRef(_vm, &cls->_super); // cls.
   compiler->parser.parsing_class = true;
 
-  // Check count exceeded.
   checkMaxConstantsReached(compiler, cls_index);
 
   skipNewLines(compiler);
-  while (!match(compiler, TK_END)) {
+  while (!match(compiler, TK_END) && !match(compiler, TK_EOF)) {
     // At the top level the stack size should be 0, before and after compiling
     // a top level statement, since there aren't any locals at the top level.
     ASSERT(compiler->parser.has_errors ||
            compiler->func->stack_size == 0, OOPS);
 
     consume(compiler, TK_DEF, "Expected method definition.");
+    if (compiler->parser.has_syntax_error) break;
+
     int fn_index = compileFunction(compiler, FUNC_METHOD);
+    if (compiler->parser.has_syntax_error) break;
+
     Var fn_var = compiler->module->constants.data[fn_index];
     ASSERT(IS_OBJ_TYPE(fn_var, OBJ_FUNC), OOPS);
 
@@ -2410,6 +2477,7 @@ static int compileClass(Compiler* compiler) {
            compiler->func->stack_size == 0, OOPS);
 
     skipNewLines(compiler);
+    if (compiler->parser.has_syntax_error) break;
   }
 
   compiler->parser.parsing_class = false;
@@ -2437,6 +2505,7 @@ static int compileFunction(Compiler* compiler, FuncType fn_type) {
   int fn_index;
   Function* func = newFunction(compiler->parser.vm, name, name_length,
                                compiler->module, false, NULL, &fn_index);
+
   checkMaxConstantsReached(compiler, fn_index);
 
   if (fn_type != FUNC_LITERAL) {
@@ -2482,7 +2551,8 @@ static int compileFunction(Compiler* compiler, FuncType fn_type) {
         }
       }
       if (predefined) {
-        parseError(compiler, "Multiple definition of a parameter.");
+        semanticError(compiler, compiler->parser.previous,
+                      "Multiple definition of a parameter.");
       }
 
       compilerAddVariable(compiler, param_name, param_len,
@@ -2585,8 +2655,10 @@ static Module* importFile(Compiler* compiler, const char* path) {
   }
 
   if (resolved.string == NULL) {
-    parseError(compiler, "Cannot resolve path '%s' from '%s'", path,
-               compiler->module->path->data);
+    semanticError(compiler, compiler->parser.previous,
+      "Cannot resolve path '%s' from '%s'", path,
+      compiler->module->path->data);
+    return NULL;
   }
 
   // Create new string for the resolved path. And free the resolved path.
@@ -2611,15 +2683,17 @@ static Module* importFile(Compiler* compiler, const char* path) {
   // The script not exists in the VM, make sure we have the script loading
   // api function.
   if (vm->config.load_script_fn == NULL) {
-    parseError(compiler, "Cannot import. The hosting application haven't "
-               "registered the script loading API");
+    semanticError(compiler, compiler->parser.previous,
+                  "Cannot import. The hosting application haven't  registered "
+                  "the script loading API");
     return NULL;
   }
 
   // Load the script at the path.
   PkStringPtr source = vm->config.load_script_fn(vm, path_->data);
   if (source.string == NULL) {
-    parseError(compiler, "Error loading script at \"%s\"", path_->data);
+    semanticError(compiler, compiler->parser.previous,
+                  "Error loading script at \"%s\"", path_->data);
     return NULL;
   }
 
@@ -2656,8 +2730,8 @@ static Module* importFile(Compiler* compiler, const char* path) {
   if (source.on_done != NULL) source.on_done(vm, source);
 
   if (result != PK_RESULT_SUCCESS) {
-    parseError(compiler, "Compilation of imported script '%s' failed",
-               path_->data);
+    semanticError(compiler, compiler->parser.previous,
+                  "Compilation of imported script '%s' failed", path_->data);
   }
 
   return module;
@@ -2677,7 +2751,8 @@ static Module* importCoreLib(Compiler* compiler, const char* name_start,
 
   Module* imported = vmGetModule(compiler->parser.vm, module_name);
   if (imported == NULL) {
-    parseError(compiler, "No module named '%s' exists.", module_name->data);
+    semanticError(compiler, compiler->parser.previous,
+                  "No module named '%s' exists.", module_name->data);
     return NULL;
   }
 
