Map tuple to tuple in Scala

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?

As it stands, this does not compile for me (and I would not expect it to - the type Pair / Tuple2 doesn't have a map method). Is there a List or Map involved here that you have tried to take out to simplify the question? — Shadowlands
– Shadowlands, Commented Nov 9, 2015 at 12:03
@rahilb it was added at 2.0 version as mentioned in Miles's answer — Odomontois
– Odomontois, Commented Nov 9, 2015 at 15:15

vvg · Accepted Answer · 2015-11-09 12:04:44Z

7

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,"")

answered Nov 9, 2015 at 12:04

vvg

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2 Comments

stanislav.chetvertkov Over a year ago

it seems i got 'map' from scalaz imported, which is probably misled me)

Odomontois Over a year ago

@stanislav.chetvertkov map in scalaz treats Tuple2 as monad. During map it will change only second part as defined here

Odomontois · Accepted Answer · 2015-11-09 15:13:11Z

0

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,)

answered Nov 9, 2015 at 15:13

Odomontois

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