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POScala: referencing companion object from a child class
 

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    <p>I'm thinking of a following Scala class layout. I have a basic trait that represents an Item - an interface of what ought to be an immutable object that we can query for name, weight and do some object-specific stuff by invoking methods like <code>equip</code>:</p> <pre><code>trait Item { def name: String def weight: Int def equip // ... more abstract methods } </code></pre> <p>I can create <code>Item</code> implementations, creating case classes by hand, but I'd like to have some sort of "dictionary"-based items - i.e. a static map holds mapping from type ID to values, <code>name</code> and <code>weight</code> methods just query the dictionary with a stored type ID:</p> <pre><code>object Weapon { final val NameDict = Map(1 -&gt; "short sword", 2 -&gt; "long sword") final val WeightDict = Map(1 -&gt; 15, 2 -&gt; 30) } case class Weapon(typ: Int) extends Item { import Weapon._ def name = NameDict(typ) def weight = WeightDict(typ) def equip = ... // implementation for weapon } </code></pre> <p>So far, so good. I can use <code>Weapon(1)</code> to reference an item object for "short sword". However, the same basic principle applies to any other item types, such as <code>Armor</code>, which uses exactly the same <code>name</code> and <code>weight</code> implementation, but completely different <code>equip</code> and other abstract methods implementation, for example:</p> <pre><code>object Armor { final val NameDict = Map(3 -&gt; "bronze armor", 4 -&gt; "iron armor") final val WeightDict = Map(3 -&gt; 100, 4 -&gt; 200) } case class Armor(typ: Int) extends Item { import Armor._ def name = NameDict(typ) def weight = WeightDict(typ) def equip = ... // implementation for armor } </code></pre> <p>Looks pretty similar to <code>Weapon</code>, isn't it? I'd like to factor out the common pattern (i.e. implementation of lookups in companion object dictionaries and common <code>typ</code> value) in something like that:</p> <pre><code>abstract class DictionaryItem(typ: Int) extends Item { def name = ???.NameDict(typ) def weight = ???.WeightDict(typ) } object Weapon { final val NameDict = Map(1 -&gt; "sword", 2 -&gt; "bow") final val WeightDict = Map(1 -&gt; 15, 2 -&gt; 30) } case class Weapon(typ: Int) extends DictionaryItem(typ) { def equip = ... // implementation for weapon } </code></pre> <p>But how do I reference a children's companion object (like <code>Weapon.NameDict</code>) from parent class - i.e. what should I use instead of <code>???.NameDict(typ)</code> and how do I explain to the compiler that children's companion object must include these dictionaries? Is there a better, more Scala-esque approach to such a problem?</p>
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    1. USGreyCat
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    1. COPerhaps this is a related question you want to check out http://stackoverflow.com/questions/12179092/typing-over-companion-object-in-scala
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      1. USKane
    2. COThanks, Kane! It *is* related, yes.
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