@@ -2707,7 +2782,8 @@ static inline Module* compilerImport(Compiler* compiler) {
   }
 
   // Invalid token after import/from keyword.
-  parseError(compiler, "Expected a module name or path to import.");
+  syntaxError(compiler, compiler->parser.previous,
+              "Expected a module name or path to import.");
   return NULL;
 }
 
@@ -2725,6 +2801,7 @@ static int compilerImportName(Compiler* compiler, int line,
       return compilerAddVariable(compiler, name, length, line);
 
     case NAME_LOCAL_VAR:
+    case NAME_UPVALUE:
       UNREACHABLE();
 
     case NAME_GLOBAL_VAR:
@@ -2735,7 +2812,9 @@ static int compilerImportName(Compiler* compiler, int line,
     // should pass) and allow imported entries to have the same name of
     // builtin functions.
     case NAME_BUILTIN_FN:
-      parseError(compiler, "Name '%.*s' already exists.", length, name);
+    case NAME_BUILTIN_TY:
+      semanticError(compiler, compiler->parser.previous,
+                    "Name '%.*s' already exists.", length, name);
       return -1;
   }
 
@@ -3061,7 +3140,8 @@ static void compileStatement(Compiler* compiler) {
 
   if (match(compiler, TK_BREAK)) {
     if (compiler->loop == NULL) {
-      parseError(compiler, "Cannot use 'break' outside a loop.");
+      syntaxError(compiler, compiler->parser.previous,
+                  "Cannot use 'break' outside a loop.");
       return;
     }
 
@@ -3078,7 +3158,8 @@ static void compileStatement(Compiler* compiler) {
 
   } else if (match(compiler, TK_CONTINUE)) {
     if (compiler->loop == NULL) {
-      parseError(compiler, "Cannot use 'continue' outside a loop.");
+      syntaxError(compiler, compiler->parser.previous,
+                  "Cannot use 'continue' outside a loop.");
       return;
     }
 
@@ -3091,7 +3172,8 @@ static void compileStatement(Compiler* compiler) {
   } else if (match(compiler, TK_RETURN)) {
 
     if (compiler->scope_depth == DEPTH_GLOBAL) {
-      parseError(compiler, "Invalid 'return' outside a function.");
+      syntaxError(compiler, compiler->parser.previous,
+                  "Invalid 'return' outside a function.");
       return;
     }
 
@@ -3109,7 +3191,8 @@ static void compileStatement(Compiler* compiler) {
     } else {
 
       if (compiler->func->type == FUNC_CONSTRUCTOR) {
-        parseError(compiler, "Cannor 'return' a value from constructor.");
+        syntaxError(compiler, compiler->parser.previous,
+                    "Cannor 'return' a value from constructor.");
       }
 
       compileExpression(compiler); //< Return value is at stack top.
@@ -3180,8 +3263,8 @@ static void compileTopLevelStatement(Compiler* compiler) {
     compileRegularImport(compiler);
 
   } else if (match(compiler, TK_MODULE)) {
-    parseError(compiler, "Module name must be the first statement "
-                          "of the script.");
+    syntaxError(compiler, compiler->parser.previous,
+                "Module name must be the first statement of the script.");
 
   } else {
     compileStatement(compiler);
@@ -3196,6 +3279,8 @@ static void compileTopLevelStatement(Compiler* compiler) {
 PkResult compile(PKVM* vm, Module* module, const char* source,
                  const PkCompileOptions* options) {
 
+  ASSERT(module != NULL, OOPS);
+
   // Skip utf8 BOM if there is any.
   if (strncmp(source, "\xEF\xBB\xBF", 3) == 0) source += 3;
 
@@ -3213,6 +3298,7 @@ PkResult compile(PKVM* vm, Module* module, const char* source,
   // the native api function (pkNewModule() that'll return a module without a
   // main function) so just create and add the function here.
   if (module->body == NULL) moduleAddMain(vm, module);
+  ASSERT(module->body != NULL, OOPS);
 
   // If we're compiling for a module that was already compiled (when running
   // REPL or evaluating an expression) we don't need the old main anymore.
@@ -3228,8 +3314,8 @@ PkResult compile(PKVM* vm, Module* module, const char* source,
   compilerPushFunc(compiler, &curr_fn, module->body->fn, FUNC_MAIN);
 
