Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

1 files changed, 9 insertions, 66 deletions

diff --git a/hugolib/collections.go b/hugolib/collections.go
index cf75d3732..a794a9866 100644
--- a/hugolib/collections.go
+++ b/hugolib/collections.go
@@ -1,4 +1,4 @@
-// Copyright 2018 The Hugo Authors. All rights reserved.
+// Copyright 2019 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.
@@ -14,19 +14,13 @@
 package hugolib
 
 import (
-	"fmt"
-
-	"github.com/gohugoio/hugo/resources/resource"
-
 	"github.com/gohugoio/hugo/common/collections"
+	"github.com/gohugoio/hugo/resources/page"
 )
 
 var (
-	_ collections.Grouper         = (*Page)(nil)
-	_ collections.Slicer          = (*Page)(nil)
-	_ collections.Slicer          = PageGroup{}
-	_ collections.Slicer          = WeightedPage{}
-	_ resource.ResourcesConverter = Pages{}
+	_ collections.Grouper = (*pageState)(nil)
+	_ collections.Slicer  = (*pageState)(nil)
 )
 
 // collections.Slicer implementations below. We keep these bridge implementations
@@ -35,50 +29,8 @@ var (
 
 // 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(in interface{}) (interface{}, error) {
-	switch items := in.(type) {
-	case PageGroup:
-		return items, nil
-	case []interface{}:
-		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
-	default:
-		return nil, fmt.Errorf("invalid slice type %T", items)
-	}
-}
-
-// Slice is not meant to be used externally. It's a bridge function
-// for the template functions. See collections.Slice.
-func (p WeightedPage) Slice(in interface{}) (interface{}, error) {
-	switch items := in.(type) {
-	case WeightedPages:
-		return items, nil
-	case []interface{}:
-		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
-	default:
-		return nil, fmt.Errorf("invalid slice type %T", items)
-	}
+func (p *pageState) Slice(items interface{}) (interface{}, error) {
+	return page.ToPages(items)
 }
 
 // collections.Grouper  implementations below
@@ -86,19 +38,10 @@ func (p WeightedPage) Slice(in interface{}) (interface{}, error) {
 // 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)
+func (p *pageState) Group(key interface{}, in interface{}) (interface{}, error) {
+	pages, err := page.ToPages(in)
 	if err != nil {
 		return nil, err
 	}
-	return PageGroup{Key: key, Pages: pages}, nil
-}
-
-// ToResources wraps resource.ResourcesConverter
-func (pages Pages) ToResources() resource.Resources {
-	r := make(resource.Resources, len(pages))
-	for i, p := range pages {
-		r[i] = p
-	}
-	return r
+	return page.PageGroup{Key: key, Pages: pages}, nil
 }