Adding to scala mutable map fails

Question

I don't understand why += works when I define a tuple mapEntry beforehand and then add it to a map, while trying to add directly bypassing creating of unnecessary variable mapEntry fails. Most likely I am missing something obvious. Thanks in advance.

This works: map += mapEntry while this fails: map += ("key-to-tom", Person("Tom", "Marx"))

Here is my complete scala REPL session

Welcome to Scala version 2.10.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_45).
Type in expressions to have them evaluated.
Type :help for more information.

scala> case class Person(val fname:String, val lname:String)
defined class Person

scala> val map = scala.collection.mutable.Map[String,Person]()
map: scala.collection.mutable.Map[String,Person] = Map()

scala> val mapEntry = ("key-to-joe", Person("Joe", "Smith") )
mapEntry: (String, Person) = (key-to-joe,Person(Joe,Smith))

scala> map += mapEntry
res0: map.type = Map(key-to-joe -> Person(Joe,Smith))

scala> val mapEntry2 = ("key-to-ann", Person("Ann", "Kline") )
mapEntry2: (String, Person) = (key-to-ann,Person(Ann,Kline))

scala> map += mapEntry2
res1: map.type = Map(key-to-joe -> Person(Joe,Smith), key-to-ann -> Person(Ann,Kline))

scala> map +=  ("key-to-tom", Person("Tom", "Marx") )
<console>:11: error: type mismatch;
 found   : String("key-to-tom")
 required: (String, Person)
              map +=  ("key-to-tom", Person("Tom", "Marx") )
                   ^

scala>

Vidya · Accepted Answer · 2013-11-16 05:27:12Z

3

Your last statement fails because you need to enclose the tuple in parentheses to convey that you are adding a tuple:

map +=  ( ("key-to-tom", Person("Tom", "Marx")) )

The problem is that += is overloaded. Aside from a single tuple, += also can take two or more arguments. The signature is this:

def +=(elem1: (A, B), elem2: (A, B), elems: (A, B)*): Map.this.type

Your tuple is a Tuple2 (which is the same as a Pair but still just a single parameter), and this overloaded method takes 2 arguments (at least). Scala chooses the latter as the closer match.

edited Nov 16, 2013 at 5:27

answered Nov 16, 2013 at 5:22

Vidya

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2 Comments

Sergey Passichenko Over a year ago

Also, you can use map += "key-to-tom" -> Person("Tom", "Marx") syntax

Lu Tahmazyan Over a year ago

Thanks for clear explanation. I was mesmerized by "def +=(kv: (A, B)): Map.this.type" and did not look down further.

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