1 files changed, 74 insertions, 0 deletions
diff --git a/tpl/collections/append.go b/tpl/collections/append.go
new file mode 100644
index 000000000..20afa0e72
--- /dev/null
+++ b/tpl/collections/append.go
@@ -0,0 +1,74 @@
+// 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 collections
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+// Append appends the arguments up to the last one to the slice in the last argument.
+// This construct allows template constructs like this:
+//     {{ $pages = $pages | append $p2 $p1 }}
+// Note that with 2 arguments where both are slices of the same type,
+// the first slice will be appended to the second:
+//     {{ $pages = $pages | append .Site.RegularPages }}
+func (ns *Namespace) Append(args ...interface{}) (interface{}, error) {
+	if len(args) < 2 {
+		return nil, errors.New("need at least 2 arguments to append")
+	}
+
+	to := args[len(args)-1]
+	from := args[:len(args)-1]
+
+	tov, toIsNil := indirect(reflect.ValueOf(to))
+
+	toIsNil = toIsNil || to == nil
+	var tot reflect.Type
+
+	if !toIsNil {
+		if tov.Kind() != reflect.Slice {
+			return nil, fmt.Errorf("expected a slice, got %T", to)
+		}
+
+		tot = tov.Type().Elem()
+		toIsNil = tov.Len() == 0
+
+		if len(from) == 1 {
+			// If we get []string []string, we append the from slice to to
+			fromv := reflect.ValueOf(from[0])
+			if fromv.Kind() == reflect.Slice {
+				fromt := reflect.TypeOf(from[0]).Elem()
+				if tot == fromt {
+					return reflect.AppendSlice(tov, fromv).Interface(), nil
+				}
+			}
+		}
+	}
+
+	if toIsNil {
+		return ns.Slice(from...), nil
+	}
+
+	for _, f := range from {
+		fv := reflect.ValueOf(f)
+		if tot != fv.Type() {
+			return nil, fmt.Errorf("append element type mismatch: expected %v, got %v", tot, fv.Type())
+		}
+		tov = reflect.Append(tov, fv)
+	}
+
+	return tov.Interface(), nil
+}