hugo/tpl/collections/append.go

// 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
}