When / Then / Match expression DSL

Test.class.php

<?php

use util\cmd\Console;

class One extends \lang\Object {

}

class Two extends \lang\Object {

}

class Three extends \lang\Object {

}

interface Result {

  public function get();

  public function present();

  public function when($expr);

  public function then($value);

}

class Next extends \lang\Object implements Result {

  public function __construct($match, $expr) {
    $this->match= $match;
    $this->expr= $expr;
  }

  public function get() { throw new \lang\IllegalStateException('Nothing matched'); }

  public function present() { return false; }

  public function when($expr) {
    $this->expr= $expr;
    return $this;
  }

  public function then($value) { 
    if ($this->expr) {
      return new Matched($value);
    } else {
      return $this;
    }
  }

  public function toString() { return '(nothing matched)'; }
}

class Matched extends \lang\Object implements Result {

  public function __construct($value) { $this->value= $value; }

  public function get() { return $this->value; }

  public function present() { return true; }

  public function when($expr) { return $this; }

  public function then($value) { return $this; }

  public function toString() { return \xp::stringOf($this->value); }
}

class Match extends \lang\Object {

  public static function of() { 
    return new self();
  }

  public function when($expr) {
    return new Next($this, $expr);
  }
}

class Test extends \lang\Object {
  
  public static function main($args) {
    $element= new One();
    $result= Match::of()
      ->when($element instanceof One)->then('One')
      ->when($element instanceof Two)->then('Two')
    ;
    Console::writeLine('Instance of ', $element->getClassName(), ' => ', $result);
  }
}

Inspired by http://tutorials.jenkov.com/scala/match.html