Merge branch 'colorize-err-msg'

.circleci/config.yml

@@ -12,7 +12,7 @@ jobs:
           command: |
               g++ --version
               cd tests/
-              g++ -std=c++11 -O2 -Wall -Wextra -Werror -I../ check_toml_test.cpp -o check_toml_test
+              g++ -std=c++11 -O2 -Wall -Wextra -Werror -DTOML11_COLORIZE_ERROR_MESSAGE -I../ check_toml_test.cpp -o check_toml_test
               go get github.com/BurntSushi/toml-test
               $GOPATH/bin/toml-test ./check_toml_test
   test_serialization:
@@ -24,7 +24,7 @@ jobs:
           command: |
               g++ --version
               cd tests/
-              g++ -std=c++11 -O2 -Wall -Wextra -Wpedantic -Werror -I../ check_serialization.cpp -o check_serialization
+              g++ -std=c++11 -O2 -Wall -Wextra -Wpedantic -Werror -DTOML11_COLORIZE_ERROR_MESSAGE -I../ check_serialization.cpp -o check_serialization
               git clone https://github.com/BurntSushi/toml-test.git
               cp check_serialization toml-test/tests/valid
               cd toml-test/tests/valid
@@ -47,7 +47,7 @@ jobs:
           command: |
               g++ --version
               cd tests/
-              g++ -std=c++11 -O2 -Wall -Wextra -Wpedantic -Werror -I../ check.cpp -o check
+              g++ -std=c++11 -O2 -Wall -Wextra -Wpedantic -Werror -DTOML11_COLORIZE_ERROR_MESSAGE -I../ check.cpp -o check
               git clone https://github.com/BurntSushi/toml-test.git
               cp check toml-test/tests/invalid
               cp check toml-test/tests/valid

README.md

@@ -70,6 +70,7 @@ int main()
 - [Formatting user-defined error messages](#formatting-user-defined-error-messages)
 - [Obtaining location information](#obtaining-location-information)
 - [Exceptions](#exceptions)
+- [Colorize Error Messages](#colorize-error-messages)
 - [Serializing TOML data](#serializing-toml-data)
 - [Underlying types](#underlying-types)
 - [Unreleased TOML features](#unreleased-toml-features)
@@ -1375,6 +1376,63 @@ struct exception : public std::exception
 
 It represents where the error occurs.
 
+## Colorize Error Messages
+
+By defining `TOML11_COLORIZE_ERROR_MESSAGE`, the error messages from
+`toml::parse` and `toml::find|get` will be colorized. By default, this feature
+is turned off.
+
+With the following toml file taken from `toml-lang/toml/tests/hard_example.toml`,
+
+```toml
+[error]
+array = [
+         "This might most likely happen in multiline arrays",
+         Like here,
+         "or here,
+         and here"
+        ]     End of array comment, forgot the #
+```
+
+the error message would be like this.
+
+![error-message-1](https://github.com/ToruNiina/toml11/blob/misc/misc/toml11-err-msg-1.png)
+
+With the following,
+
+```toml
+[error]
+# array = [
+#          "This might most likely happen in multiline arrays",
+#          Like here,
+#          "or here,
+#          and here"
+#         ]     End of array comment, forgot the #
+number = 3.14  pi <--again forgot the #
+```
+
+the error message would be like this.
+
+![error-message-2](https://github.com/ToruNiina/toml11/blob/misc/misc/toml11-err-msg-2.png)
+
+The message would be messy when it is written to a file, not a terminal because
+it uses [ANSI escape code](https://en.wikipedia.org/wiki/ANSI_escape_code).
+
+Without `TOML11_COLORIZE_ERROR_MESSAGE`, you can still colorize user-defined
+error message by passing `true` to the `toml::format_error` function.
+If you define `TOML11_COLORIZE_ERROR_MESSAGE`, the value is `true` by default.
+If not, the defalut value would be `false`.
+
+```cpp
+std::cerr << toml::format_error("[error] value should be positive",
+                                data.at("num"), "positive number required",
+                                hints, /*colorize = */ true) << std::endl;
+```
+
+Note: It colorize `[error]` in red. That means that it detects `[error]` prefix
+at the front of the error message. If there is no `[error]` prefix,
+`format_error` adds it to the error message.
+
 ## Serializing TOML data
 
 toml11 enables you to serialize data into toml format.

toml/color.hpp

@@ -0,0 +1,64 @@
+#ifndef TOML11_COLOR_HPP
+#define TOML11_COLOR_HPP
+#include <ostream>
+#include <cstdint>
+
+#ifdef TOML11_COLORIZE_ERROR_MESSAGE
+#define TOML11_ERROR_MESSAGE_COLORIZED true
+#else
+#define TOML11_ERROR_MESSAGE_COLORIZED false
+#endif
+
+namespace toml
+{
+
+// put ANSI escape sequence to ostream
+namespace color_ansi
+{
+namespace detail
+{
+inline int colorize_index()
+{
+    static const int index = std::ios_base::xalloc();
+    return index;
+}
+} // detail
+
+inline std::ostream& colorize(std::ostream& os)
+{
+    // by default, it is zero.
+    os.iword(detail::colorize_index()) = 1;
+    return os;
+}
+inline std::ostream& nocolorize(std::ostream& os)
+{
+    os.iword(detail::colorize_index()) = 0;
+    return os;
+}
+inline std::ostream& reset  (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[00m";} return os;}
+inline std::ostream& bold   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[01m";} return os;}
+inline std::ostream& grey   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[30m";} return os;}
+inline std::ostream& red    (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[31m";} return os;}
+inline std::ostream& green  (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[32m";} return os;}
+inline std::ostream& yellow (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[33m";} return os;}
+inline std::ostream& blue   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[34m";} return os;}
+inline std::ostream& magenta(std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[35m";} return os;}
+inline std::ostream& cyan   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[36m";} return os;}
+inline std::ostream& white  (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[37m";} return os;}
+} // color_ansi
+
+// ANSI escape sequence is the only and default colorization method currently
+namespace color = color_ansi;
+
+} // toml
+#endif// TOML11_COLOR_HPP

