What is the apply function in Scala

Question

I never understood it from the contrived unmarshalling and verbing nouns   an AddTwo class has an apply that adds two   examples   I understand that it s syntactic sugar  so  I deduced from context  it must have been designed to make some code more intuitive   What meaning does a class with an apply function give  What is it used for  and what purposes does it make code better  unmarshalling  verbing nouns etc    how does it help when used in a companion object

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TLDR for people comming from c   It s just overloaded operator of     parentheses So in scala  class X      def apply param1  Int  param2  Int  param3  Int    Int             Do something         Is same as this in c    class X      int operator   int param1  int param2  int param3             do something

User · Answer

1 - Treat functions as objects  2 - The apply method is similar to   call    in Python  which allows you to use an instance of a given class as a function

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It comes from the idea that you often want to apply something to an object  The more accurate example is the one of factories  When you have a factory  you want to apply parameter to it to create an object   Scala guys thought that  as it occurs in many situation  it could be nice to have a shortcut to call apply  Thus when you give parameters directly to an object  it s desugared as if you pass these parameters to the apply function of that object   class MyAdder x  Int      def apply y  Int    x   y    val adder   new MyAdder 2  val result   adder 4     equivalent to x apply 4    It s often use in companion object  to provide a nice factory method for a class or a trait  here is an example   trait A     val x  Int   def myComplexStrategy  Int    object A     def apply x  Int   A   new MyA x     private class MyA val x  Int  extends A       val myComplexStrategy   42         From the scala standard library  you might look at how scala collection Seq is implemented  Seq is a trait  thus new Seq 1  2  won t compile but thanks to companion object and apply  you can call Seq 1  2  and the implementation is chosen by the companion object

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Here is a small example for those who want to peruse quickly   object ApplyExample01 extends App       class Greeter1 var message  String        println  A greeter-1 is being instantiated with message     message           class Greeter2         def apply message  String            println  A greeter-2 is being instantiated with message     message               val g1  Greeter1   new Greeter1  hello     val g2  Greeter2   new Greeter2      g2  world             output      A greeter-1 is being instantiated with message hello      A greeter-2 is being instantiated with message world

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Mathematicians have their own little funny ways  so instead of saying  then we call function f passing it x as a parameter  as we programmers would say  they talk about  applying function f to its argument x        In mathematics and computer science  Apply is a function that applies   functions to arguments     Wikipedia   apply serves the purpose of closing the gap between Object-Oriented and Functional paradigms in Scala  Every function in Scala can be represented as an object  Every function also has an OO type  for instance  a function that takes an Int parameter and returns an Int will have OO type of Function1 Int Int        define a function in scala   x Int    gt  x   1      assign an object representing the function to a variable  val f    x Int    gt  x   1   Since everything is an object in Scala f can now be treated as a reference to Function1 Int Int  object  For example  we can call toString method inherited from Any  that would have been impossible for a pure function  because functions don t have methods     f toString   Or we could define another  Function1 Int Int  object by calling compose method on f and chaining two different functions together    val f2   f compose  x Int    gt  x - 1    Now if we want to actually execute the function  or as mathematician say  apply a function to its arguments  we would call the apply method on the Function1 Int Int  object    f2 apply 2    Writing f apply args  every time you want to execute a function represented as an object is the Object-Oriented way  but would add a lot of clutter to the code without adding much additional information and it would be nice to be able to use more standard notation  such as f args   That s where Scala compiler steps in and whenever we have a reference f to a function object and write f  args  to apply arguments to the represented function the compiler silently expands f  args  to the object method call f apply  args    Every function in Scala can be treated as an object and it works the other way too - every object can be treated as a function  provided it has the apply method  Such objects can be used in the function notation      we will be able to use this object as a function  as well as an object object Foo     var y   5   def apply  x  Int    x   y     Foo  1     using Foo object in function notation    There are many usage cases when we would want to treat an object as a function  The most common scenario is a factory pattern  Instead of adding clutter to the code using a factory method we can apply object to a set of arguments to create a new instance of an associated class   List 1 2 3     same as List apply 1 2 3  but less clutter  functional notation     the way the factory method invocation would have looked    in other languages with OO notation - needless clutter List instanceOf 1 2 3     So apply method is just a handy way of closing the gap between functions and objects in Scala

[scala] What is the apply function in Scala?

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