tpl: Add docshelper for template funcs

And fix some other minor related issues.

Updates #3418

tpl/internal/templatefuncsRegistry.go

@@ -16,6 +16,12 @@
 package internal
 
 import (
+	"encoding/json"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+
 	"github.com/spf13/hugo/deps"
 )
 
@@ -32,12 +38,46 @@ type TemplateFuncsNamespace struct {
 	// This is the method receiver.
 	Context interface{}
 
+	// Additional info, aliases and examples, per method name.
+	MethodMappings map[string]TemplateFuncMethodMapping
+}
+
+func (t *TemplateFuncsNamespace) AddMethodMapping(m interface{}, aliases []string, examples [][2]string) {
+	if t.MethodMappings == nil {
+		t.MethodMappings = make(map[string]TemplateFuncMethodMapping)
+	}
+
+	name := methodToName(m)
+
+	// sanity check
+	for _, e := range examples {
+		if e[0] == "" {
+			panic(t.Name + ": Empty example for " + name)
+		}
+	}
+	for _, a := range aliases {
+		if a == "" {
+			panic(t.Name + ": Empty alias for " + name)
+		}
+	}
+
+	t.MethodMappings[name] = TemplateFuncMethodMapping{
+		Method:   m,
+		Aliases:  aliases,
+		Examples: examples,
+	}
+
+}
+
+type TemplateFuncMethodMapping struct {
+	Method interface{}
+
 	// Any template funcs aliases. This is mainly motivated by keeping
 	// backwards compability, but some new template funcs may also make
 	// sense to give short and snappy aliases.
 	// Note that these aliases are global and will be merged, so the last
 	// key will win.
-	Aliases map[string]interface{}
+	Aliases []string
 
 	// A slice of input/expected examples.
 	// We keep it a the namespace level for now, but may find a way to keep track
@@ -45,3 +85,44 @@ type TemplateFuncsNamespace struct {
 	// Some of these, hopefully just a few, may depend on some test data to run.
 	Examples [][2]string
 }
+
+func methodToName(m interface{}) string {
+	name := runtime.FuncForPC(reflect.ValueOf(m).Pointer()).Name()
+	name = filepath.Ext(name)
+	name = strings.TrimPrefix(name, ".")
+	name = strings.TrimSuffix(name, "-fm")
+	return name
+}
+
+func (t *TemplateFuncsNamespace) MarshalJSON() ([]byte, error) {
+	type Func struct {
+		Name        string
+		Description string // TODO(bep)
+		Aliases     []string
+		Examples    [][2]string
+	}
+	// TODO(bep) map/lookup from docs template Namespace + Func name.
+	var funcs []Func
+
+	ctx := t.Context.(func() interface{})()
+	ctxType := reflect.TypeOf(ctx)
+	for i := 0; i < ctxType.NumMethod(); i++ {
+		method := ctxType.Method(i)
+		f := Func{
+			Name: method.Name,
+		}
+		if mapping, ok := t.MethodMappings[method.Name]; ok {
+			f.Aliases = mapping.Aliases
+			f.Examples = mapping.Examples
+		}
+		funcs = append(funcs, f)
+	}
+
+	return json.Marshal(&struct {
+		Name  string
+		Funcs []Func
+	}{
+		Name:  t.Name,
+		Funcs: funcs,
+	})
+}