Replace some comparison operators with three-way comparison operators

Signed-off-by: yamacir-kit <[email protected]>

README.md

@@ -90,7 +90,7 @@ Procedures for each standard are provided by the following R7RS-style libraries:
 cmake -B build -DCMAKE_BUILD_TYPE=Release
 cd build
 make package
-sudo apt install build/meevax_0.5.268_amd64.deb
+sudo apt install build/meevax_0.5.269_amd64.deb
 ```
 
 or
@@ -122,9 +122,9 @@ sudo rm -rf /usr/local/share/meevax
 
 | Target Name | Description
 |-------------|-------------
-| `all`       | Build shared-library `libmeevax.0.5.268.so` and executable `meevax`
+| `all`       | Build shared-library `libmeevax.0.5.269.so` and executable `meevax`
 | `test`      | Test executable `meevax`
-| `package`   | Generate debian package `meevax_0.5.268_amd64.deb`
+| `package`   | Generate debian package `meevax_0.5.269_amd64.deb`
 | `install`   | Copy files into `/usr/local` directly
 
 ## Usage

VERSION

@@ -1 +1 @@
-0.5.268
+0.5.269

include/meevax/kernel/string.hpp

@@ -50,11 +50,7 @@ inline namespace kernel
 
     operator std::string() const;
 
-    friend auto operator ==(string const& lhs, string const& rhs) { return static_cast<std::vector<character> const&>(lhs) == static_cast<std::vector<character> const&>(rhs); }
-    friend auto operator < (string const& lhs, string const& rhs) { return static_cast<std::vector<character> const&>(lhs) <  static_cast<std::vector<character> const&>(rhs); }
-    friend auto operator > (string const& lhs, string const& rhs) { return static_cast<std::vector<character> const&>(lhs) >  static_cast<std::vector<character> const&>(rhs); }
-    friend auto operator <=(string const& lhs, string const& rhs) { return static_cast<std::vector<character> const&>(lhs) <= static_cast<std::vector<character> const&>(rhs); }
-    friend auto operator >=(string const& lhs, string const& rhs) { return static_cast<std::vector<character> const&>(lhs) >= static_cast<std::vector<character> const&>(rhs); }
+    auto operator <=>(string const&) const = default;
   };
 
   auto operator <<(std::ostream &, string const&) -> std::ostream &;

include/meevax/kernel/symbol.hpp

@@ -33,19 +33,15 @@ inline namespace kernel
       : name { std::forward<decltype(xs)>(xs)... }
     {}
 
+    auto operator <=>(symbol const&) const = default;
+
     operator std::string() const noexcept
     {
       return name;
     }
   };
 
   auto operator + (symbol const&, symbol const&) -> std::string;
-  auto operator ==(symbol const&, symbol const&) -> bool;
-  auto operator !=(symbol const&, symbol const&) -> bool;
-  auto operator < (symbol const&, symbol const&) -> bool;
-  auto operator <=(symbol const&, symbol const&) -> bool;
-  auto operator > (symbol const&, symbol const&) -> bool;
-  auto operator >=(symbol const&, symbol const&) -> bool;
 
   auto operator <<(std::ostream &, symbol const&) -> std::ostream &;
 

src/kernel/symbol.cpp

@@ -28,13 +28,6 @@ inline namespace kernel
     return a.name + b.name;
   }
 
-  auto operator ==(symbol const& a, symbol const& b) -> bool { return a.name == b.name; }
-  auto operator !=(symbol const& a, symbol const& b) -> bool { return a.name != b.name; }
-  auto operator < (symbol const& a, symbol const& b) -> bool { return a.name <  b.name; }
-  auto operator <=(symbol const& a, symbol const& b) -> bool { return a.name <= b.name; }
-  auto operator > (symbol const& a, symbol const& b) -> bool { return a.name >  b.name; }
-  auto operator >=(symbol const& a, symbol const& b) -> bool { return a.name >= b.name; }
-
   auto operator <<(std::ostream & os, symbol const& datum) -> std::ostream &
   {
     if (datum.name.empty())