Add LookupTag function

Author: Dean Karn

errors.go

@@ -75,3 +75,24 @@ func HasType(err error, typ string) bool {
 	}
 	return false
 }
+
+// LookupTag recursively searches for the provided tag and returns it's value or nil
+func LookupTag(err error, key string) interface{} {
+	switch t := err.(type) {
+	case Chain:
+		for i := len(t) - 1; i >= 0; i-- {
+			for j := 0; j < len(t[i].Tags); j++ {
+				if t[i].Tags[j].Key == key {
+					return t[i].Tags[j].Value
+				}
+			}
+		}
+	case *Link:
+		for i := 0; i < len(t.Tags); i++ {
+			if t.Tags[i].Key == key {
+				return t.Tags[i].Value
+			}
+		}
+	}
+	return nil
+}

errors_test.go

@@ -104,7 +104,7 @@ func TestCause(t *testing.T) {
 	}
 }
 
-func TestIsErr(t *testing.T) {
+func TestCause2(t *testing.T) {
 	err := Wrap(io.EOF, "prefix")
 	err = Wrap(err, "prefix2")
 
@@ -132,3 +132,10 @@ func TestHelpers(t *testing.T) {
 		t.Errorf("Expected to have type 'Test'")
 	}
 }
+
+func TestLookupTag(t *testing.T) {
+	err := Wrap(io.EOF, "prefix").WithTag("Key", "Value")
+	if LookupTag(err, "Key").(string) != "Value" {
+		t.Fatalf("want %s got %v", "Value", LookupTag(err, "Key"))
+	}
+}