   // Lex initial tokens. current <-- next.
-  lexToken(&(compiler->parser));
-  lexToken(&(compiler->parser));
+  lexToken(compiler);
+  lexToken(compiler);
   skipNewLines(compiler);
 
   if (match(compiler, TK_MODULE)) {
@@ -3238,7 +3324,8 @@ PkResult compile(PKVM* vm, Module* module, const char* source,
     // application module attribute might already set. In that case make it
     // Compile error.
     if (module->name != NULL) {
-      parseError(compiler, "Module name already defined.");
+      syntaxError(compiler, compiler->parser.previous,
+                  "Module name already defined.");
 
     } else {
       consume(compiler, TK_NAME, "Expected a name for the module.");
@@ -3249,7 +3336,7 @@ PkResult compile(PKVM* vm, Module* module, const char* source,
     }
   }
 
-  while (!match(compiler, TK_EOF)) {
+  while (!match(compiler, TK_EOF) && !compiler->parser.has_syntax_error) {
     compileTopLevelStatement(compiler);
     skipNewLines(compiler);
   }
@@ -3257,20 +3344,23 @@ PkResult compile(PKVM* vm, Module* module, const char* source,
   emitFunctionEnd(compiler);
 
   // Resolve forward names (function names that are used before defined).
-  for (int i = 0; i < compiler->parser.forwards_count; i++) {
-    ForwardName* forward = &compiler->parser.forwards[i];
-    const char* name = forward->name;
-    int length = forward->length;
-    int index = moduleGetGlobalIndex(compiler->module, name, (uint32_t)length);
-    if (index != -1) {
-      patchForward(compiler, forward->func, forward->instruction, index);
-    } else {
-      // need_more_lines is only true for unexpected EOF errors. For syntax
-      // errors it'll be false by now but. Here it's a semantic errors, so
-      // we're overriding it to false.
-      compiler->parser.need_more_lines = false;
-      resolveError(compiler, forward->line, "Name '%.*s' is not defined.",
-                   length, name);
+  if (!compiler->parser.has_syntax_error) {
+    for (int i = 0; i < compiler->parser.forwards_count; i++) {
+      ForwardName* forward = &compiler->parser.forwards[i];
+      const char* name = forward->tkname.start;
+      int length = forward->tkname.length;
+      int index = moduleGetGlobalIndex(compiler->module, name,
+                                       (uint32_t)length);
+      if (index != -1) {
+        patchForward(compiler, forward->func, forward->instruction, index);
+      } else {
+        // need_more_lines is only true for unexpected EOF errors. For syntax
+        // errors it'll be false by now but. Here it's a semantic errors, so
+        // we're overriding it to false.
+        compiler->parser.need_more_lines = false;
+        resolveError(compiler, forward->tkname, "Name '%.*s' is not defined.",
+                     length, name);
+      }
     }
   }
 
@@ -3296,11 +3386,20 @@ PkResult compile(PKVM* vm, Module* module, const char* source,
   return PK_RESULT_SUCCESS;
 }
 
+// FIXME:
+// move this to pk_core or create new pk_public.c to implement all
+// public functions.
 PkResult pkCompileModule(PKVM* vm, PkHandle* module_handle, PkStringPtr source,
                          const PkCompileOptions* options) {
-  __ASSERT(module_handle != NULL, "Argument module was NULL.");
-  __ASSERT(IS_OBJ_TYPE(module_handle->value, OBJ_MODULE),
+
+  ASSERT(source.string != NULL, OOPS);
+
+  // FIXME:
+  // CHECK_NULL, CHECK_TYPE marcros can be use after this function is moved.
+  ASSERT(module_handle != NULL, "Argument module was NULL.");
+  ASSERT(IS_OBJ_TYPE(module_handle->value, OBJ_MODULE),
            "Given handle is not a module.");
+
   Module* module = (Module*)AS_OBJ(module_handle->value);
 
   PkResult result = compile(vm, module, source.string, options);
diff --git a/src/pk_core.c b/src/pk_core.c
index cf94153..1ed7228 100644
--- a/src/pk_core.c
+++ b/src/pk_core.c
@@ -480,8 +480,8 @@ DEF(coreHelp,
 
   if (argc == 0) {
     // If there ins't an io function callback, we're done.
-    if (vm->config.write_fn == NULL) RET(VAR_NULL);
-    vm->config.write_fn(vm, "TODO: print help here\n");
+    if (vm->config.stdout_write == NULL) RET(VAR_NULL);
+    vm->config.stdout_write(vm, "TODO: print help here\n");
 