toml/get.hpp

@@ -186,7 +186,7 @@ get(const basic_value<C, M, V>& v)
         }
         default:
         {
-            throw type_error(detail::format_underline("[error] toml::value "
+            throw type_error(detail::format_underline("toml::value: "
                 "bad_cast to std::chrono::system_clock::time_point", {
                     {std::addressof(detail::get_region(v)),
                      concat_to_string("the actual type is ", v.type())}
@@ -301,7 +301,7 @@ get(const basic_value<C, M, V>& v)
     if(ar.size() != container.size())
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[erorr] toml::get specified container size is ", container.size(),
+            "toml::get: specified container size is ", container.size(),
             " but there are ", ar.size(), " elements in toml array."), {
                 {std::addressof(detail::get_region(v)), "here"}
             }));
@@ -326,7 +326,7 @@ get(const basic_value<C, M, V>& v)
     if(ar.size() != 2)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[erorr] toml::get specified std::pair but there are ", ar.size(),
+            "toml::get: specified std::pair but there are ", ar.size(),
             " elements in toml array."), {
                 {std::addressof(detail::get_region(v)), "here"}
             }));
@@ -357,7 +357,7 @@ get(const basic_value<C, M, V>& v)
     if(ar.size() != std::tuple_size<T>::value)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] toml::get specified std::tuple with ",
+            "toml::get: specified std::tuple with ",
             std::tuple_size<T>::value, " elements, but there are ", ar.size(),
             " elements in toml array."), {
                 {std::addressof(detail::get_region(v)), "here"}
@@ -430,7 +430,7 @@ basic_value<C, M, V> const& find(const basic_value<C, M, V>& v, const key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), {
+            "key \"", ky, "\" not found"), {
                 {std::addressof(detail::get_region(v)), "in this table"}
             }));
     }
@@ -444,7 +444,7 @@ basic_value<C, M, V>& find(basic_value<C, M, V>& v, const key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), {
+            "key \"", ky, "\" not found"), {
                 {std::addressof(detail::get_region(v)), "in this table"}
             }));
     }
@@ -458,7 +458,7 @@ basic_value<C, M, V> find(basic_value<C, M, V>&& v, const key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), {
+            "key \"", ky, "\" not found"), {
                 {std::addressof(detail::get_region(v)), "in this table"}
             }));
     }
@@ -477,7 +477,7 @@ find(const basic_value<C, M, V>& v, const key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), {
+            "key \"", ky, "\" not found"), {
                 {std::addressof(detail::get_region(v)), "in this table"}
             }));
     }
@@ -493,7 +493,7 @@ find(basic_value<C, M, V>& v, const key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), {
+            "key \"", ky, "\" not found"), {
                 {std::addressof(detail::get_region(v)), "in this table"}
             }));
     }
@@ -509,7 +509,7 @@ find(basic_value<C, M, V>&& v, const key& ky)
     if(tab.count(ky) == 0)
     {
         throw std::out_of_range(detail::format_underline(concat_to_string(
-            "[error] key \"", ky, "\" not found"), {
+            "key \"", ky, "\" not found"), {
                 {std::addressof(detail::get_region(v)), "in this table"}
             }));
     }

toml/parser.hpp

@@ -30,13 +30,13 @@ parse_boolean(location<Container>& loc)
         else // internal error.
         {
             throw internal_error(format_underline(
-                "[error] toml::parse_boolean: internal error",
+                "toml::parse_boolean: internal error",
                 {{std::addressof(reg), "invalid token"}}),
                 source_location(std::addressof(reg)));
         }
     }
     loc.reset(first); //rollback
-    return err(format_underline("[error] toml::parse_boolean: ",
+    return err(format_underline("toml::parse_boolean: ",
                {{std::addressof(loc), "the next token is not a boolean"}}));
 }
 
@@ -58,7 +58,7 @@ parse_binary_integer(location<Container>& loc)
             else // internal error.
             {
                 throw internal_error(format_underline(
-                    "[error] toml::parse_integer: internal error",
+                    "toml::parse_integer: internal error",
                     {{std::addressof(token.unwrap()), "invalid token"}}),
                     source_location(std::addressof(loc)));
             }
@@ -66,7 +66,7 @@ parse_binary_integer(location<Container>& loc)
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.reset(first);
-    return err(format_underline("[error] toml::parse_binary_integer:",
+    return err(format_underline("toml::parse_binary_integer:",
                {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
@@ -87,7 +87,7 @@ parse_octal_integer(location<Container>& loc)
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.reset(first);
-    return err(format_underline("[error] toml::parse_octal_integer:",
+    return err(format_underline("toml::parse_octal_integer:",
                {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
@@ -108,7 +108,7 @@ parse_hexadecimal_integer(location<Container>& loc)
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.reset(first);
-    return err(format_underline("[error] toml::parse_hexadecimal_integer",
+    return err(format_underline("toml::parse_hexadecimal_integer",
                {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
@@ -131,13 +131,13 @@ parse_integer(location<Container>& loc)
 
         if(std::isdigit(*second))
         {
-            return err(format_underline("[error] toml::parse_integer: "
+            return err(format_underline("toml::parse_integer: "
                 "leading zero in an Integer is not allowed.",
                 {{std::addressof(loc), "leading zero"}}));
         }
         else if(std::isalpha(*second))
         {
-             return err(format_underline("[error] toml::parse_integer: "
+             return err(format_underline("toml::parse_integer: "
                 "unknown integer prefix appeared.",
                 {{std::addressof(loc), "none of 0x, 0o, 0b"}}));
         }
@@ -154,7 +154,7 @@ parse_integer(location<Container>& loc)
         return ok(std::make_pair(retval, token.unwrap()));
     }
     loc.reset(first);
-    return err(format_underline("[error] toml::parse_integer: ",
+    return err(format_underline("toml::parse_integer: ",
                {{std::addressof(loc), "the next token is not an integer"}}));
 }
 
@@ -243,7 +243,7 @@ parse_floating(location<Container>& loc)
         return ok(std::make_pair(v, token.unwrap()));
     }
     loc.reset(first);
-    return err(format_underline("[error] toml::parse_floating: ",
+    return err(format_underline("toml::parse_floating: ",
                {{std::addressof(loc), "the next token is not a float"}}));
 }
 
