Databases
StackOverflow2013
Postsdbo
POScala: Ordering contravariance
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POScala: Ordering contravariance
    primarykey
    data
    text
    <p>Is there any reason why Scala's <code>Ordering</code> trait is not contravariant? A motivating example follows.</p> <p>Suppose I want to perform an ordered insert. I may have a function with the signature</p> <pre><code>def insert[A, B &gt;: A](list: List[A], item: A)(implicit ord: Ordering[B]): List[A] </code></pre> <p>Here, I have an <code>Ordering</code> which accepts super types of type <code>A</code>. I imagine this is useful when you're dealing with <code>case classes</code>. For example:</p> <pre><code>abstract class CodeTree case class Fork(left: CodeTree, right: CodeTree, chars: List[Char], weight: Int) extends CodeTree case class Leaf(char: Char, weight: Int) extends CodeTree def weight(tree: CodeTree): Int def chars(tree: CodeTree): List[Char] implicit object CodeTreeOrdering extends Ordering[CodeTree] { def compare(a: CodeTree, b: CodeTree): Int = weight(a) compare weight(b) } </code></pre> <p>I would want my insert function to work with types <code>List[CodeTree]</code>, <code>List[Leaf]</code> or <code>List[Fork]</code>. However, as <code>Ordering</code> isn't contravariant, I need to define implicit <code>Orderings</code> for each <code>case</code>.</p> <p>If I define</p> <pre><code>trait MyOrdering[-A] { def compare(a: A, b: A): Int } </code></pre> <p>everything works as expected.</p> <p>Is there any other way to achieve my goal?</p> <p>EDIT:</p> <p>My current solution is to define insert as </p> <pre><code>def insert[A](list: List[A], item: A)(implicit ord: Ordering[A]): List[A] </code></pre> <p>which works fine when dealing with <code>List[CodeTree]</code>. I also define (inspired by the scalaz library):</p> <pre><code>trait Contravariant[F[_]] { def contramap[A, B](r: F[A], f: B =&gt; A): F[B] } implicit object OrderingContravariant extends Contravariant[Ordering] { def contramap[A, B](r: Ordering[A], f: B =&gt; A) = r.on(f) } implicit def orderingCodeTree[A &lt;: CodeTree]: Ordering[A] = implicitly[Contravariant[Ordering]].contramap(CodeTreeOrdering, identity) </code></pre> <p>I'm defining an implicit factory function for <code>Ordering[A &lt;: CodeTree]</code> instances.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USDuarte Nunes
    1. USDuarte Nunes
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POScala: Ordering contravariance
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POScala: Ordering contravariance
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POScala: Ordering contravariance
      1. This table or related slice is empty.
      1. VTUpMod
    1. COLooks like it's a bit of a technical issue relating to the type inferencer failing to find the most specific ordering. See http://scala-programming-language.1934581.n4.nabble.com/Contravariant-Ordering-T-java-util-Comparator-and-uncheckedVariance-td1955224.html for details.
      singulars
      1. POScala: Ordering contravariance
      1. USImpredicative
    2. CO@Impredicative I edited the post with a nasty workaround.
      singulars
      1. POScala: Ordering contravariance
      1. USDuarte Nunes
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload