tpl/collections: Add collections.Append

Before this commit you would typically use `.Scratch.Add` to manually create slices in a loop.

With variable overwrite in Go 1.11, we can do better. This commit adds the `append` template func.

A made-up example:

```bash
{{ $p1 := index .Site.RegularPages 0 }}{{ $p2 := index .Site.RegularPages 1 }}
{{ $pages := slice }}
{{ if true }}
  {{ $pages = $pages | append $p2 $p1 }}
{{ end }}
```

Note that with 2 slices as arguments, the two examples below will give the same result:

```bash
{{ $s1 := slice "a" "b" | append (slice "c" "d") }}
{{ $s2 := slice "a" "b" | append "c" "d" }}
```

Both of the above will give `[]string{a, b, c, d}`.

This commit also improves the type handling in the `slice` template function. Now `slice "a" "b"` will give a `[]string` slice. The old behaviour was to return a `[]interface{}`.

Fixes #5190

2 files changed, 36 insertions, 0 deletions

diff --git a/resource/resource.go b/resource/resource.go
index 9a974e912..dd9cbbd41 100644
--- a/resource/resource.go
+++ b/resource/resource.go
@@ -28,6 +28,7 @@ import (
 	"github.com/gohugoio/hugo/output"
 	"github.com/gohugoio/hugo/tpl"
 
+	"github.com/gohugoio/hugo/common/collections"
 	"github.com/gohugoio/hugo/common/hugio"
 	"github.com/gohugoio/hugo/common/loggers"
 
@@ -49,6 +50,7 @@ var (
 	_ Cloner                  = (*genericResource)(nil)
 	_ ResourcesLanguageMerger = (*Resources)(nil)
 	_ permalinker             = (*genericResource)(nil)
+	_ collections.Slicer      = (*genericResource)(nil)
 )
 
 var noData = make(map[string]interface{})
@@ -150,6 +152,11 @@ type ReadSeekCloserResource interface {
 // I.e. both pages and images etc.
 type Resources []Resource
 
+// ResourcesConverter converts a given slice of Resource objects to Resources.
+type ResourcesConverter interface {
+	ToResources() Resources
+}
+
 // ByType returns resources of a given resource type (ie. "image").
 func (r Resources) ByType(tp string) Resources {
 	var filtered Resources
@@ -550,6 +557,7 @@ func (l *publishOnce) publish(s Source) error {
 
 // genericResource represents a generic linkable resource.
 type genericResource struct {
+	commonResource
 	resourcePathDescriptor
 
 	title  string
@@ -586,6 +594,9 @@ type genericResource struct {
 	*publishOnce
 }
 
+type commonResource struct {
+}
+
 func (l *genericResource) Data() interface{} {
 	return noData
 }
@@ -621,6 +632,27 @@ func (l genericResource) WithNewBase(base string) Resource {
 	return &l
 }
 
+// Slice is not meant to be used externally. It's a bridge function
+// for the template functions. See collections.Slice.
+func (commonResource) Slice(in interface{}) (interface{}, error) {
+	switch items := in.(type) {
+	case Resources:
+		return items, nil
+	case []interface{}:
+		groups := make(Resources, len(items))
+		for i, v := range items {
+			g, ok := v.(Resource)
+			if !ok {
+				return nil, fmt.Errorf("type %T is not a Resource", v)
+			}
+			groups[i] = g
+		}
+		return groups, nil
+	default:
+		return nil, fmt.Errorf("invalid slice type %T", items)
+	}
+}
+
 func (l *genericResource) initHash() error {
 	var err error
 	l.hashInit.Do(func() {
diff --git a/resource/transform.go b/resource/transform.go
index 9e6215b9e..01b05b73e 100644
--- a/resource/transform.go
+++ b/resource/transform.go
@@ -19,6 +19,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/gohugoio/hugo/common/collections"
 	"github.com/gohugoio/hugo/common/errors"
 	"github.com/gohugoio/hugo/common/hugio"
 	"github.com/gohugoio/hugo/helpers"
@@ -37,6 +38,7 @@ import (
 var (
 	_ ContentResource        = (*transformedResource)(nil)
 	_ ReadSeekCloserResource = (*transformedResource)(nil)
+	_ collections.Slicer     = (*transformedResource)(nil)
 )
 
 func (s *Spec) Transform(r Resource, t ResourceTransformation) (Resource, error) {
@@ -166,6 +168,8 @@ type transformedResourceMetadata struct {
 }
 
 type transformedResource struct {
+	commonResource
+
 	cache *ResourceCache
 
 	// This is the filename inside resources/_gen/assets