@@ -276,7 +276,7 @@ std::string read_utf8_codepoint(const region<Container>& reg,
     {
         if(0xD800 <= codepoint && codepoint <= 0xDFFF)
         {
-            throw syntax_error(format_underline("[error] "
+            throw syntax_error(format_underline(
                 "toml::read_utf8_codepoint: codepoints in the range "
                 "[0xD800, 0xDFFF] are not valid UTF-8.", {{
                     std::addressof(loc), "not a valid UTF-8 codepoint"
@@ -298,7 +298,7 @@ std::string read_utf8_codepoint(const region<Container>& reg,
     }
     else // out of UTF-8 region
     {
-        throw syntax_error(format_underline("[error] toml::read_utf8_codepoint:"
+        throw syntax_error(format_underline("toml::read_utf8_codepoint:"
             " input codepoint is too large.",
             {{std::addressof(loc), "should be in [0x00..0x10FFFF]"}}),
             source_location(std::addressof(loc)));
@@ -312,7 +312,7 @@ result<std::string, std::string> parse_escape_sequence(location<Container>& loc)
     const auto first = loc.iter();
     if(first == loc.end() || *first != '\\')
     {
-        return err(format_underline("[error]: toml::parse_escape_sequence: ", {{
+        return err(format_underline("toml::parse_escape_sequence: ", {{
             std::addressof(loc), "the next token is not a backslash \"\\\""}}));
     }
     loc.advance();
@@ -333,7 +333,7 @@ result<std::string, std::string> parse_escape_sequence(location<Container>& loc)
             }
             else
             {
-                return err(format_underline("[error] parse_escape_sequence: "
+                return err(format_underline("parse_escape_sequence: "
                            "invalid token found in UTF-8 codepoint uXXXX.",
                            {{std::addressof(loc), "here"}}));
             }
@@ -346,14 +346,14 @@ result<std::string, std::string> parse_escape_sequence(location<Container>& loc)
             }
             else
             {
-                return err(format_underline("[error] parse_escape_sequence: "
+                return err(format_underline("parse_escape_sequence: "
                            "invalid token found in UTF-8 codepoint Uxxxxxxxx",
                            {{std::addressof(loc), "here"}}));
             }
         }
     }
 
-    const auto msg = format_underline("[error] parse_escape_sequence: "
+    const auto msg = format_underline("parse_escape_sequence: "
            "unknown escape sequence appeared.", {{std::addressof(loc),
            "escape sequence is one of \\, \", b, t, n, f, r, uxxxx, Uxxxxxxxx"}},
            /* Hints = */{"if you want to write backslash as just one backslash, "
@@ -378,7 +378,7 @@ parse_ml_basic_string(location<Container>& loc)
         auto delim = lex_ml_basic_string_delim::invoke(inner_loc);
         if(!delim)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "parse_ml_basic_string: invalid token",
                 {{std::addressof(inner_loc), "should be \"\"\""}}),
                 source_location(std::addressof(inner_loc)));
@@ -405,7 +405,7 @@ parse_ml_basic_string(location<Container>& loc)
             }
             if(inner_loc.iter() == inner_loc.end())
             {
-                throw internal_error(format_underline("[error] "
+                throw internal_error(format_underline(
                     "parse_ml_basic_string: unexpected end of region",
                     {{std::addressof(inner_loc), "not sufficient token"}}),
                     source_location(std::addressof(inner_loc)));
@@ -417,7 +417,7 @@ parse_ml_basic_string(location<Container>& loc)
     else
     {
         loc.reset(first);
-        return err(format_underline("[error] toml::parse_ml_basic_string: "
+        return err(format_underline("toml::parse_ml_basic_string: "
                    "the next token is not a valid multiline string",
                    {{std::addressof(loc), "here"}}));
     }
@@ -436,7 +436,7 @@ parse_basic_string(location<Container>& loc)
         auto quot = lex_quotation_mark::invoke(inner_loc);
         if(!quot)
         {
-            throw internal_error(format_underline("[error] parse_basic_string: "
+            throw internal_error(format_underline("parse_basic_string: "
                 "invalid token", {{std::addressof(inner_loc), "should be \""}}),
                 source_location(std::addressof(inner_loc)));
         }
@@ -458,7 +458,7 @@ parse_basic_string(location<Container>& loc)
             }
             if(inner_loc.iter() == inner_loc.end())
             {
-                throw internal_error(format_underline("[error] "
+                throw internal_error(format_underline(
                     "parse_basic_string: unexpected end of region",
                     {{std::addressof(inner_loc), "not sufficient token"}}),
                     source_location(std::addressof(inner_loc)));
@@ -470,7 +470,7 @@ parse_basic_string(location<Container>& loc)
     else
     {
         loc.reset(first); // rollback
-        return err(format_underline("[error] toml::parse_basic_string: "
+        return err(format_underline("toml::parse_basic_string: "
                    "the next token is not a valid string",
                    {{std::addressof(loc), "here"}}));
     }
@@ -488,7 +488,7 @@ parse_ml_literal_string(location<Container>& loc)
         const auto open = lex_ml_literal_string_delim::invoke(inner_loc);
         if(!open)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "parse_ml_literal_string: invalid token",
                 {{std::addressof(inner_loc), "should be '''"}}),
                 source_location(std::addressof(inner_loc)));
@@ -501,7 +501,7 @@ parse_ml_literal_string(location<Container>& loc)
         const auto close = lex_ml_literal_string_delim::invoke(inner_loc);
         if(!close)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "parse_ml_literal_string: invalid token",
                 {{std::addressof(inner_loc), "should be '''"}}),
                 source_location(std::addressof(inner_loc)));
@@ -513,7 +513,7 @@ parse_ml_literal_string(location<Container>& loc)
     else
     {
         loc.reset(first); // rollback
-        return err(format_underline("[error] toml::parse_ml_literal_string: "
+        return err(format_underline("toml::parse_ml_literal_string: "
                    "the next token is not a valid multiline literal string",
                    {{std::addressof(loc), "here"}}));
     }
@@ -531,7 +531,7 @@ parse_literal_string(location<Container>& loc)
         const auto open = lex_apostrophe::invoke(inner_loc);
         if(!open)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "parse_literal_string: invalid token",
                 {{std::addressof(inner_loc), "should be '"}}),
                 source_location(std::addressof(inner_loc)));
@@ -542,7 +542,7 @@ parse_literal_string(location<Container>& loc)
         const auto close = lex_apostrophe::invoke(inner_loc);
         if(!close)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "parse_literal_string: invalid token",
                 {{std::addressof(inner_loc), "should be '"}}),
                 source_location(std::addressof(inner_loc)));
@@ -554,7 +554,7 @@ parse_literal_string(location<Container>& loc)
     else
     {
         loc.reset(first); // rollback
-        return err(format_underline("[error] toml::parse_literal_string: "
+        return err(format_underline("toml::parse_literal_string: "
                    "the next token is not a valid literal string",
                    {{std::addressof(loc), "here"}}));
     }
@@ -588,7 +588,7 @@ parse_string(location<Container>& loc)
             return parse_literal_string(loc);
         }
     }
-    return err(format_underline("[error] toml::parse_string: ",
+    return err(format_underline("toml::parse_string: ",
                 {{std::addressof(loc), "the next token is not a string"}}));
 }
 