   } else if (argc == 1) {
 
@@ -492,15 +492,15 @@ DEF(coreHelp,
     if (!validateArgClosure(vm, 1, &closure)) return;
 
     // If there ins't an io function callback, we're done.
-    if (vm->config.write_fn == NULL) RET(VAR_NULL);
+    if (vm->config.stdout_write == NULL) RET(VAR_NULL);
 
     if (closure->fn->docstring != NULL) {
-      vm->config.write_fn(vm, closure->fn->docstring);
-      vm->config.write_fn(vm, "\n\n");
+      vm->config.stdout_write(vm, closure->fn->docstring);
+      vm->config.stdout_write(vm, "\n\n");
     } else {
-      vm->config.write_fn(vm, "function '");
-      vm->config.write_fn(vm, closure->fn->name);
-      vm->config.write_fn(vm, "()' doesn't have a docstring.\n");
+      vm->config.stdout_write(vm, "function '");
+      vm->config.stdout_write(vm, closure->fn->name);
+      vm->config.stdout_write(vm, "()' doesn't have a docstring.\n");
     }
   }
 }
@@ -649,14 +649,14 @@ DEF(corePrint,
 
   // If the host application doesn't provide any write function, discard the
   // output.
-  if (vm->config.write_fn == NULL) return;
+  if (vm->config.stdout_write == NULL) return;
 
   for (int i = 1; i <= ARGC; i++) {
-    if (i != 1) vm->config.write_fn(vm, " ");
-    vm->config.write_fn(vm, toString(vm, ARG(i))->data);
+    if (i != 1) vm->config.stdout_write(vm, " ");
+    vm->config.stdout_write(vm, toString(vm, ARG(i))->data);
   }
 
-  vm->config.write_fn(vm, "\n");
+  vm->config.stdout_write(vm, "\n");
 }
 
 DEF(coreInput,
@@ -670,13 +670,13 @@ DEF(coreInput,
   }
 
   // If the host application doesn't provide any write function, return.
-  if (vm->config.read_fn == NULL) return;
+  if (vm->config.stdin_read == NULL) return;
 
   if (argc == 1) {
-    vm->config.write_fn(vm, toString(vm, ARG(1))->data);
+    vm->config.stdout_write(vm, toString(vm, ARG(1))->data);
   }
 
-  PkStringPtr result = vm->config.read_fn(vm);
+  PkStringPtr result = vm->config.stdin_read(vm);
   String* line = newString(vm, result.string);
   if (result.on_done) result.on_done(vm, result);
   RET(VAR_OBJ(line));
@@ -920,7 +920,7 @@ DEF(stdLangWrite,
 
   // If the host application doesn't provide any write function, discard the
   // output.
-  if (vm->config.write_fn == NULL) return;
+  if (vm->config.stdout_write == NULL) return;
 
   String* str; //< Will be cleaned by garbage collector;
 
@@ -933,7 +933,7 @@ DEF(stdLangWrite,
       str = toString(vm, arg);
     }
 
-    vm->config.write_fn(vm, str->data);
+    vm->config.stdout_write(vm, str->data);
   }
 }
 
diff --git a/src/pk_debug.c b/src/pk_debug.c
index 2525731..2806081 100644
--- a/src/pk_debug.c
+++ b/src/pk_debug.c
@@ -11,6 +11,209 @@
 #include "pk_value.h"
 #include "pk_vm.h"
 
