gohugoio/hugo · error

grouping not supported for type %T %T

Error message

grouping not supported for type %T %T

What it means

Returned by the collections.Group template function when neither items nor its slice element implements collections.Grouper. Group first checks items for the Grouper interface, then newSliceElement(items); if both fail it errors with %T %T showing the outer and element types. Grouping is currently only supported for Pages (and types implementing Grouper).

Solutions

1. Pass a Pages collection (e.g. `.Site.RegularPages`) to group; that is the supported input.
2. For non-page data, pre-group in your template using `slice`/`range`/a partial instead of the `group` function.
3. If grouping a custom type, implement the collections.Grouper interface on its slice element so Group can dispatch to it.

Example fix

// before
{{ $groups := group "date" .Params.posts }}

// after
{{ $groups := group "date" .Site.RegularPages }}

// Guard that items implements Grouper (Pages do) before grouping.
func isGrouper(v any) bool {
    _, ok1 := v.(collections.Grouper)
    if ok1 { return true }
    // also check slice element
    rv := reflect.ValueOf(v)
    if rv.Kind() == reflect.Slice && rv.Len() > 0 {
        _, ok2 := rv.Index(0).Interface().(collections.Grouper)
        return ok2
    }
    return false
}

// Go type guard for Grouper interface.
func asGrouper(v any) (collections.Grouper, bool) {
    g, ok := v.(collections.Grouper)
    return g, ok
}

When it happens

Trigger: Calling `group key items` in a template where items is not a Pages collection or a type that implements Grouper — e.g. a slice of plain maps, strings, or a single Page. The function requires a collection whose element type knows how to group itself.

Common situations: Trying to group a slice of dict/strings, grouping a single object instead of .Pages, or grouping a custom data list that doesn't implement the Grouper interface.

Related errors

• argument must be a slice
• can't apply over
• can't iterate over %T
• incompatible map types, got %T to %T
• nil is not a valid key to group by

AI-assisted analysis of gohugoio/hugo@52c9bd7908 (2026-08-09). Data as JSON: /api/errors/e43959e22586e983. Report an issue: GitHub.

Appendix: source

Thrown at tpl/collections/collections.go:327

// Group groups a set of items by the given key.
// This is currently only supported for Pages.
func (ns *Namespace) Group(key any, items any) (any, error) {
	if key == nil {
		return nil, errors.New("nil is not a valid key to group by")
	}

	if g, ok := items.(collections.Grouper); ok {
		return g.Group(key, items)
	}

	in := newSliceElement(items)

	if g, ok := in.(collections.Grouper); ok {
		return g.Group(key, items)
	}

	return nil, fmt.Errorf("grouping not supported for type %T %T", items, in)
}

// IsSet returns whether a given array, channel, slice, or map in c has the given key
// defined.
func (ns *Namespace) IsSet(c any, key any) (bool, error) {
	av := reflect.ValueOf(c)
	kv := reflect.ValueOf(key)

	switch av.Kind() {
	case reflect.Array, reflect.Chan, reflect.Slice:
		k, err := cast.ToIntE(key)
		if err != nil {
			return false, fmt.Errorf("isset unable to use key of type %T as index", key)
		}
		if av.Len() > k {
			return true, nil
		}
	case reflect.Map:

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