What are all the uses of an underscore in Scala

Question

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

User · Answer

There is one usage I can see everyone here seems to have forgotten to list...

Rather than doing this:

List("foo", "bar", "baz").map(n => n.toUpperCase())

You could can simply do this:

List("foo", "bar", "baz").map(_.toUpperCase())

User · Answer

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   gt            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     gt  5               Discarded parameter case     gt             Wild card pattern -- matches anything val  a        1  2     same thing for     lt - 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  lt  gt            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

User · Answer

An excellent explanation of the uses of the underscore is Scala    underscore  magic   Examples    def matchTest x  Int   String   x match        case 1   gt   one       case 2   gt   two       case     gt   anything other than one and two       expr match        case List 1        gt    a list with three element and the first element is 1       case List        gt    a list with zero or more elements        case Map        gt    matches a map with any key type and any value type        case     gt       List 1 2 3 4 5  foreach print         Doing the same without underscore    List 1 2 3 4 5  foreach  a   gt  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   gt  Bar           Imports all the members except Foo  To exclude a member rename it to   import com test Fun   Foo   gt            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 gt 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

User · Answer

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       gt  println  Hi       Ignored names of self types  trait MySeq      Seq      gt      Wildcard patterns  Some 5  match   case Some      gt  println  Yes       Wildcard patterns in interpolations   abc  match   case s a  c    gt      Sequence wildcard in patterns  C 1  2  3  match   case C vs         gt  vs foreach f         Wildcard imports  import java util     Hiding imports  import java util  ArrayList   gt          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    gt  Int        gt  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   gt  Unit     val set  Set     gt  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      gt  Unit    gt     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     gt  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

User · Answer

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

User · Answer

There is a specific example that     be used     type StringMatcher   String   gt   String   gt  Boolean     def starts  StringMatcher    prefix String    gt    startsWith prefix   may be equal to      def starts  StringMatcher    prefix String    gt   s   gt s startsWith prefix   Applying         in some scenarios will automatically convert to     x n     x n

User · Answer

Here are some more examples where   is used   val nums   List 1 2 3 4 5 6 7 8 9 10   nums filter      2    0   nums reduce          nums exists    gt  5   nums takeWhile    lt  8    In all above examples one underscore represents an element in the list  for reduce the first underscore represents the accumulator

[scala] What are all the uses of an underscore in Scala?

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