@@ -604,7 +604,7 @@ parse_local_date(location<Container>& loc)
         const auto y = lex_date_fullyear::invoke(inner_loc);
         if(!y || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_inner_local_date: invalid year format",
                 {{std::addressof(inner_loc), "should be `-`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -613,7 +613,7 @@ parse_local_date(location<Container>& loc)
         const auto m = lex_date_month::invoke(inner_loc);
         if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_date: invalid month format",
                 {{std::addressof(inner_loc), "should be `-`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -622,7 +622,7 @@ parse_local_date(location<Container>& loc)
         const auto d = lex_date_mday::invoke(inner_loc);
         if(!d)
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_date: invalid day format",
                 {{std::addressof(inner_loc), "here"}}),
                 source_location(std::addressof(inner_loc)));
@@ -637,7 +637,7 @@ parse_local_date(location<Container>& loc)
     else
     {
         loc.reset(first);
-        return err(format_underline("[error]: toml::parse_local_date: ",
+        return err(format_underline("toml::parse_local_date: ",
             {{std::addressof(loc), "the next token is not a local_date"}}));
     }
 }
@@ -654,7 +654,7 @@ parse_local_time(location<Container>& loc)
         const auto h = lex_time_hour::invoke(inner_loc);
         if(!h || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_time: invalid year format",
                 {{std::addressof(inner_loc), "should be `:`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -663,7 +663,7 @@ parse_local_time(location<Container>& loc)
         const auto m = lex_time_minute::invoke(inner_loc);
         if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_time: invalid month format",
                 {{std::addressof(inner_loc), "should be `:`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -672,7 +672,7 @@ parse_local_time(location<Container>& loc)
         const auto s = lex_time_second::invoke(inner_loc);
         if(!s)
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_time: invalid second format",
                 {{std::addressof(inner_loc), "here"}}),
                 source_location(std::addressof(inner_loc)));
@@ -709,7 +709,7 @@ parse_local_time(location<Container>& loc)
         {
             if(before_secfrac != inner_loc.iter())
             {
-                throw internal_error(format_underline("[error]: "
+                throw internal_error(format_underline(
                     "toml::parse_local_time: invalid subsecond format",
                     {{std::addressof(inner_loc), "here"}}),
                 source_location(std::addressof(inner_loc)));
@@ -720,7 +720,7 @@ parse_local_time(location<Container>& loc)
     else
     {
         loc.reset(first);
-        return err(format_underline("[error]: toml::parse_local_time: ",
+        return err(format_underline("toml::parse_local_time: ",
             {{std::addressof(loc), "the next token is not a local_time"}}));
     }
 }
@@ -736,7 +736,7 @@ parse_local_datetime(location<Container>& loc)
         const auto date = parse_local_date(inner_loc);
         if(!date || inner_loc.iter() == inner_loc.end())
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_datetime: invalid datetime format",
                 {{std::addressof(inner_loc), "date, not datetime"}}),
                 source_location(std::addressof(inner_loc)));
@@ -744,7 +744,7 @@ parse_local_datetime(location<Container>& loc)
         const char delim = *(inner_loc.iter());
         if(delim != 'T' && delim != 't' && delim != ' ')
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_datetime: invalid datetime format",
                 {{std::addressof(inner_loc), "should be `T` or ` ` (space)"}}),
                 source_location(std::addressof(inner_loc)));
@@ -753,7 +753,7 @@ parse_local_datetime(location<Container>& loc)
         const auto time = parse_local_time(inner_loc);
         if(!time)
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_local_datetime: invalid datetime format",
                 {{std::addressof(inner_loc), "invalid time fomrat"}}),
                 source_location(std::addressof(inner_loc)));
@@ -765,7 +765,7 @@ parse_local_datetime(location<Container>& loc)
     else
     {
         loc.reset(first);
-        return err(format_underline("[error]: toml::parse_local_datetime: ",
+        return err(format_underline("toml::parse_local_datetime: ",
             {{std::addressof(loc), "the next token is not a local_datetime"}}));
     }
 }
@@ -781,7 +781,7 @@ parse_offset_datetime(location<Container>& loc)
         const auto datetime = parse_local_datetime(inner_loc);
         if(!datetime || inner_loc.iter() == inner_loc.end())
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_offset_datetime: invalid datetime format",
                 {{std::addressof(inner_loc), "date, not datetime"}}),
                 source_location(std::addressof(inner_loc)));
@@ -803,7 +803,7 @@ parse_offset_datetime(location<Container>& loc)
         }
         else if(*inner_loc.iter() != 'Z' && *inner_loc.iter() != 'z')
         {
-            throw internal_error(format_underline("[error]: "
+            throw internal_error(format_underline(
                 "toml::parse_offset_datetime: invalid datetime format",
                 {{std::addressof(inner_loc), "should be `Z` or `+HH:MM`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -814,7 +814,7 @@ parse_offset_datetime(location<Container>& loc)
     else
     {
         loc.reset(first);
-        return err(format_underline("[error]: toml::parse_offset_datetime: ",
+        return err(format_underline("toml::parse_offset_datetime: ",
             {{std::addressof(loc), "the next token is not a offset_datetime"}}));
     }
 }
@@ -836,7 +836,7 @@ parse_simple_key(location<Container>& loc)
         const auto reg = bare.unwrap();
         return ok(std::make_pair(reg.str(), reg));
     }
-    return err(format_underline("[error] toml::parse_simple_key: ",
+    return err(format_underline("toml::parse_simple_key: ",
             {{std::addressof(loc), "the next token is not a simple key"}}));
 }
 
@@ -862,7 +862,7 @@ parse_key(location<Container>& loc)
             }
             else
             {
-                throw internal_error(format_underline("[error] "
+                throw internal_error(format_underline(
                     "toml::detail::parse_key: dotted key contains invalid key",
                     {{std::addressof(inner_loc), k.unwrap_err()}}),
                     source_location(std::addressof(inner_loc)));
@@ -879,7 +879,7 @@ parse_key(location<Container>& loc)
             }
             else
             {
-                throw internal_error(format_underline("[error] toml::parse_key: "
+                throw internal_error(format_underline("toml::parse_key: "
                     "dotted key contains invalid key ",
                     {{std::addressof(inner_loc), "should be `.`"}}),
                     source_location(std::addressof(inner_loc)));
@@ -895,7 +895,7 @@ parse_key(location<Container>& loc)
         return ok(std::make_pair(std::vector<key>(1, smpl.unwrap().first),
                                  smpl.unwrap().second));
     }
-    return err(format_underline("[error] toml::parse_key: ",
+    return err(format_underline("toml::parse_key: ",
                 {{std::addressof(loc), "is not a valid key"}}));
 }
 
