gohugoio/hugo · error

expected slice of Resource objects, received %T instead

Error message

expected slice of Resource objects, received %T instead

What it means

`resources.Concat` requires its second argument to be a `resource.Resources` slice (or something convertible via `ResourcesConverter`). Passing a single Resource, a plain slice built with `slice` containing non-resources, or any other type hits the default case of the type switch and fails with the received type.

Source

Thrown at tpl/resources/resources.go:215

}

// Concat concatenates a slice of Resource objects. These resources must
// (currently) be of the same Media Type.
func (ns *Namespace) Concat(targetPathIn any, r any) (resource.Resource, error) {
	targetPath, err := cast.ToStringE(targetPathIn)
	if err != nil {
		return nil, err
	}

	var rr resource.Resources

	switch v := r.(type) {
	case resource.Resources:
		rr = v
	case resource.ResourcesConverter:
		rr = v.ToResources()
	default:
		return nil, fmt.Errorf("expected slice of Resource objects, received %T instead", r)
	}

	if len(rr) == 0 {
		return nil, errors.New("must provide one or more Resource objects to concat")
	}

	return ns.bundlerClient.Concat(targetPath, rr)
}

// FromString creates a Resource from a string published to the relative target path.
func (ns *Namespace) FromString(targetPathIn, contentIn any) (resource.Resource, error) {
	targetPath, err := cast.ToStringE(targetPathIn)
	if err != nil {
		return nil, err
	}
	content, err := cast.ToStringE(contentIn)
	if err != nil {
		return nil, err

View on GitHub ↗ (pinned to 8a468df065)

Solutions

1. Wrap resources in `slice`: `resources.Concat "js/bundle.js" (slice $a $b)`.
2. Use `resources.Match "js/*.js"` to get a proper Resources slice directly.
3. Ensure every element is a Resource — check `resources.Get` results for nil before appending.

Example fix

<!-- before -->
{{ $bundle := resources.Concat "js/bundle.js" $main }}
<!-- after -->
{{ $bundle := resources.Concat "js/bundle.js" (slice $main) }}

{{ $css := slice (resources.Get "a.css") (resources.Get "b.css") }}{{ $bundle := $css | resources.Concat "bundle.css" }}

{{ $allResources := true }}{{ range $r := $items }}{{ if not (reflect.IsMap (dict)) }}{{ end }}{{ end }}{{/* practically: ensure every element came from resources.Get / .Resources */}}

{{ with try (resources.Concat "bundle.css" $items) }}{{ with .Err }}{{ errorf "concat failed: %s" . }}{{ end }}{{ end }}

When it happens

Trigger: `{{ resources.Concat "js/bundle.js" $oneResource }}` passing a single resource instead of a slice; passing a `[]any` that contains strings/paths rather than Resource objects; passing the result of `readDir` or a string glob rather than `resources.Match`.

Common situations: Forgetting to wrap a single resource in `slice`; building the list from file paths instead of `resources.Get` results; mixing nil entries from failed `resources.Get` calls into the slice (making it a generic slice).

Related errors

• type %T not supported in Resource transformations
• must provide one or more Resource objects to concat
• resource %T does not implement resource.ReadSeekerCloserReso
• failed to copy resources to vendor dir: %w
• arguments to complement must be slices or arrays

AI-assisted analysis of gohugoio/hugo@8a468df065 (2026-07-31). Data as JSON: /data/errors/cd2c347f28043650.json. Report an issue: GitHub ↗.