+// FIXME:
+// Refactor this. Maybe move to a module, Rgb values are hardcoded ?!
+// Should check stderr/stdout etc.
+static void _printRed(PKVM* vm, const char* msg) {
+  if (vm->config.use_ansi_color) {
+    vm->config.stderr_write(vm, "\033[38;2;220;100;100m");
+    vm->config.stderr_write(vm, msg);
+    vm->config.stderr_write(vm, "\033[0m");
+  } else {
+    vm->config.stderr_write(vm, msg);
+  }
+}
+
+void reportCompileTimeError(PKVM* vm, const char* path, int line,
+                            const char* source, const char* at, int length,
+                            const char* fmt, va_list args) {
+
+  pkWriteFn writefn = vm->config.stderr_write;
+  if (writefn == NULL) return;
+
+  pkByteBuffer buff;
+  pkByteBufferInit(&buff);
+  {
+    // Initial size set to 512. Will grow if needed.
+    pkByteBufferReserve(&buff, vm, 512);
+
+    buff.count = 0;
+    writefn(vm, path);
+    writefn(vm, ":");
+    snprintf((char*)buff.data, buff.capacity, "%d", line);
+    writefn(vm, (char*)buff.data);
+    _printRed(vm, " error: ");
+
+    // Print the error message.
+    buff.count = 0;
+    int size = vsnprintf(NULL, 0, fmt, args) + 1;
+    ASSERT(size >= 0, "vnsprintf() failed.");
+    pkByteBufferReserve(&buff, vm, size);
+    vsnprintf((char*)buff.data, size, fmt, args);
+    writefn(vm, (char*)buff.data);
+    writefn(vm, "\n");
+
+    // Print the lines. (TODO: Optimize it).
+
+    int start = line - 2; // First line.
+    if (start < 1) start = 1;
+    int end = start + 5; // Exclisive last line.
+
+    int line_number_width = 5;
+
+    int curr_line = line;
+    const char* c = at;
+
+    // Get the first character of the [start] line.
+    if (c != source) {
+      do {
+        c--;
+        if (*c == '\n') curr_line--;
+        if (c == source) break;
+      } while (curr_line >= start);
+    }
+
+    curr_line = start;
+    if (c != source) {
+      ASSERT(*c == '\n', OOPS);
+      c++; // Enter the line.
+    }
+
+    // Print each lines.
+    while (curr_line < end) {
+
+      buff.count = 0;
+      snprintf((char*)buff.data, buff.capacity,
+               "%*d", line_number_width, curr_line);
+      writefn(vm, (char*)buff.data);
+      writefn(vm, " | ");
+
+      if (curr_line != line) {
+        // Run to the line end.
+        const char* line_start = c;
+        while (*c != '\0' && *c != '\n') c++;
+
+        buff.count = 0;
+        pkByteBufferAddString(&buff, vm, line_start,
+                              (uint32_t)(c - line_start));
+        pkByteBufferWrite(&buff, vm, '\0');
+        writefn(vm, (char*)buff.data);
+        writefn(vm, "\n");
+
+      } else {
+
+        const char* line_start = c;
+
+        // Print line till error.
+        buff.count = 0;
+        pkByteBufferAddString(&buff, vm, line_start,
+                              (uint32_t)(at - line_start));
+        pkByteBufferWrite(&buff, vm, '\0');
+        writefn(vm, (char*)buff.data);
+
+        // Print error token - if the error token is a new line ignore it.
+        if (*at != '\n') {
+          buff.count = 0;
+          pkByteBufferAddString(&buff, vm, at, length);
+          pkByteBufferWrite(&buff, vm, '\0');
+          _printRed(vm, (char*)buff.data);
+
+          // Run to the line end. Note that tk.length is not reliable and
+          // sometimes longer than the actual string which will cause a
+          // buffer overflow.
+          const char* tail_start = at;
+          for (int i = 0; i < length; i++) {
+            if (*tail_start == '\0') break;
+            tail_start++;
+          }
+
+          c = tail_start;
+          while (*c != '\0' && *c != '\n') c++;
+
+          // Print rest of the line.
+          if (c != tail_start) {
+            buff.count = 0;
+            pkByteBufferAddString(&buff, vm, tail_start,
+                                  (uint32_t)(c - tail_start));
+            pkByteBufferWrite(&buff, vm, '\0');
+            writefn(vm, (char*)buff.data);
+          }
+        } else {
+          c = at; // Run 'c' to the end of the line.
+        }
+        writefn(vm, "\n");
+
+        // White space before error token.
+        buff.count = 0;
+        pkByteBufferFill(&buff, vm, ' ', line_number_width);
+        pkByteBufferAddString(&buff, vm, " | ", 3);
+
+        for (const char* c2 = line_start; c2 < at; c2++) {
+          char white_space = (*c2 == '\t') ? '\t' : ' ';
+          pkByteBufferWrite(&buff, vm, white_space);
+        }
+
+        pkByteBufferWrite(&buff, vm, '\0');
+        writefn(vm, (char*)buff.data);
+
+        // Error token underline.
+        buff.count = 0;
+        pkByteBufferFill(&buff, vm, '~', (uint32_t)(length ? length : 1));
+        pkByteBufferWrite(&buff, vm, '\0');
+        _printRed(vm, (char*)buff.data);
+        writefn(vm, "\n");
+
+      }
+
+      if (*c == '\0') break;
+      curr_line++; c++;
+    }
+  }
+  pkByteBufferClear(&buff, vm);
+}
+
+void reportRuntimeError(PKVM* vm, Fiber* fiber) {
+
+  pkWriteFn writefn = vm->config.stderr_write;
+  if (writefn == NULL) return;
+
+  // Error message.
+  _printRed(vm, "Error: ");
+  writefn(vm, fiber->error->data);
+  writefn(vm, "\n");
+
+  // Stack trace.
+  for (int i = fiber->frame_count - 1; i >= 0; i--) {
+    CallFrame* frame = &fiber->frames[i];
+    const Function* fn = frame->closure->fn;
+    ASSERT(!fn->is_native, OOPS);
+    int line = fn->fn->oplines.data[frame->ip - fn->fn->opcodes.data - 1];
+
+    if (fn->owner->path == NULL) {
+
+      writefn(vm, "  [at:");
+      char buff[STR_INT_BUFF_SIZE];
+      sprintf(buff, "%2d", line);
+      writefn(vm, buff);
+      writefn(vm, "] ");
+      writefn(vm, fn->name);
+      writefn(vm, "()\n");
+
+    } else {
+      writefn(vm, "  ");
+      writefn(vm, fn->name);
+      writefn(vm, "() [");
+      writefn(vm, fn->owner->path->data);
+      writefn(vm, ":");
+      char buff[STR_INT_BUFF_SIZE];
+      sprintf(buff, "%d", line);
+      writefn(vm, buff);
+      writefn(vm, "]\n");
+    }
+
+  }
+}
+
 // Opcode names array.
 static const char* op_names[] = {
   #define OPCODE(name, params, stack) #name,
@@ -19,20 +222,20 @@ static const char* op_names[] = {
 };
 
