add Cause function

find the root error

error.go

@@ -1,6 +1,7 @@
 package errorx
 
 import (
+	"errors"
 	"fmt"
 )
 
@@ -24,3 +25,18 @@ func Tracef(err error, format string, args ...interface{}) error {
 	}
 	return fmt.Errorf("%s %s\n%w", getLocation(), fmt.Sprintf(format, args...), err)
 }
+
+func Cause(err error) error {
+	if err == nil {
+		return nil
+	}
+
+	for {
+		next := errors.Unwrap(err)
+		if next == nil {
+			return err
+		}
+
+		err = next
+	}
+}

error_test.go

@@ -29,6 +29,9 @@ func TestTrace(t *testing.T) {
 		assert.True(errors.Is(err, errNotFound))
 		t.Logf("\n%+v", err)
 
+		got := errors.Unwrap(err)
+		t.Log(got)
+
 		errNone := errors.New("none")
 		assert.False(errors.Is(err, errNone))
 	})
@@ -41,6 +44,49 @@ func TestTrace(t *testing.T) {
 	})
 }
 
+func TestCause(t *testing.T) {
+	assert := assert.New(t)
+
+	tds := []struct {
+		name string
+		err  error
+		want error
+	}{
+		{
+			name: "nil",
+			err:  nil,
+			want: nil,
+		},
+		{
+			name: "standard error",
+			err:  errors.New("err1"),
+			want: errors.New("err1"),
+		},
+		{
+			name: "trace err",
+			err:  Trace(errors.New("err1")),
+			want: errors.New("err1"),
+		},
+		{
+			name: "more trace err",
+			err:  Trace(Trace(errors.New("err1"))),
+			want: errors.New("err1"),
+		},
+		{
+			name: "tracef err",
+			err:  Tracef(errors.New("err1"), "description"),
+			want: errors.New("err1"),
+		},
+	}
+
+	for _, td := range tds {
+		t.Run(td.name, func(t *testing.T) {
+			got := Cause(td.err)
+			assert.Equal(td.want, got)
+		})
+	}
+}
+
 type PointerError struct {
 	code int32
 	msg  string