I've taken a look at the list of surveys taken on scala-lang.org and noticed a curious question: "Can you name all the uses of “_”?". Can you? If yes, please do so here. Explanatory examples are appreciated.
7 Answers
The ones I can think of are
Existential types
def foo(l: List[Option[_]]) = ...
Higher kinded type parameters
case class A[K[_],T](a: K[T])
Ignored variables
val _ = 5
Ignored parameters
List(1, 2, 3) foreach { _ => println("Hi") }
Ignored names of self types
trait MySeq { _: Seq[_] => }
Wildcard patterns
Some(5) match { case Some(_) => println("Yes") }
Wildcard patterns in interpolations
"abc" match { case s"a$_c" => }
Sequence wildcard in patterns
C(1, 2, 3) match { case C(vs @ _*) => vs.foreach(f(_)) }
Wildcard imports
import java.util._
Hiding imports
import java.util.{ArrayList => _, _}
Joining letters to operators
def bang_!(x: Int) = 5
Assignment operators
def foo_=(x: Int) { ... }
Placeholder syntax
List(1, 2, 3) map (_ + 2)
Method values
List(1, 2, 3) foreach println _
Converting call-by-name parameters to functions
def toFunction(callByName: => Int): () => Int = callByName _
Default initializer
var x: String = _ // unloved syntax may be eliminated
There may be others I have forgotten!
Example showing why foo(_)
and foo _
are different:
This example comes from 0__:
trait PlaceholderExample {
def process[A](f: A => Unit)
val set: Set[_ => Unit]
set.foreach(process _) // Error
set.foreach(process(_)) // No Error
}
In the first case, process _
represents a method; Scala takes the polymorphic method and attempts to make it monomorphic by filling in the type parameter, but realizes that there is no type that can be filled in for A
that will give the type (_ => Unit) => ?
(Existential _
is not a type).
In the second case, process(_)
is a lambda; when writing a lambda with no explicit argument type, Scala infers the type from the argument that foreach
expects, and _ => Unit
is a type (whereas just plain _
isn't), so it can be substituted and inferred.
This may well be the trickiest gotcha in Scala I have ever encountered.
Note that this example compiles in 2.13. Ignore it like it was assigned to underscore.
From (my entry) in the FAQ, which I certainly do not guarantee to be complete (I added two entries just two days ago):
import scala._ // Wild card -- all of Scala is imported
import scala.{ Predef => _, _ } // Exception, everything except Predef
def f[M[_]] // Higher kinded type parameter
def f(m: M[_]) // Existential type
_ + _ // Anonymous function placeholder parameter
m _ // Eta expansion of method into method value
m(_) // Partial function application
_ => 5 // Discarded parameter
case _ => // Wild card pattern -- matches anything
val (a, _) = (1, 2) // same thing
for (_ <- 1 to 10) // same thing
f(xs: _*) // Sequence xs is passed as multiple parameters to f(ys: T*)
case Seq(xs @ _*) // Identifier xs is bound to the whole matched sequence
var i: Int = _ // Initialization to the default value
def abc_<>! // An underscore must separate alphanumerics from symbols on identifiers
t._2 // Part of a method name, such as tuple getters
1_000_000 // Numeric literal separator (Scala 2.13+)
This is also part of this question.
An excellent explanation of the uses of the underscore is Scala _ [underscore] magic.
Examples:
def matchTest(x: Int): String = x match {
case 1 => "one"
case 2 => "two"
case _ => "anything other than one and two"
}
expr match {
case List(1,_,_) => " a list with three element and the first element is 1"
case List(_*) => " a list with zero or more elements "
case Map[_,_] => " matches a map with any key type and any value type "
case _ =>
}
List(1,2,3,4,5).foreach(print(_))
// Doing the same without underscore:
List(1,2,3,4,5).foreach( a => print(a))
In Scala, _
acts similar to *
in Java while importing packages.
// Imports all the classes in the package matching
import scala.util.matching._
// Imports all the members of the object Fun (static import in Java).
import com.test.Fun._
// Imports all the members of the object Fun but renames Foo to Bar
import com.test.Fun.{ Foo => Bar , _ }
// Imports all the members except Foo. To exclude a member rename it to _
import com.test.Fun.{ Foo => _ , _ }
In Scala, a getter and setter will be implicitly defined for all non-private vars in a object. The getter name is same as the variable name and _=
is added for the setter name.
class Test {
private var a = 0
def age = a
def age_=(n:Int) = {
require(n>0)
a = n
}
}
Usage:
val t = new Test
t.age = 5
println(t.age)
If you try to assign a function to a new variable, the function will be invoked and the result will be assigned to the variable. This confusion occurs due to the optional braces for method invocation. We should use _ after the function name to assign it to another variable.
class Test {
def fun = {
// Some code
}
val funLike = fun _
}
Besides the usages that JAiro mentioned, I like this one:
def getConnectionProps = {
( Config.getHost, Config.getPort, Config.getSommElse, Config.getSommElsePartTwo )
}
If someone needs all connection properties, he can do:
val ( host, port, sommEsle, someElsePartTwo ) = getConnectionProps
If you need just a host and a port, you can do:
val ( host, port, _, _ ) = getConnectionProps
There is a specific example that "_" be used:
type StringMatcher = String => (String => Boolean)
def starts: StringMatcher = (prefix:String) => _ startsWith prefix
may be equal to :
def starts: StringMatcher = (prefix:String) => (s)=>s startsWith prefix
Applying “_” in some scenarios will automatically convert to “(x$n) => x$n ”