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up vote 6 down vote favorite

In my opinion, one of the greatest things about Scala is its interoperability with Java and its similar syntax. One thing that I found strange is the use of the _ operator for package wilcard imports instead of the * operator that is used in Java.

Is there a technical reason for using _ instead of *? If not, then why was this change done?

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This might just be a functional thing. I know Haskell uses _ in a few places to act as a wildcard/not-specified value. –  KChaloux Apr 11 '13 at 19:17
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4 Answers

up vote 20 down vote accepted

In Scala, the * is a valid identifier. One could write:

val * = "trollin'"
println(*)

With the result being:

trollin'

One could write a class named * as such:

class * {
  def test():String = {
    "trollin'"
  }
}

So with that being the case, when I have a class * in the package us.hexcoder and I write:

import us.hexcoder.*

You would be saying that you wish to import a class with the name *. Because of this, Scala needed to use another symbol to indicate a wildcard import. For whatever reason, they decided to use _ as the wildcard symbol.

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3  
The question becomes then, why was * chosen as a valid identifier and _ as wildcard? –  Mike Partridge Apr 11 '13 at 20:01
2  
Even though I have more upvotes, this is the more correct answer. Never heard of * as an identifier before. –  Mike Brown Apr 11 '13 at 20:12
 
@MikeBrown It's also valid in Lisp. Here is an example in Scheme –  Glenn Nelson Apr 11 '13 at 21:02
7  
@MikePatridge Almost definitely because Scala defines all of its operators as functions, and allows the developer to define their own operators as such. It doesn't do special-case syntax for operators, and * needed to be included for multiplication. So another less common character needed to be chosen as a reserved character. –  KChaloux Apr 11 '13 at 23:08
1  
The _ character resembles a blank (as in "fill in the blank"), so whether Scala borrowed it from somewhere else or came up with it themselves, it makes sense to use it as a wildcard. –  Michael Shaw Apr 12 '13 at 3:11
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up vote 12 down vote

In functional languages, the _ character is commonly used to say, "I don't care about this parameter" or "anything can go here". Extending that value to namespace imports only makes sense.

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up vote 1 down vote

In addition to Gleen's answer, import is a valid statement anywhere in Scala and you can import an object or an instance members into scope. As * is obviously a member of many classes, it can not be used as a wildcard for the import statement. So you end up with a string which must not be a valid identifier. _ comes to mind. Indeed other places where it is used for a different meaning (existential type, function as an instance) is also due to the same issue.

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up vote 0 down vote

In Scala there are dozens of places where the underscore is used. Using it for wildcard imports is just a logical consequence.

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