 static void dumpValue(PKVM* vm, Var value) {
-  if (!vm->config.write_fn) return;
+  if (!vm->config.stdout_write) return;
   String* repr = toRepr(vm, value);
-  vm->config.write_fn(vm, repr->data);
+  vm->config.stdout_write(vm, repr->data);
   // String repr will be garbage collected - No need to clean.
 }
 
 void dumpFunctionCode(PKVM* vm, Function* func) {
 
-  if (!vm->config.write_fn) return;
+  if (!vm->config.stdout_write) return;
 
 #define _INDENTATION "  "
 #define _INT_WIDTH 5 // Width of the integer string to print.
 
-#define PRINT(str) vm->config.write_fn(vm, str)
+#define PRINT(str) vm->config.stdout_write(vm, str)
 #define NEWLINE() PRINT("\n")
 
 #define _PRINT_INT(value, width)                                     \
diff --git a/src/pk_debug.h b/src/pk_debug.h
index 39703ae..879f724 100644
--- a/src/pk_debug.h
+++ b/src/pk_debug.h
@@ -10,6 +10,14 @@
 #include "pk_internal.h"
 #include "pk_value.h"
 
+// Pretty print compile time error.
+void reportCompileTimeError(PKVM* vm, const char* path, int line,
+                            const char* source, const char* at, int length,
+                            const char* fmt, va_list args);
+
+// Pretty print runtime error.
+void reportRuntimeError(PKVM* vm, Fiber* fiber);
+
 // Dump opcodes of the given function to the stdout.
 void dumpFunctionCode(PKVM* vm, Function* func);
 
diff --git a/src/pk_internal.h b/src/pk_internal.h
index 1ae739a..db1f9e0 100644
--- a/src/pk_internal.h
+++ b/src/pk_internal.h
@@ -36,6 +36,8 @@
 #elif defined(__clang__)
   #pragma clang diagnostic ignored "-Wint-to-pointer-cast"
   #pragma clang diagnostic ignored "-Wunused-parameter"
+#elif defined(_MSC_VER)
+  #pragma warning(disable:26812)
 #endif
 
