Scala implicit string conversions utility

Question

I'm trying to write some string utils to be able to do implicit conversions of the form "myString".doSpecialConversion and have a specialValue based on which doSpecialConversion works.

Attempt-1: Use a trait:

trait ConversionUtils {

  // Need an overridable value                                                                                                                                                                                                                                                 
  lazy val specialValue = ","

  implicit class StringConversions(val s: String) {
    def doSpecialConversion: Option[String] = if (s == specialValue) None else Some(s)
  }
}

Trait works just fine but the problem is that its not static so multiple StringConversions will be created which is undesired. So I try to extend AnyVal which can't be done for a trait because Another limitation that is a result of supporting only one parameter to a class is that a value class must be top-level or a member of a statically accessible object.

Attempt-2: Use a singleton:

object ConversionUtils {

  // Need an overridable value                                                                                                                                                                                                                                                 
  lazy val specialValue = ","

  implicit class StringConversions(val s: String) extends AnyVal {
    def doSpecialConversion: Option[String] = if (s == specialValue) None else Some(s)
  }
}

Question: How do I provide a Util to be able to override the specialValue for StringConversions and be true-ly static?

dk14 · Accepted Answer · 2016-03-03 09:16:25Z

1

You can ask for an implicit parameter:

object ConversionUtils {

  case class SpecialValue(str: String)

  implicit class StringConversions(val s: String) extends AnyVal {
    def doSpecialConversion(implicit sv: SpecialValue): Option[String] = if (s == sv.str) None else Some(s)
  }
}

Usage:

scala> implicit val sp = SpecialValue(",")
sp: ConversionUtils.SpecialValue = SpecialValue(,)

scala> "aaa".doSpecialConversion
res0: Option[String] = Some(aaa)

scala> ",".doSpecialConversion
res1: Option[String] = None

In general case, macro-libraries, like machinist might help to get rid of boilerplate.

edited Mar 3, 2016 at 9:16

answered Mar 3, 2016 at 8:34

dk14

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