tomnomnom/gron · error

unexpected data type for merge: `%s`

Error message

unexpected data type for merge: `%s`

What it means

recursiveMerge received a value of an unhandled dynamic type for its first argument (a): it handles maps, slices and scalar kinds (string, int, float64, bool, nil, json.Number), so anything else — an unexpected concrete type in the parsed tree — falls to the default branch. This signals an internal datastructure bug rather than bad user input.

Source

Thrown at ungron.go:465

		bMap, ok := b.(map[string]interface{})
		if !ok {
			return nil, fmt.Errorf("cannot merge object with non-object")
		}
		return recursiveMapMerge(a.(map[string]interface{}), bMap)

	case []interface{}:
		bSlice, ok := b.([]interface{})
		if !ok {
			return nil, fmt.Errorf("cannot merge array with non-array")
		}
		return recursiveSliceMerge(a.([]interface{}), bSlice)

	case string, int, float64, bool, nil, json.Number:
		// Can't merge them, second one wins
		return b, nil

	default:
		return nil, fmt.Errorf("unexpected data type for merge: `%s`", reflect.TypeOf(a))
	}
}

// recursiveMapMerge recursively merges map[string]interface{} values
func recursiveMapMerge(a, b map[string]interface{}) (map[string]interface{}, error) {
	// Merge keys from b into a
	for k, v := range b {
		_, exists := a[k]
		if !exists {
			// Doesn't exist in a, just add it in
			a[k] = v
		} else {
			// Does exist, merge the values
			merged, err := recursiveMerge(a[k], b[k])
			if err != nil {
				return nil, err
			}

View on GitHub (pinned to 88a6234ea2)

Solutions

1. Report/inspect the value's reflect.Type shown in the message to find where the unexpected type entered the parsed tree
2. Extend recursiveMerge to handle the new type if it is legitimately produced by parsing

When it happens

Trigger: Thrown at ungron.go:465 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of tomnomnom/gron@88a6234ea2 (2026-09-06). Data as JSON: /api/errors/c269fa5152818526. Report an issue: GitHub.