@@ -913,11 +913,11 @@ parse_array(location<Container>& loc)
     const auto first = loc.iter();
     if(loc.iter() == loc.end())
     {
-        return err("[error] toml::parse_array: input is empty");
+        return err("toml::parse_array: input is empty");
     }
     if(*loc.iter() != '[')
     {
-        return err("[error] toml::parse_array: token is not an array");
+        return err("toml::parse_array: token is not an array");
     }
     loc.advance();
 
@@ -944,7 +944,7 @@ parse_array(location<Container>& loc)
                 auto array_start_loc = loc;
                 array_start_loc.reset(first);
 
-                throw syntax_error(format_underline("[error] toml::parse_array: "
+                throw syntax_error(format_underline("toml::parse_array: "
                     "type of elements should be the same each other.", {
                         {std::addressof(array_start_loc), "array starts here"},
                         {
@@ -965,7 +965,7 @@ parse_array(location<Container>& loc)
             auto array_start_loc = loc;
             array_start_loc.reset(first);
 
-            throw syntax_error(format_underline("[error] toml::parse_array: "
+            throw syntax_error(format_underline("toml::parse_array: "
                 "value having invalid format appeared in an array", {
                     {std::addressof(array_start_loc), "array starts here"},
                     {std::addressof(loc), "it is not a valid value."}
@@ -988,7 +988,7 @@ parse_array(location<Container>& loc)
                 auto array_start_loc = loc;
                 array_start_loc.reset(first);
 
