Scala map with partial function as value

Question

In Twitter's Scala school collections section, they show a Map with a partial function as a value:

// timesTwo() was defined earlier.
def timesTwo(i: Int): Int = i * 2
Map("timesTwo" -> timesTwo(_))

If I try to compile this with Scala 2.9.1 and sbt I get the following:

[error] ... missing parameter type for expanded function ((x$1) => "timesTwo".$minus$greater(timesTwo(x$1)))
[error]     Map("timesTwo" -> timesTwo(_))
[error]                                ^
[error] one error found

If I add the parameter type:

Map("timesTwo" -> timesTwo(_: Int))

I then get the following compiler error:

[error] ... type mismatch;
[error]  found   : Int => (java.lang.String, Int)
[error]  required: (?, ?)
[error]     Map("timesTwo" -> timesTwo(_: Int))
[error]                    ^
[error] one error found

I'm stumped. What am I missing?

Community · Accepted Answer · 2017-05-23 12:11:25Z

5

It thinks you want to do this:

 Map((x: Int) => "timesTwo".->timesTwo(x))

When you want this:

 Map("timesTwo" -> { (x: Int) => timesTwo(x) })

So this works:

 Map( ("timesTwo", timesTwo(_)) )
 Map("timesTwo" -> { timesTwo(_) })

Note this is not an usual error, see

(and probably more)

edited May 23, 2017 at 12:11

CommunityBot

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answered Mar 23, 2012 at 4:21

huynhjl

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2 Comments

Luigi Plinge Over a year ago

Or Map("timesTwo" -> (timesTwo _))

kiritsuku Over a year ago

or Map[String, Int => Int]("timesTwo" -> timesTwo)

oxbow_lakes · Accepted Answer · 2012-03-24 17:45:46Z

2

You are missing telling scalac that you want to lift the method timesTwo into a function. This can be done with an underscore as follows

scala> Map("timesTwo" -> timesTwo _)
res0: scala.collection.immutable.Map[java.lang.String,Int => Int] = Map(timesTwo -> <function1>)

answered Mar 24, 2012 at 17:45

oxbow_lakes

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