+ Operator on lists in Scala

Question

I'm pretty new to Scala programming and now trying to understand the exact behavior of the operator + for the scala.collection.immutable.List. Here is what I wrote:

val _1 = List(1, 2) println((_1 + "2") foreach print

I couldn't write the following

_1 + 2 //error: expected String

That's clear, because we're trying to execute the + operator defined within the String class.

But what is not clear it's why they didn't just override the operator + for the List, but instead defined operator ++ for concatenating?

Was there some technical reason for that?

What does that mean?

I think it is a design flaw of the Scala authors to make + attached to Strings. — Arseniy Zhizhelev
– Arseniy Zhizhelev, Commented Jul 20, 2015 at 17:23
What do you mean by "operator + for the List"? scala.ollection.immutable.List does not provide the + operator. And what you want would be :+ instead of + or ++. — yanana
– yanana, Commented Jul 20, 2015 at 17:26
@yanana I know. That's exactly what I was confused by. What was the reason for not providing an overriden version for List? — stella
– stella, Commented Jul 20, 2015 at 17:27
There is a + for everything convertible to String. So the List is first converted to String and then there is a + operator. — Arseniy Zhizhelev
– Arseniy Zhizhelev, Commented Jul 20, 2015 at 17:27
IMHO, to provide a + operator to the List-like data structure is just a bad habit as the + operation in mathematics is commutative, but List nor even String is not. — yanana
– yanana, Commented Jul 20, 2015 at 17:43

0__ · Accepted Answer · 2015-07-20 20:12:04Z

why they didn't just override the operator + for the List, but instead defined operator ++ for concatenating?

Methods that take not individual elements but collections usually indicate that by a "plural" operator, e.g. ++ to concatenate collections, ::: to prepend lists. The + method is for example used to add individual elements to non-linear collections Set and Map:

Set(1, 2, 3) + 4 Map(1 -> "foo", 2 -> "bar") + (3 -> "baz")

And in "plural":

Set(1, 2, 3) ++ Set(4, 5, 6)

Linear collections indicate the "direction" by adding a colon, so :+ for append and +: for prepend:

1 +: Seq(2, 3) :+ 4

Was there some technical reason for that?

No. It is possible to define the + method on any type for any reason. It is just not advised.

List(1, 2) + "3" // "List(1, 2)3"

This is a common puzzler in Scala. If the right hand operand is a String, there is an implicit conversion of the left operand to another String using the toString method, glueing the two strings together.

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