-                throw syntax_error(format_underline("[error] toml::parse_array:"
+                throw syntax_error(format_underline("toml::parse_array:"
                     " missing array separator `,` after a value", {
                         {std::addressof(array_start_loc), "array starts here"},
                         {std::addressof(loc),             "should be `,`"}
@@ -997,7 +997,7 @@ parse_array(location<Container>& loc)
         }
     }
     loc.reset(first);
-    throw syntax_error(format_underline("[error] toml::parse_array: "
+    throw syntax_error(format_underline("toml::parse_array: "
             "array did not closed by `]`",
             {{std::addressof(loc), "should be closed"}}),
             source_location(std::addressof(loc)));
@@ -1019,7 +1019,7 @@ parse_key_value_pair(location<Container>& loc)
         if(const auto keyval_sep = lex_keyval_sep::invoke(loc))
         {
             loc.reset(first);
-            msg = format_underline("[error] toml::parse_key_value_pair: "
+            msg = format_underline("toml::parse_key_value_pair: "
                 "empty key is not allowed.",
                 {{std::addressof(loc), "key expected before '='"}});
         }
@@ -1035,7 +1035,7 @@ parse_key_value_pair(location<Container>& loc)
         const auto line_end = std::find(loc.iter(), loc.end(), '\n');
         if(std::find(loc.iter(), line_end, '=') != line_end)
         {
-            msg = format_underline("[error] toml::parse_key_value_pair: "
+            msg = format_underline("toml::parse_key_value_pair: "
                 "invalid format for key",
                 {{std::addressof(loc), "invalid character in key"}},
                 {"Did you forget '.' to separate dotted-key?",
@@ -1043,7 +1043,7 @@ parse_key_value_pair(location<Container>& loc)
         }
         else // if not, the error is lack of key-value separator.
         {
-            msg = format_underline("[error] toml::parse_key_value_pair: "
+            msg = format_underline("toml::parse_key_value_pair: "
                 "missing key-value separator `=`",
                 {{std::addressof(loc), "should be `=`"}});
         }
@@ -1061,7 +1061,7 @@ parse_key_value_pair(location<Container>& loc)
         if(sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>::invoke(loc))
         {
             loc.reset(after_kvsp);
-            msg = format_underline("[error] toml::parse_key_value_pair: "
+            msg = format_underline("toml::parse_key_value_pair: "
                     "missing value after key-value separator '='",
                     {{std::addressof(loc), "expected value, but got nothing"}});
         }
@@ -1196,7 +1196,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                     {
                         // show special err msg for conflicting table
                         throw syntax_error(format_underline(concat_to_string(
-                            "[error] toml::insert_value: array of table (\"",
+                            "toml::insert_value: array of table (\"",
                             format_dotted_keys(first, last),
                             "\") cannot be defined"), {
                                 {std::addressof(get_region(tab->at(k))),
@@ -1208,7 +1208,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                     else if(!(tab->at(k).is_array()))
                     {
                         throw syntax_error(format_underline(concat_to_string(
-                            "[error] toml::insert_value: array of table (\"",
+                            "toml::insert_value: array of table (\"",
                             format_dotted_keys(first, last), "\") collides with"
                             " existing value"), {
                                 {std::addressof(get_region(tab->at(k))),
@@ -1223,7 +1223,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                     if(!(a.front().is_table()))
                     {
                         throw syntax_error(format_underline(concat_to_string(
-                            "[error] toml::insert_value: array of table (\"",
+                            "toml::insert_value: array of table (\"",
                             format_dotted_keys(first, last), "\") collides with"
                             " existing value"), {
                                 {std::addressof(get_region(tab->at(k))),
@@ -1251,7 +1251,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                     if(detail::get_region(a.front()).str().substr(0,2) != "[[")
                     {
                         throw syntax_error(format_underline(concat_to_string(
-                            "[error] toml::insert_value: array of table (\"",
+                            "toml::insert_value: array of table (\"",
                             format_dotted_keys(first, last), "\") collides with"
                             " existing array-of-tables"), {
                                 {std::addressof(get_region(tab->at(k))),
@@ -1280,7 +1280,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                                 tab->at(k), first, iter, last))
                     {
                         throw syntax_error(format_underline(concat_to_string(
-                            "[error] toml::insert_value: table (\"",
+                            "toml::insert_value: table (\"",
                             format_dotted_keys(first, last),
                             "\") already exists."), {
                                 {std::addressof(get_region(tab->at(k))),
@@ -1308,7 +1308,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                         tab->at(k).as_array().front().is_table())
                 {
                     throw syntax_error(format_underline(concat_to_string(
-                        "[error] toml::insert_value: array of tables (\"",
+                        "toml::insert_value: array of tables (\"",
                         format_dotted_keys(first, last), "\") already exists."), {
                             {std::addressof(get_region(tab->at(k))),
                              "array of tables defined here"},
@@ -1319,7 +1319,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                 else
                 {
                     throw syntax_error(format_underline(concat_to_string(
-                        "[error] toml::insert_value: value (\"",
+                        "toml::insert_value: value (\"",
                         format_dotted_keys(first, last), "\") already exists."), {
                             {std::addressof(get_region(tab->at(k))),
                              "value already exists here"},
@@ -1355,7 +1355,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
                 if(!a.back().is_table())
                 {
                     throw syntax_error(format_underline(concat_to_string(
-                        "[error] toml::insert_value: target (",
+                        "toml::insert_value: target (",
                         format_dotted_keys(first, std::next(iter)),
                         ") is neither table nor an array of tables"), {
                             {std::addressof(get_region(a.back())),
@@ -1368,7 +1368,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
             else
             {
                 throw syntax_error(format_underline(concat_to_string(
-                    "[error] toml::insert_value: target (",
+                    "toml::insert_value: target (",
                     format_dotted_keys(first, std::next(iter)),
                     ") is neither table nor an array of tables"), {
                         {std::addressof(get_region(tab->at(k))),
@@ -1392,7 +1392,7 @@ parse_inline_table(location<Container>& loc)
     table_type retval;
     if(!(loc.iter() != loc.end() && *loc.iter() == '{'))
     {
-        return err(format_underline("[error] toml::parse_inline_table: ",
+        return err(format_underline("toml::parse_inline_table: ",
             {{std::addressof(loc), "the next token is not an inline table"}}));
     }
     loc.advance();
@@ -1421,7 +1421,7 @@ parse_inline_table(location<Container>& loc)
             insert_nested_key(retval, val, keys.begin(), keys.end(), key_reg);
         if(!inserted)
         {
-            throw internal_error("[error] toml::parse_inline_table: "
+            throw internal_error("toml::parse_inline_table: "
                 "failed to insert value into table: " + inserted.unwrap_err(),
                 source_location(std::addressof(loc)));
         }
@@ -1439,14 +1439,14 @@ parse_inline_table(location<Container>& loc)
             }
             else if(*loc.iter() == '#' || *loc.iter() == '\r' || *loc.iter() == '\n')
             {
-                throw syntax_error(format_underline("[error] "
+                throw syntax_error(format_underline(
                     "toml::parse_inline_table: missing curly brace `}`",
                     {{std::addressof(loc), "should be `}`"}}),
                     source_location(std::addressof(loc)));
             }
             else
             {
-                throw syntax_error(format_underline("[error] "
+                throw syntax_error(format_underline(
                     "toml::parse_inline_table: missing table separator `,` ",
                     {{std::addressof(loc), "should be `,`"}}),
                     source_location(std::addressof(loc)));
@@ -1454,7 +1454,7 @@ parse_inline_table(location<Container>& loc)
         }
     }
     loc.reset(first);
-    throw syntax_error(format_underline("[error] toml::parse_inline_table: "
+    throw syntax_error(format_underline("toml::parse_inline_table: "
             "inline table did not closed by `}`",
             {{std::addressof(loc), "should be closed"}}),
             source_location(std::addressof(loc)));
@@ -1480,7 +1480,7 @@ result<value_t, std::string> guess_number_type(const location<Container>& l)
         if(loc.iter() != loc.end() && (*loc.iter() == '+' || *loc.iter() == '-'
                     || *loc.iter() == 'Z' || *loc.iter() == 'z'))
         {
-            return err(format_underline("[error] bad offset: should be [+-]HH:MM or Z",
+            return err(format_underline("bad offset: should be [+-]HH:MM or Z",
                         {{std::addressof(loc), "[+-]HH:MM or Z"}},
                         {"pass: +09:00, -05:30", "fail: +9:00, -5:30"}));
         }
@@ -1500,14 +1500,14 @@ result<value_t, std::string> guess_number_type(const location<Container>& l)
             const auto c = *loc.iter();
             if(c == 'T' || c == 't')
             {
-                return err(format_underline("[error] bad time: should be HH:MM:SS.subsec",
+                return err(format_underline("bad time: should be HH:MM:SS.subsec",
                         {{std::addressof(loc), "HH:MM:SS.subsec"}},
                         {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
                          "fail: 1979-05-27T7:32:00, 1979-05-27 17:32"}));
             }
             if('0' <= c && c <= '9')
             {
-                return err(format_underline("[error] bad time: missing T",
+                return err(format_underline("bad time: missing T",
                         {{std::addressof(loc), "T or space required here"}},
                         {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
                          "fail: 1979-05-27T7:32:00, 1979-05-27 7:32"}));
@@ -1516,7 +1516,7 @@ result<value_t, std::string> guess_number_type(const location<Container>& l)
                 ('0' <= *std::next(loc.iter()) && *std::next(loc.iter())<= '9'))
             {
                 loc.advance();
-                return err(format_underline("[error] bad time: should be HH:MM:SS.subsec",
+                return err(format_underline("bad time: should be HH:MM:SS.subsec",
                         {{std::addressof(loc), "HH:MM:SS.subsec"}},
                         {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
                          "fail: 1979-05-27T7:32:00, 1979-05-27 7:32"}));
@@ -1533,7 +1533,7 @@ result<value_t, std::string> guess_number_type(const location<Container>& l)
     {
         if(loc.iter() != loc.end() && *loc.iter() == '_')
         {
-            return err(format_underline("[error] bad float: `_` should be surrounded by digits",
+            return err(format_underline("bad float: `_` should be surrounded by digits",
                         {{std::addressof(loc), "here"}},
                         {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
                          "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
@@ -1549,7 +1549,7 @@ result<value_t, std::string> guess_number_type(const location<Container>& l)
             const auto c = *loc.iter();
             if(c == '_')
             {
-                return err(format_underline("[error] bad integer: `_` should be surrounded by digits",
+                return err(format_underline("bad integer: `_` should be surrounded by digits",
                             {{std::addressof(loc), "here"}},
                             {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
                              "fail: 1__000, 0123"}));
@@ -1558,21 +1558,21 @@ result<value_t, std::string> guess_number_type(const location<Container>& l)
             {
                 // leading zero. point '0'
                 loc.retrace();
-                return err(format_underline("[error] bad integer: leading zero",
+                return err(format_underline("bad integer: leading zero",
                             {{std::addressof(loc), "here"}},
                             {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
                              "fail: 1__000, 0123"}));
             }
             if(c == ':' || c == '-')
             {
-                return err(format_underline("[error] bad datetime: invalid format",
+                return err(format_underline("bad datetime: invalid format",
                             {{std::addressof(loc), "here"}},
                             {"pass: 1979-05-27T07:32:00-07:00, 1979-05-27 07:32:00.999999Z",
                              "fail: 1979-05-27T7:32:00-7:00, 1979-05-27 7:32-00:30"}));
             }
             if(c == '.' || c == 'e' || c == 'E')
             {
-                return err(format_underline("[error] bad float: invalid format",
+                return err(format_underline("bad float: invalid format",
                             {{std::addressof(loc), "here"}},
                             {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
                              "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
@@ -1582,19 +1582,19 @@ result<value_t, std::string> guess_number_type(const location<Container>& l)
     }
     if(loc.iter() != loc.end() && *loc.iter() == '.')
     {
-        return err(format_underline("[error] bad float: invalid format",
+        return err(format_underline("bad float: invalid format",
                 {{std::addressof(loc), "integer part required before this"}},
                 {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
                  "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
     }
     if(loc.iter() != loc.end() && *loc.iter() == '_')
     {
-        return err(format_underline("[error] bad number: `_` should be surrounded by digits",
+        return err(format_underline("bad number: `_` should be surrounded by digits",
                 {{std::addressof(loc), "`_` is not surrounded by digits"}},
                 {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
                  "fail: 1__000, 0123"}));
     }
-    return err(format_underline("[error] bad format: unknown value appeared",
+    return err(format_underline("bad format: unknown value appeared",
                 {{std::addressof(loc), "here"}}));
 }
 
