diff --git a/README.md b/README.md
index 1ba4d0014..faf473275 100644
--- a/README.md
+++ b/README.md
@@ -105,9 +105,9 @@ sudo rm -rf /usr/local/share/meevax
 
 | Target Name        | Description
 |:-------------------|:--
-| `all` (default)    | Build shared-library `libmeevax.0.4.230.so` and executable `meevax`.
+| `all` (default)    | Build shared-library `libmeevax.0.4.231.so` and executable `meevax`.
 | `test`             | Test executable `meevax`.
-| `package`          | Generate debian package `meevax_0.4.230_amd64.deb`.
+| `package`          | Generate debian package `meevax_0.4.231_amd64.deb`.
 | `install`          | Copy files into `/usr/local` __(1)__.
 | `install.deb`      | `all` + `package` + `sudo apt install <meevax>.deb`
 | `safe-install.deb` | `all` + `test` + `package` + `sudo apt install <meevax>.deb`
@@ -122,7 +122,7 @@ __(1)__ Meevax installed by `make install` cannot be uninstalled by the system's
 ## Usage
 
 ```
-Meevax Lisp System, version 0.4.230
+Meevax Lisp System, version 0.4.231
 
 Usage: meevax [OPTION...] [FILE...]
 
diff --git a/VERSION b/VERSION
index 3e3534c19..5942fea1a 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.4.230
+0.4.231
diff --git a/include/meevax/kernel/complex.hpp b/include/meevax/kernel/complex.hpp
index c08ff3c56..6c6698342 100644
--- a/include/meevax/kernel/complex.hpp
+++ b/include/meevax/kernel/complex.hpp
@@ -37,8 +37,6 @@ inline namespace kernel
 
     auto imag() const noexcept -> const_reference;
 
-    static auto pattern() -> std::regex const&;
-
     auto real() const noexcept -> const_reference;
 
     explicit operator std::complex<double>();
diff --git a/src/kernel/complex.cpp b/src/kernel/complex.cpp
index 30c17d3ae..e571547c7 100644
--- a/src/kernel/complex.cpp
+++ b/src/kernel/complex.cpp
@@ -23,10 +23,22 @@ inline namespace kernel
 {
   complex::complex(std::string const& token, int radix)
   {
-    if (std::smatch result; std::regex_match(token, result, pattern()))
+    std::regex static const rectangular { R"(([+-]?.*)([+-].*)i)" };
+
+    std::regex static const polar { R"(([+-]?.*)@([+-]?.*))" };
+
+    if (std::smatch result; std::regex_match(token, result, rectangular))
+    {
+      std::get<0>(*this) = string_to_real(result[1].length() == 0 ?                 "0" : result.str(1), radix);
+      std::get<1>(*this) = string_to_real(result[2].length() == 1 ? result.str(2) + "1" : result.str(2), radix);
+    }
+    else if (std::regex_match(token, result, polar))
     {
-      std::get<0>(*this) = string_to_real(result.str(1), radix);
-      std::get<1>(*this) = string_to_real(result.str(2), radix);
+      auto const magnitude = string_to_real(result.str(1), radix);
+      auto const angle     = string_to_real(result.str(2), radix);
+
+      std::get<0>(*this) = magnitude * apply<cos>(angle);
+      std::get<1>(*this) = magnitude * apply<sin>(angle);
     }
     else
     {
@@ -51,12 +63,6 @@ inline namespace kernel
     return second;
   }
 
-  auto complex::pattern() -> std::regex const&
-  {
-    std::regex static const pattern { R"(([+-]?.*)([+-].*)[ij])" };
-    return pattern;
-  }
-
   auto complex::real() const noexcept -> const_reference
   {
     return first;
diff --git a/src/kernel/number.cpp b/src/kernel/number.cpp
index d0f102538..126ad3649 100644
--- a/src/kernel/number.cpp
+++ b/src/kernel/number.cpp
@@ -217,7 +217,7 @@ inline namespace kernel
   auto operator * (complex const& a, complex const& b) -> complex { return complex(a.real() * b.real() - a.imag() * b.imag(), a.imag() * b.real() + a.real() * b.imag()); }
   auto operator / (complex const& a, complex const& b) -> complex { auto x = a.real() * b.real() + a.imag() * b.imag(); auto y = a.imag() * b.real() - a.real() * b.imag(); auto d = b.real() * b.real() + b.imag() * b.imag(); return complex(x / d, y / d); }
   auto operator % (complex const&  , complex const&  ) -> complex { throw std::invalid_argument("unsupported operation"); }
-  auto operator ==(complex const& a, complex const& b) -> bool    { return apply<equal_to>(a.real(), b.real()) and apply<equal_to>(a.imag(), b.imag()); }
+  auto operator ==(complex const& a, complex const& b) -> bool    { return apply<equal_to>(a.real(), b.real()).as<bool>() and apply<equal_to>(a.imag(), b.imag()).as<bool>(); }
   auto operator !=(complex const& a, complex const& b) -> bool    { return not (a == b); }
   auto operator < (complex const&  , complex const&  ) -> bool    { throw std::invalid_argument("unsupported operation"); }
   auto operator <=(complex const&  , complex const&  ) -> bool    { throw std::invalid_argument("unsupported operation"); }
diff --git a/test/r7rs.ss b/test/r7rs.ss
index ced304e7c..3b59e0e64 100644
--- a/test/r7rs.ss
+++ b/test/r7rs.ss
@@ -982,7 +982,7 @@
 
 (check (sqrt 9) => 3)
 
-; (check (sqrt -1) => +i)
+(check (sqrt -1) => +i)
 
 ; (check (exact-integer-sqrt 4) => #,(values 2 0))
 
@@ -1555,4 +1555,4 @@
 
 (check-report)
 
-(exit (check-passed? 391))
+(exit (check-passed? 392))