diff --git a/generator/root_test.go b/generator/root_test.go
index c550185..a1651c4 100644
--- a/generator/root_test.go
+++ b/generator/root_test.go
@@ -491,7 +491,7 @@ func TestShouldGenerateCodeRaiseErrorWhenCodeFormatterIsExited(t *testing.T) {
 		).EnableSyntaxChecking()
 
 		_, err := generator.Generate(0)
-		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command="gofmt -e".+`), err.Error())
+		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command='gofmt -e'.+`), err.Error())
 	}
 
 	{
@@ -500,7 +500,7 @@ func TestShouldGenerateCodeRaiseErrorWhenCodeFormatterIsExited(t *testing.T) {
 		).Gofmt()
 
 		_, err := generator.Generate(0)
-		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command="gofmt".+`), err.Error())
+		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command='gofmt'.+`), err.Error())
 	}
 
 	{
@@ -509,11 +509,11 @@ func TestShouldGenerateCodeRaiseErrorWhenCodeFormatterIsExited(t *testing.T) {
 		).Goimports()
 
 		_, err := generator.Generate(0)
-		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command="goimports".+`), err.Error())
+		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command='goimports'.+`), err.Error())
 	}
 
 	{
 		_, err := applyCodeFormatter("", "not-existed-cmd")
-		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command="not-existed-cmd".+`), err.Error())
+		assert.Regexp(t, regexp.MustCompile(`^\[GOWRTR-13\] code formatter raises error: command='not-existed-cmd'.+`), err.Error())
 	}
 }
diff --git a/go.mod b/go.mod
index 15584e9..2c7e12b 100644
--- a/go.mod
+++ b/go.mod
@@ -2,11 +2,12 @@ module github.com/moznion/gowrtr
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
-	github.com/moznion/go-errgen v1.3.2
+	github.com/iancoleman/strcase v0.0.0-20191112232945-16388991a334 // indirect
+	github.com/moznion/go-errgen v1.8.1
 	github.com/pkg/errors v0.9.1
 	github.com/stretchr/testify v1.3.0
-	golang.org/x/lint v0.0.0-20181217174547-8f45f776aaf1
-	golang.org/x/tools v0.0.0-20190121143147-24cd39ecf745 // indirect
+	golang.org/x/lint v0.0.0-20191125180803-fdd1cda4f05f
+	golang.org/x/tools v0.0.0-20200117220505-0cba7a3a9ee9 // indirect
 )
 
 go 1.13
diff --git a/internal/errmsg/errmsg.go b/internal/errmsg/errmsg.go
index b2f77f2..c9064d5 100644
--- a/internal/errmsg/errmsg.go
+++ b/internal/errmsg/errmsg.go
@@ -14,7 +14,7 @@ type errs struct {
 	FuncSignatureIsNilError                           error `errmsg:"func signature must not be nil, bit it gets nil (caused at %s)" vars:"caller string"`
 	AnonymousFuncSignatureIsNilError                  error `errmsg:"anonymous func signature must not be nil, bit it gets nil (caused at %s)" vars:"caller string"`
 	FuncInvocationParameterIsEmptyError               error `errmsg:"a parameter of function invocation must not be nil, but it gets nil (caused at %s)" vars:"caller string"`
-	CodeFormatterError                                error `errmsg:"code formatter raises error: command=\"%s\", err=\"%s\", msg=\"%s\"" vars:"cmd string, msg string, fmterr error"`
+	CodeFormatterError                                error `errmsg:"code formatter raises error: command='%s', err='%s', msg='%s'" vars:"cmd string, msg string, fmterr error"`
 	CaseConditionIsEmptyError                         error `errmsg:"condition of case must not be empty, but it gets empty (caused at %s)" vars:"caller string"`
 	IfConditionIsEmptyError                           error `errmsg:"condition of if must not be empty, but it gets empty (caused at %s)" vars:"caller string"`
 	UnnamedReturnTypeAppearsAfterNamedReturnTypeError error `errmsg:"unnamed return type appears after named return type (caused at %s)" vars:"caller string"`
diff --git a/internal/errmsg/errs_errmsg_gen.go b/internal/errmsg/errs_errmsg_gen.go
index 9b14886..9b7d831 100644
--- a/internal/errmsg/errs_errmsg_gen.go
+++ b/internal/errmsg/errs_errmsg_gen.go
@@ -14,164 +14,218 @@ import (
 func StructNameIsNilErr(caller string) error {
 	return fmt.Errorf(`[GOWRTR-1] struct name must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// StructNameIsNilErrWrap wraps the error.
 func StructNameIsNilErrWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-1] struct name must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-1] struct name must not be empty, but it gets empty (caused at %s)")
 }
 