@@ -1623,7 +1623,7 @@ result<Value, std::string> parse_value(location<Container>& loc)
     const auto first = loc.iter();
     if(first == loc.end())
     {
-        return err(format_underline("[error] toml::parse_value: input is empty",
+        return err(format_underline("toml::parse_value: input is empty",
                    {{std::addressof(loc), ""}}));
     }
 
@@ -1646,7 +1646,7 @@ result<Value, std::string> parse_value(location<Container>& loc)
         case value_t::table          : {return parse_inline_table<value_type>(loc);}
         default:
         {
-            const auto msg = format_underline("[error] toml::parse_value: "
+            const auto msg = format_underline("toml::parse_value: "
                     "unknown token appeared", {{std::addressof(loc), "unknown"}});
             loc.reset(first);
             return err(msg);
@@ -1665,7 +1665,7 @@ parse_table_key(location<Container>& loc)
         const auto open = lex_std_table_open::invoke(inner_loc);
         if(!open || inner_loc.iter() == inner_loc.end())
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "toml::parse_table_key: no `[`",
                 {{std::addressof(inner_loc), "should be `[`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -1676,7 +1676,7 @@ parse_table_key(location<Container>& loc)
         const auto keys = parse_key(inner_loc);
         if(!keys)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "toml::parse_table_key: invalid key",
                 {{std::addressof(inner_loc), "not key"}}),
                 source_location(std::addressof(inner_loc)));
@@ -1687,7 +1687,7 @@ parse_table_key(location<Container>& loc)
         const auto close = lex_std_table_close::invoke(inner_loc);
         if(!close)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "toml::parse_table_key: no `]`",
                 {{std::addressof(inner_loc), "should be `]`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -1701,7 +1701,7 @@ parse_table_key(location<Container>& loc)
             const auto nl = lex_newline_after_table_key::invoke(loc);
             if(!nl)
             {
-                throw syntax_error(format_underline("[error] "
+                throw syntax_error(format_underline(
                     "toml::parse_table_key: newline required after [table.key]",
                     {{std::addressof(loc), "expected newline"}}),
                     source_location(std::addressof(loc)));
@@ -1711,7 +1711,7 @@ parse_table_key(location<Container>& loc)
     }
     else
     {
-        return err(format_underline("[error] toml::parse_table_key: "
+        return err(format_underline("toml::parse_table_key: "
             "not a valid table key", {{std::addressof(loc), "here"}}));
     }
 }
@@ -1727,7 +1727,7 @@ parse_array_table_key(location<Container>& loc)
         const auto open = lex_array_table_open::invoke(inner_loc);
         if(!open || inner_loc.iter() == inner_loc.end())
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "toml::parse_array_table_key: no `[[`",
                 {{std::addressof(inner_loc), "should be `[[`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -1736,7 +1736,7 @@ parse_array_table_key(location<Container>& loc)
         const auto keys = parse_key(inner_loc);
         if(!keys)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "toml::parse_array_table_key: invalid key",
                 {{std::addressof(inner_loc), "not a key"}}),
                 source_location(std::addressof(inner_loc)));
@@ -1745,7 +1745,7 @@ parse_array_table_key(location<Container>& loc)
         const auto close = lex_array_table_close::invoke(inner_loc);
         if(!close)
         {
-            throw internal_error(format_underline("[error] "
+            throw internal_error(format_underline(
                 "toml::parse_table_key: no `]]`",
                 {{std::addressof(inner_loc), "should be `]]`"}}),
                 source_location(std::addressof(inner_loc)));
@@ -1759,7 +1759,7 @@ parse_array_table_key(location<Container>& loc)
             const auto nl = lex_newline_after_table_key::invoke(loc);
             if(!nl)
             {
-                throw syntax_error(format_underline("[error] toml::"
+                throw syntax_error(format_underline("toml::"
                     "parse_array_table_key: newline required after [[table.key]]",
                     {{std::addressof(loc), "expected newline"}}),
                     source_location(std::addressof(loc)));
@@ -1769,7 +1769,7 @@ parse_array_table_key(location<Container>& loc)
     }
     else
     {
-        return err(format_underline("[error] toml::parse_array_table_key: "
+        return err(format_underline("toml::parse_array_table_key: "
             "not a valid table key", {{std::addressof(loc), "here"}}));
     }
 }
@@ -1844,7 +1844,7 @@ parse_ml_table(location<Container>& loc)
         {
             const auto before2 = loc.iter();
             lex_ws::invoke(loc); // skip whitespace
-            const auto msg = format_underline("[error] toml::parse_table: "
+            const auto msg = format_underline("toml::parse_table: "
                 "invalid line format", {{std::addressof(loc), concat_to_string(
                 "expected newline, but got '", show_char(*loc.iter()), "'.")}});
             loc.reset(before2);
@@ -1958,7 +1958,7 @@ result<Value, std::string> parse_toml_file(location<Container>& loc)
 
             continue;
         }
-        return err(format_underline("[error]: toml::parse_toml_file: "
+        return err(format_underline("toml::parse_toml_file: "
             "unknown line appeared", {{std::addressof(loc), "unknown format"}}));
     }
 

toml/region.hpp

@@ -9,6 +9,7 @@
 #include <iterator>
 #include <iomanip>
 #include <cassert>
+#include "color.hpp"
 
 namespace toml
 {
@@ -418,7 +419,8 @@ struct region final : public region_base
 // to show a better error message.
 inline std::string format_underline(const std::string& message,
         const std::vector<std::pair<region_base const*, std::string>>& reg_com,
-        const std::vector<std::string>& helps = {})
+        const std::vector<std::string>& helps = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
 {
     assert(!reg_com.empty());
 