 /*****************************************************************************/
diff --git a/src/pk_value.c b/src/pk_value.c
index b388137..52c0a4c 100644
--- a/src/pk_value.c
+++ b/src/pk_value.c
@@ -1630,6 +1630,8 @@ bool toBool(Var v) {
     case OBJ_RANGE: // [[FALLTHROUGH]]
     case OBJ_MODULE:
     case OBJ_FUNC:
+    case OBJ_CLOSURE:
+    case OBJ_UPVALUE:
     case OBJ_FIBER:
     case OBJ_CLASS:
     case OBJ_INST:
diff --git a/src/pk_vm.c b/src/pk_vm.c
index a9ab792..0471a89 100644
--- a/src/pk_vm.c
+++ b/src/pk_vm.c
@@ -27,12 +27,14 @@ PkConfiguration pkNewConfiguration(void) {
   PkConfiguration config;
   config.realloc_fn = defaultRealloc;
 
-  config.error_fn = NULL;
-  config.write_fn = NULL;
-  config.read_fn = NULL;
+  config.stdout_write = NULL;
+  config.stderr_write = NULL;
+  config.stdin_read = NULL;
 
   config.load_script_fn = NULL;
   config.resolve_path_fn = NULL;
+
+  config.use_ansi_color = false;
   config.user_data = NULL;
 
   return config;
@@ -674,25 +676,15 @@ static void closeUpvalues(Fiber* fiber, Var* top) {
 
     fiber->open_upvalues = upvalue->next;
   }
-
 }
 
 static void reportError(PKVM* vm) {
   ASSERT(VM_HAS_ERROR(vm), "runtimeError() should be called after an error.");
+
   // TODO: pass the error to the caller of the fiber.
 
-  // Print the Error message and stack trace.
-  if (vm->config.error_fn == NULL) return;
-  Fiber* fiber = vm->fiber;
-  vm->config.error_fn(vm, PK_ERROR_RUNTIME, NULL, -1, fiber->error->data);
-  for (int i = fiber->frame_count - 1; i >= 0; i--) {
-    CallFrame* frame = &fiber->frames[i];
-    const Function* fn = frame->closure->fn;
-    ASSERT(!fn->is_native, OOPS);
-    int line = fn->fn->oplines.data[frame->ip - fn->fn->opcodes.data - 1];
-    vm->config.error_fn(vm, PK_ERROR_STACKTRACE, fn->owner->path->data, line,
-                        fn->name);
-  }
+  if (vm->config.stderr_write == NULL) return;
+  reportRuntimeError(vm, vm->fiber);
 }
 
 /******************************************************************************
@@ -1701,11 +1693,11 @@ L_do_call:
 
     OPCODE(REPL_PRINT):
     {
-      if (vm->config.write_fn != NULL) {
+      if (vm->config.stdout_write != NULL) {
         Var tmp = PEEK(-1);
         if (!IS_NULL(tmp)) {
-          vm->config.write_fn(vm, toRepr(vm, tmp)->data);
-          vm->config.write_fn(vm, "\n");
+          vm->config.stdout_write(vm, toRepr(vm, tmp)->data);
+          vm->config.stdout_write(vm, "\n");
         }
       }
       DISPATCH();
diff --git a/tests/native/example1.c b/tests/native/example1.c
index 12446fa..43c7cb5 100644
--- a/tests/native/example1.c
+++ b/tests/native/example1.c
@@ -48,7 +48,7 @@ int main(int argc, char** argv) {
 
   // Pocket VM configuration.
   PkConfiguration config = pkNewConfiguration();
-  config.write_fn = stdoutCallback;
+  config.stdout_write = stdoutCallback;
 
   // Create a new pocket VM.
   PKVM* vm = pkNewVM(&config);
diff --git a/tests/native/example2.c b/tests/native/example2.c
index 2d0b238..2082648 100644
--- a/tests/native/example2.c
+++ b/tests/native/example2.c
@@ -96,7 +96,7 @@ void stdoutCallback(PKVM* vm, const char* text) {
 int main(int argc, char** argv) {
 
   PkConfiguration config = pkNewConfiguration();
-  config.write_fn = stdoutCallback;
+  config.stdout_write = stdoutCallback;
 
   PKVM* vm = pkNewVM(&config);
   registerVector(vm);