 // StructFieldNameIsEmptyErr returns the error.
 func StructFieldNameIsEmptyErr(caller string) error {
 	return fmt.Errorf(`[GOWRTR-2] field name must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// StructFieldNameIsEmptyErrWrap wraps the error.
 func StructFieldNameIsEmptyErrWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-2] field name must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-2] field name must not be empty, but it gets empty (caused at %s)")
 }
 
 // StructFieldTypeIsEmptyErr returns the error.
 func StructFieldTypeIsEmptyErr(caller string) error {
 	return fmt.Errorf(`[GOWRTR-3] field type must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// StructFieldTypeIsEmptyErrWrap wraps the error.
 func StructFieldTypeIsEmptyErrWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-3] field type must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-3] field type must not be empty, but it gets empty (caused at %s)")
 }
 
 // FuncParameterNameIsEmptyErr returns the error.
 func FuncParameterNameIsEmptyErr(caller string) error {
 	return fmt.Errorf(`[GOWRTR-4] func parameter name must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// FuncParameterNameIsEmptyErrWrap wraps the error.
 func FuncParameterNameIsEmptyErrWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-4] func parameter name must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-4] func parameter name must not be empty, but it gets empty (caused at %s)")
 }
 
 // LastFuncParameterTypeIsEmptyErr returns the error.
 func LastFuncParameterTypeIsEmptyErr(caller string) error {
 	return fmt.Errorf(`[GOWRTR-5] the last func parameter type must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// LastFuncParameterTypeIsEmptyErrWrap wraps the error.
 func LastFuncParameterTypeIsEmptyErrWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-5] the last func parameter type must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-5] the last func parameter type must not be empty, but it gets empty (caused at %s)")
 }
 
 // FuncNameIsEmptyError returns the error.
 func FuncNameIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-6] name of func must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// FuncNameIsEmptyErrorWrap wraps the error.
 func FuncNameIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-6] name of func must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-6] name of func must not be empty, but it gets empty (caused at %s)")
 }
 
 // InterfaceNameIsEmptyError returns the error.
 func InterfaceNameIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-7] name of interface must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// InterfaceNameIsEmptyErrorWrap wraps the error.
 func InterfaceNameIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-7] name of interface must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-7] name of interface must not be empty, but it gets empty (caused at %s)")
 }
 
 // FuncReceiverNameIsEmptyError returns the error.
 func FuncReceiverNameIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-8] name of func receiver must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// FuncReceiverNameIsEmptyErrorWrap wraps the error.
 func FuncReceiverNameIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-8] name of func receiver must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-8] name of func receiver must not be empty, but it gets empty (caused at %s)")
 }
 
 // FuncReceiverTypeIsEmptyError returns the error.
 func FuncReceiverTypeIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-9] type of func receiver must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// FuncReceiverTypeIsEmptyErrorWrap wraps the error.
 func FuncReceiverTypeIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-9] type of func receiver must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-9] type of func receiver must not be empty, but it gets empty (caused at %s)")
 }
 
 // FuncSignatureIsNilError returns the error.
 func FuncSignatureIsNilError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-10] func signature must not be nil, bit it gets nil (caused at %s)`, caller)
 }
+
+// FuncSignatureIsNilErrorWrap wraps the error.
 func FuncSignatureIsNilErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-10] func signature must not be nil, bit it gets nil (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-10] func signature must not be nil, bit it gets nil (caused at %s)")
 }
 
 // AnonymousFuncSignatureIsNilError returns the error.
 func AnonymousFuncSignatureIsNilError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-11] anonymous func signature must not be nil, bit it gets nil (caused at %s)`, caller)
 }
+
+// AnonymousFuncSignatureIsNilErrorWrap wraps the error.
 func AnonymousFuncSignatureIsNilErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-11] anonymous func signature must not be nil, bit it gets nil (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-11] anonymous func signature must not be nil, bit it gets nil (caused at %s)")
 }
 
 // FuncInvocationParameterIsEmptyError returns the error.
 func FuncInvocationParameterIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-12] a parameter of function invocation must not be nil, but it gets nil (caused at %s)`, caller)
 }
+
+// FuncInvocationParameterIsEmptyErrorWrap wraps the error.
 func FuncInvocationParameterIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-12] a parameter of function invocation must not be nil, but it gets nil (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-12] a parameter of function invocation must not be nil, but it gets nil (caused at %s)")
 }
 