@@ -432,7 +434,27 @@ inline std::string format_underline(const std::string& message,
     )->first->line_num().size());
 
     std::ostringstream retval;
-    retval << message << '\n';
+
+    if(colorize)
+    {
+        retval << color::colorize; // turn on ANSI color
+    }
+
+    // XXX
+    // Here, before `colorize` support, it does not output `[error]` prefix
+    // automatically. So some user may output it manually and this change may
+    // duplicate the prefix. To avoid it, check the first 7 characters and
+    // if it is "[error]", it removes that part from the message shown.
+    if(message.size() > 7 && message.substr(0, 7) == "[error]")
+    {
+        retval << color::bold << color::red << "[error]" << color::reset
+               << color::bold << message.substr(7) << color::reset << '\n';
+    }
+    else
+    {
+        retval << color::bold << color::red << "[error] " << color::reset
+               << color::bold << message << color::reset << '\n';
+    }
 
     for(auto iter = reg_com.begin(); iter != reg_com.end(); ++iter)
     {
@@ -440,34 +462,41 @@ inline std::string format_underline(const std::string& message,
         if(iter != reg_com.begin() &&
            std::prev(iter)->first->name() == iter->first->name())
         {
-            retval << "\n ...\n";
+            retval << color::bold << color::blue << "\n ...\n" << color::reset;
         }
         else // if filename differs, print " --> filename.toml"
         {
             if(iter != reg_com.begin()) {retval << '\n';}
-            retval << " --> " << iter->first->name() << '\n';
+            retval << color::bold << color::blue << " --> " << color::reset
+                   << iter->first->name() << '\n';
+            // add one almost-empty line for readability
+            retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ')
+                   << color::bold << color::blue << " | "  << color::reset << '\n';
         }
         const region_base* const reg = iter->first;
         const std::string&   comment = iter->second;
 
-        retval << ' ' << std::setw(line_num_width) << reg->line_num();
+        retval << ' ' << std::setw(line_num_width) << color::bold << color::blue
-        retval << " | " << reg->line() << '\n';
+               << reg->line_num() << " | "  << color::reset << reg->line() << '\n';
-        retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ');
+
-        retval << " | " << make_string(reg->before(), ' ');
+        retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ')
+               << color::bold << color::blue << " | " << color::reset
+               << make_string(reg->before(), ' ');
 
         if(reg->size() == 1)
         {
             // invalid
             // ^------
-            retval << '^';
+            retval << color::bold << color::red
-            retval << make_string(reg->after(), '-');
+                   << '^' << make_string(reg->after(), '-') << color::reset;
         }
         else
         {
             // invalid
             // ~~~~~~~
             const auto underline_len = std::min(reg->size(), reg->line().size());
-            retval << make_string(underline_len, '~');
+            retval << color::bold << color::red
+                   << make_string(underline_len, '~') << color::reset;
         }
         retval << ' ';
         retval << comment;
@@ -477,10 +506,10 @@ inline std::string format_underline(const std::string& message,
     {
         retval << '\n';
         retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ');
-        retval << " | ";
+        retval << color::bold << color::blue << " | " << color::reset;
         for(const auto help : helps)
         {
-            retval << "\nHint: ";
+            retval << color::bold << "\nHint: " << color::reset;
             retval << help;
         }
     }

toml/value.hpp

@@ -32,7 +32,7 @@ template<value_t Expected,
 throw_bad_cast(value_t actual, const ::toml::basic_value<C, T, A>& v)
 {
     throw type_error(detail::format_underline(concat_to_string(
-        "[error] toml::value bad_cast to ", Expected), {
+        "toml::value: bad_cast to ", Expected), {
             {std::addressof(get_region(v)),
              concat_to_string("the actual type is ", actual)}
         }), v.location());
@@ -1860,25 +1860,27 @@ operator>=(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
 template<typename C, template<typename ...> class T, template<typename ...> class A>
 inline std::string format_error(const std::string& err_msg,
         const basic_value<C, T, A>& v, const std::string& comment,
-        std::vector<std::string> hints = {})
+        std::vector<std::string> hints = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
 {
     return detail::format_underline(err_msg,
         std::vector<std::pair<detail::region_base const*, std::string>>{
             {std::addressof(detail::get_region(v)), comment}
-        }, std::move(hints));
+        }, std::move(hints), colorize);
 }
 
 template<typename C, template<typename ...> class T, template<typename ...> class A>
 inline std::string format_error(const std::string& err_msg,
         const toml::basic_value<C, T, A>& v1, const std::string& comment1,
         const toml::basic_value<C, T, A>& v2, const std::string& comment2,
-        std::vector<std::string> hints = {})
+        std::vector<std::string> hints = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
 {
     return detail::format_underline(err_msg,
         std::vector<std::pair<detail::region_base const*, std::string>>{
             {std::addressof(detail::get_region(v1)), comment1},
             {std::addressof(detail::get_region(v2)), comment2}
-        }, std::move(hints));
+        }, std::move(hints), colorize);
 }
 
 template<typename C, template<typename ...> class T, template<typename ...> class A>
@@ -1886,14 +1888,15 @@ inline std::string format_error(const std::string& err_msg,
         const toml::basic_value<C, T, A>& v1, const std::string& comment1,
         const toml::basic_value<C, T, A>& v2, const std::string& comment2,
         const toml::basic_value<C, T, A>& v3, const std::string& comment3,
-        std::vector<std::string> hints = {})
+        std::vector<std::string> hints = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
 {
     return detail::format_underline(err_msg,
         std::vector<std::pair<detail::region_base const*, std::string>>{
             {std::addressof(detail::get_region(v1)), comment1},
             {std::addressof(detail::get_region(v2)), comment2},
             {std::addressof(detail::get_region(v3)), comment3}
-        }, std::move(hints));
+        }, std::move(hints), colorize);
 }
 
 template<typename Visitor, typename C,