An improvement to make seq's extractors work for Array and String #7108

scabug · 2013-02-10T09:18:11Z

Firstly, the following codes compile fine while run with exception:

scala> import scala.collection.SeqLike
import scala.collection.SeqLike
scala> val xs: SeqLike[Char, String] = "abcdef"
xs: scala.collection.SeqLike[Char,String] = abcdef
scala> xs match { case head +: tail => tail }
java.lang.ClassCastException: java.lang.String cannot be cast to scala.collection.SeqLike
	at .<init>(<console>:10)
	at .<clinit>(<console>)
	at .<init>(<console>:7)
	at .<clinit>(<console>)
        ...

Secondly, there are two useful extractors defined in scala.collection.SeqExtractors.scala. But sadly these two extractors does not work for Array and String, and sometimes we do need these function, especially for String.

I would like the followed codes working as expected:

// Expected Result
"abcdef" match { case p +: m :+ q => m } // expect m == "bcde"
Array(1,2,3) match { case h +: tail => tail } //expect tail == Array(2,3)

Here I got an idea to fix the issue by rewriting these two extractors as following:

// SeqExtractors.scala
package scala.collection

/** An extractor used to head/tail deconstruct sequences. */
object +: {
  def unapply[T,Coll <% SeqLike[T, Coll]](t: Coll): Option[(T, Coll)] =
    if(t.isEmpty) None
    else Some(t.head -> t.tail)
}

/** An extractor used to init/last deconstruct sequences. */
object :+ {
  /** Splits a sequence into init :+ tail.
   * @return Some((init, tail)) if sequence is non-empty. None otherwise.
   */
  def unapply[T,Coll <% SeqLike[T, Coll]](t: Coll): Option[(Coll, T)] =
    if(t.isEmpty) None
    else Some(t.init -> t.last)
}

scabug · 2013-02-10T09:18:11Z

Imported From: https://issues.scala-lang.org/browse/SI-7108?orig=1
Reporter: Eastsun (eastsun)
Affected Versions: 2.11.0-M1

scabug · 2013-02-18T18:41:24Z

@paulp said:
How can this possibly be right:

  pt1 match {
    // if at least one of the types in an intersection is checkable, use the checkable ones
    // this avoids problems as in run/matchonseq.scala, where the expected type is `Coll with scala.collection.SeqLike`
    // Coll is an abstract type, but SeqLike of course is not
    case RefinedType(ps, _) if ps.length > 1 && (ps exists infer.isCheckable) =>
      None

    case ptCheckable if infer isUncheckable ptCheckable =>
      val classTagExtractor = resolveClassTag(pos, ptCheckable)

      if (classTagExtractor != EmptyTree && unapplyMember(classTagExtractor.tpe) != NoSymbol)
        Some(classTagExtractor)
      else None

    case _ => None
  }

scabug added enhancement quickfix patmat labels Apr 7, 2017

SethTisue removed quickfix labels Feb 5, 2018

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An improvement to make seq's extractors work for Array and String #7108

An improvement to make seq's extractors work for Array and String #7108

scabug commented Feb 10, 2013 •

edited by retronym

scabug commented Feb 10, 2013

scabug commented Feb 18, 2013

An improvement to make seq's extractors work for Array and String #7108

An improvement to make seq's extractors work for Array and String #7108

Comments

scabug commented Feb 10, 2013 • edited by retronym

scabug commented Feb 10, 2013

scabug commented Feb 18, 2013

scabug commented Feb 10, 2013 •

edited by retronym