Just monad

just_monad.php

<?php

class Just {
  
  protected $value;
  
  function __construct( $value ) {
    $this->value = $value;  
  }
  
  public static function of( $value ) {
    return new self( $value );
  }
  
  public function map( callable $fn ) {
    return Just::of( $fn( $this->value ) );
  }
  
  public function value() {
    return $this->value;
  }
  
}