tpl/collections: Improve type handling in collections.Slice

Fixes #5188

hugolib/collections.go

@@ -0,0 +1,78 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+import (
+	"fmt"
+
+	"github.com/gohugoio/hugo/common/collections"
+)
+
+var (
+	_ collections.Grouper = (*Page)(nil)
+	_ collections.Slicer  = (*Page)(nil)
+	_ collections.Slicer  = PageGroup{}
+	_ collections.Slicer  = WeightedPage{}
+)
+
+// collections.Slicer implementations below. We keep these bridge implementations
+// here as it makes it easier to get an idea of "type coverage". These
+// implementations have no value on their own.
+
+// Slice is not meant to be used externally. It's a bridge function
+// for the template functions. See collections.Slice.
+func (p *Page) Slice(items []interface{}) (interface{}, error) {
+	return toPages(items)
+}
+
+// Slice is not meant to be used externally. It's a bridge function
+// for the template functions. See collections.Slice.
+func (p PageGroup) Slice(items []interface{}) (interface{}, error) {
+	groups := make(PagesGroup, len(items))
+	for i, v := range items {
+		g, ok := v.(PageGroup)
+		if !ok {
+			return nil, fmt.Errorf("type %T is not a PageGroup", v)
+		}
+		groups[i] = g
+	}
+	return groups, nil
+}
+
+// Slice is not meant to be used externally. It's a bridge function
+// for the template functions. See collections.Slice.
+func (p WeightedPage) Slice(items []interface{}) (interface{}, error) {
+	weighted := make(WeightedPages, len(items))
+	for i, v := range items {
+		g, ok := v.(WeightedPage)
+		if !ok {
+			return nil, fmt.Errorf("type %T is not a WeightedPage", v)
+		}
+		weighted[i] = g
+	}
+	return weighted, nil
+}
+
+// collections.Grouper  implementations below
+
+// Group creates a PageGroup from a key and a Pages object
+// This method is not meant for external use. It got its non-typed arguments to satisfy
+// a very generic interface in the tpl package.
+func (p *Page) Group(key interface{}, in interface{}) (interface{}, error) {
+	pages, err := toPages(in)
+	if err != nil {
+		return nil, err
+	}
+	return PageGroup{Key: key, Pages: pages}, nil
+}