 // CodeFormatterError returns the error.
 func CodeFormatterError(cmd string, msg string, fmterr error) error {
-	return fmt.Errorf(`[GOWRTR-13] code formatter raises error: command="%s", err="%s", msg="%s"`, cmd, msg, fmterr)
+	return fmt.Errorf(`[GOWRTR-13] code formatter raises error: command='%s', err='%s', msg='%s'`, cmd, msg, fmterr)
 }
+
+// CodeFormatterErrorWrap wraps the error.
 func CodeFormatterErrorWrap(cmd string, msg string, fmterr error, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-13] code formatter raises error: command="%s", err="%s", msg="%s"`, cmd, msg, fmterr), err.Error())
+	return errors.Wrap(err, "[GOWRTR-13] code formatter raises error: command='%s', err='%s', msg='%s'")
 }
 
 // CaseConditionIsEmptyError returns the error.
 func CaseConditionIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-14] condition of case must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// CaseConditionIsEmptyErrorWrap wraps the error.
 func CaseConditionIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-14] condition of case must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-14] condition of case must not be empty, but it gets empty (caused at %s)")
 }
 
 // IfConditionIsEmptyError returns the error.
 func IfConditionIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-15] condition of if must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// IfConditionIsEmptyErrorWrap wraps the error.
 func IfConditionIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-15] condition of if must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-15] condition of if must not be empty, but it gets empty (caused at %s)")
 }
 
 // UnnamedReturnTypeAppearsAfterNamedReturnTypeError returns the error.
 func UnnamedReturnTypeAppearsAfterNamedReturnTypeError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-16] unnamed return type appears after named return type (caused at %s)`, caller)
 }
+
+// UnnamedReturnTypeAppearsAfterNamedReturnTypeErrorWrap wraps the error.
 func UnnamedReturnTypeAppearsAfterNamedReturnTypeErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-16] unnamed return type appears after named return type (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-16] unnamed return type appears after named return type (caused at %s)")
 }
 
 // ValueOfCompositeLiteralIsEmptyError returns the error.
 func ValueOfCompositeLiteralIsEmptyError(caller string) error {
 	return fmt.Errorf(`[GOWRTR-17] a value of composite literal must not be empty, but it gets empty (caused at %s)`, caller)
 }
+
+// ValueOfCompositeLiteralIsEmptyErrorWrap wraps the error.
 func ValueOfCompositeLiteralIsEmptyErrorWrap(caller string, err error) error {
-	return errors.Wrap(fmt.Errorf(`[GOWRTR-17] a value of composite literal must not be empty, but it gets empty (caused at %s)`, caller), err.Error())
+	return errors.Wrap(err, "[GOWRTR-17] a value of composite literal must not be empty, but it gets empty (caused at %s)")
 }
 
+// ErrsType represents the error type.
 type ErrsType int
 
 const (
+
+	// StructNameIsNilErrType represents the error type for StructNameIsNilErr.
 	StructNameIsNilErrType ErrsType = iota
+	// StructFieldNameIsEmptyErrType represents the error type for StructFieldNameIsEmptyErr.
 	StructFieldNameIsEmptyErrType
+	// StructFieldTypeIsEmptyErrType represents the error type for StructFieldTypeIsEmptyErr.
 	StructFieldTypeIsEmptyErrType
+	// FuncParameterNameIsEmptyErrType represents the error type for FuncParameterNameIsEmptyErr.
 	FuncParameterNameIsEmptyErrType
+	// LastFuncParameterTypeIsEmptyErrType represents the error type for LastFuncParameterTypeIsEmptyErr.
 	LastFuncParameterTypeIsEmptyErrType
+	// FuncNameIsEmptyErrorType represents the error type for FuncNameIsEmptyError.
 	FuncNameIsEmptyErrorType
+	// InterfaceNameIsEmptyErrorType represents the error type for InterfaceNameIsEmptyError.
 	InterfaceNameIsEmptyErrorType
+	// FuncReceiverNameIsEmptyErrorType represents the error type for FuncReceiverNameIsEmptyError.
 	FuncReceiverNameIsEmptyErrorType
+	// FuncReceiverTypeIsEmptyErrorType represents the error type for FuncReceiverTypeIsEmptyError.
 	FuncReceiverTypeIsEmptyErrorType
+	// FuncSignatureIsNilErrorType represents the error type for FuncSignatureIsNilError.
 	FuncSignatureIsNilErrorType
+	// AnonymousFuncSignatureIsNilErrorType represents the error type for AnonymousFuncSignatureIsNilError.
 	AnonymousFuncSignatureIsNilErrorType
+	// FuncInvocationParameterIsEmptyErrorType represents the error type for FuncInvocationParameterIsEmptyError.
 	FuncInvocationParameterIsEmptyErrorType
+	// CodeFormatterErrorType represents the error type for CodeFormatterError.
 	CodeFormatterErrorType
+	// CaseConditionIsEmptyErrorType represents the error type for CaseConditionIsEmptyError.
 	CaseConditionIsEmptyErrorType
+	// IfConditionIsEmptyErrorType represents the error type for IfConditionIsEmptyError.
 	IfConditionIsEmptyErrorType
+	// UnnamedReturnTypeAppearsAfterNamedReturnTypeErrorType represents the error type for UnnamedReturnTypeAppearsAfterNamedReturnTypeError.
 	UnnamedReturnTypeAppearsAfterNamedReturnTypeErrorType
+	// ValueOfCompositeLiteralIsEmptyErrorType represents the error type for ValueOfCompositeLiteralIsEmptyError.
 	ValueOfCompositeLiteralIsEmptyErrorType
+	// ErrsUnknownType represents unknown type for Errs
 	ErrsUnknownType
 )
 
