I have to choose the more elegant interface for selectors for the treebuilder library. Here I post some alternatives to se how they look like.

current

<?php
// Current interface

$tree
    ->leaf('element', 'author')
        ->key('value', 'author_name')
        ->value('element', 'name')
    ->end()
    ->leaf('element', 'date')
        ->value(function(\DateTime $dateTime) { return $dateTime->format('d-m-Y'); })
        ->key('value', 'born in')
    ->end()
    ->leaf()
        ->key('value', 'short description')
        ->value(function($book) {
                $date = $book['date'];
                return sprintf('"%s": a book by %s written in %s',
                    $book['title'],
                    $book['author']['name'],
                    $book['date']->format('Y')
                );
            })
    ->end()
    ->each('element', 'related')
        ->key('value', 'related titles')
        ->leaf()
            ->value('element', 'title')
        ->end()
    ->end()
;

default PA but no resolver

<?php
// default PA but no resolver

$tree
    ->leaf('author')
        ->key(new Value('author_name'))
        ->value('element', 'name')
    ->end()
    ->leaf('date')
        ->value(function(\DateTime $dateTime) { return $dateTime->format('d-m-Y'); })
        ->key(new Value('born in'))
    ->end()
    ->leaf()
        ->key(new Value('short description'))
        ->value(function($book) {
                $date = $book['date'];
                return sprintf('"%s": a book by %s written in %s',
                    $book['title'],
                    $book['author']['name'],
                    $book['date']->format('Y')
                );
            })
    ->end()
    ->each('related')
        ->key(new Value('related titles'))
        ->leaf()
            ->value('title')
        ->end()
    ->end()
;