add ToString() helper (#2100)

pkg/exprhelpers/exprlib.go

@@ -121,6 +121,7 @@ func GetExprEnv(ctx map[string]interface{}) map[string]interface{} {
 		"TrimPrefix":  strings.TrimPrefix,
 		"TrimSuffix":  strings.TrimSuffix,
 		"Get":         Get,
+		"String":      ToString,
 		"Distance":    Distance,
 	}
 	for k, v := range ctx {
@@ -484,3 +485,11 @@ func ParseUnix(value string) string {
 	}
 	return t.Format(time.RFC3339)
 }
+
+func ToString(value interface{}) string {
+	s, ok := value.(string)
+	if !ok {
+		return ""
+	}
+	return s
+}

pkg/exprhelpers/exprlib_test.go

@@ -1131,3 +1131,40 @@ func TestIsIp(t *testing.T) {
 		})
 	}
 }
+
+func TestToString(t *testing.T) {
+	tests := []struct {
+		name     string
+		value    interface{}
+		expected string
+	}{
+		{
+			name:     "ToString() test: valid string",
+			value:    "foo",
+			expected: "foo",
+		},
+		{
+			name:     "ToString() test: valid string",
+			value:    interface{}("foo"),
+			expected: "foo",
+		},
+		{
+			name:     "ToString() test: invalid type",
+			value:    1,
+			expected: "",
+		},
+		{
+			name:     "ToString() test: invalid type 2",
+			value:    interface{}(nil),
+			expected: "",
+		},
+	}
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			output := ToString(tc.value)
+			require.Equal(t, tc.expected, output)
+		})
+	}
+
+}