-// ErrsList returns the list of errors.
+// ListErrs returns the list of errors.
 func ListErrs() []string {
-	return []string{"[GOWRTR-1] struct name must not be empty, but it gets empty (caused at %s)", "[GOWRTR-2] field name must not be empty, but it gets empty (caused at %s)", "[GOWRTR-3] field type must not be empty, but it gets empty (caused at %s)", "[GOWRTR-4] func parameter name must not be empty, but it gets empty (caused at %s)", "[GOWRTR-5] the last func parameter type must not be empty, but it gets empty (caused at %s)", "[GOWRTR-6] name of func must not be empty, but it gets empty (caused at %s)", "[GOWRTR-7] name of interface must not be empty, but it gets empty (caused at %s)", "[GOWRTR-8] name of func receiver must not be empty, but it gets empty (caused at %s)", "[GOWRTR-9] type of func receiver must not be empty, but it gets empty (caused at %s)", "[GOWRTR-10] func signature must not be nil, bit it gets nil (caused at %s)", "[GOWRTR-11] anonymous func signature must not be nil, bit it gets nil (caused at %s)", "[GOWRTR-12] a parameter of function invocation must not be nil, but it gets nil (caused at %s)", "[GOWRTR-13] code formatter raises error: command=\"%s\", err=\"%s\", msg=\"%s\"", "[GOWRTR-14] condition of case must not be empty, but it gets empty (caused at %s)", "[GOWRTR-15] condition of if must not be empty, but it gets empty (caused at %s)", "[GOWRTR-16] unnamed return type appears after named return type (caused at %s)", "[GOWRTR-17] a value of composite literal must not be empty, but it gets empty (caused at %s)"}
+	return []string{"[GOWRTR-1] struct name must not be empty, but it gets empty (caused at %s)", "[GOWRTR-2] field name must not be empty, but it gets empty (caused at %s)", "[GOWRTR-3] field type must not be empty, but it gets empty (caused at %s)", "[GOWRTR-4] func parameter name must not be empty, but it gets empty (caused at %s)", "[GOWRTR-5] the last func parameter type must not be empty, but it gets empty (caused at %s)", "[GOWRTR-6] name of func must not be empty, but it gets empty (caused at %s)", "[GOWRTR-7] name of interface must not be empty, but it gets empty (caused at %s)", "[GOWRTR-8] name of func receiver must not be empty, but it gets empty (caused at %s)", "[GOWRTR-9] type of func receiver must not be empty, but it gets empty (caused at %s)", "[GOWRTR-10] func signature must not be nil, bit it gets nil (caused at %s)", "[GOWRTR-11] anonymous func signature must not be nil, bit it gets nil (caused at %s)", "[GOWRTR-12] a parameter of function invocation must not be nil, but it gets nil (caused at %s)", "[GOWRTR-13] code formatter raises error: command='%s', err='%s', msg='%s'", "[GOWRTR-14] condition of case must not be empty, but it gets empty (caused at %s)", "[GOWRTR-15] condition of if must not be empty, but it gets empty (caused at %s)", "[GOWRTR-16] unnamed return type appears after named return type (caused at %s)", "[GOWRTR-17] a value of composite literal must not be empty, but it gets empty (caused at %s)"}
 }
 
 // IdentifyErrs checks the identity of an error