将元组映射到Scala中的元组

Question

I want to map a pair of options of String like the following

val pair: (Option[String], Option[String]) = (Some("a"), None)

val mapped: (String, String) = pair map {case (a:Option[String],b:Option[String]) => (a.getOrElse(""),b.getOrElse(""))}

but the output signature is different from what I expected

(Option[String],(String,String))

It seems that I'm missing something here... maybe scalaz or shapeless allows for such functionality of mapping tuples?

Answer 1

Simple change from map to match you'll get expected types.

scala>   val pair: (Option[String], Option[String]) = (Some("a"), None)
pair: (Option[String], Option[String]) = (Some(a),None)

scala>

scala>   val mapped: (String, String) = pair match {case (a:Option[String],b:Option[String]) => (a.getOrElse(""),b.getOrElse(""))}
mapped: (String, String) = (a,"")

scala>

scala>   mapped
res8: (String, String) = (a,"")

Answer 2

In case if you specifically want to do things like that with shapeless , you should make some preparations.

First tell your compiler what you want to use in case of None :

class Default[T](val value: T)

implicit object defaultString extends Default[String]("")

Now create your function to map :

import shapeless._

object extract extends Poly1 {
  implicit def withDefault[T](implicit default: Default[T]) =
    at[Option[T]](_ getOrElse default.value)
}

Now use shapeless extension for tuples :

import syntax.std.tuple._

pair.map(extract) // res0: (String, String) = (a,)