Understanding implicit in Scala

Question

I was making my way through the Scala playframework tutorial and I came across this snippet of code which had me puzzled    def newTask   Action   implicit request   gt  taskForm bindFromRequest fold          errors   gt  BadRequest views html index Task all    errors            label   gt              Task create label            Redirect routes Application tasks                       So I decided to investigate and came across this post   I still don t get it   What is the difference between this    implicit def double2Int d   Double    Int   d toInt   and  def double2IntNonImplicit d   Double    Int   d toInt   other than the obvious fact they have different method names   When should I use implicit and why

User · Answer

A very basic example of Implicits in scala.

Implicit parameters:

val value = 10
implicit val multiplier = 3
def multiply(implicit by: Int) = value * by
val result = multiply // implicit parameter wiil be passed here
println(result) // It will print 30 as a result

Note: Here multiplier will be implicitly passed into the function multiply. Missing parameters to the function call are looked up by type in the current scope meaning that code will not compile if there is no implicit variable of type Int in the scope.

Implicit conversions:

implicit def convert(a: Double): Int = a.toInt
val res = multiply(2.0) // Type conversions with implicit functions
println(res)  // It will print 20 as a result

Note: When we call multiply function passing a double value, the compiler will try to find the conversion implicit function in the current scope, which converts Int to Double (As function multiply accept Int parameter). If there is no implicit convert function then the compiler will not compile the code.

User · Answer

WARNING  contains sarcasm judiciously  YMMV     Luigi s answer is complete and correct  This one is only to extend it a bit with an example of how you can gloriously overuse implicits  as it happens quite often in Scala projects  Actually so often  you can probably even find it in one of the  Best Practice  guides   object HelloWorld     case class Text content  String    case class Prefix text  String     implicit def String2Text content  String  implicit prefix  Prefix          Text prefix text         content         def printText text  Text   Unit         println text content         def main args  Array String    Unit         printText  World              Best to hide this line somewhere below a pile of completely unrelated code       Better yet  import its package from another distant place    implicit val prefixLOL   Prefix  Hello

User · Answer

I had the exact same question as you had and I think I should share how I started to understand it by a few really simple examples  note that it only covers the common use cases    There are two common use cases in Scala using implicit    Using it on a variable Using it on a function   Examples are as follows  Using it on a variable  As you can see  if the implicit keyword is used in the last parameter list  then the closest variable will be used      Here I define a class and initiated an instance of this class case class Person val name  String  val charles  Person   Person  Charles       Here I define a function def greeting words  String  implicit person  Person    person match     case Person name  String  if name         gt  s  name   words      case     gt    words     greeting  Good morning      Charles  Good moring  val charles  Person   Person     greeting  Good morning      Good moring   Using it on a function  As you can see  if the implicit is used on the function  then the closest type conversion method will be used    val num   10    num  Int  of course      Here I define a implicit function implicit def intToString num  Int    s  num -- I am a String now    val num   10    num  Int  of course   Nothing happens yet   Compiler believes you want 10 to be an Int     Util    val num  String   10    Compiler trust you first  and it thinks you have  implicitly  told it that you had a way to covert the type from Int to String  which the function  intToString  can do     So num is now actually  10 -- I am a String now      console will print this - gt  val num  String   10 -- I am a String now    Hope this can help

User · Answer

In scala implicit works as  Converter Parameter value injector Extension method There are 3 types of use of Implicit  Implicitly type conversion   It converts the error producing assignment into intended type val x  String    quot 1 quot   val y Int   x    String is not the sub type of Int   so error happens in line 2  To resolve the error the compiler will look for such a method in the scope which has implicit keyword and takes a String as argument and returns an Int   so implicit def z a String  Int   2  val x  String    quot 1 quot   val y Int   x    compiler will use z here like val y Int z x   println y     result 2   amp  no error    Implicitly receiver conversion  We generally by receiver call object s properties  eg  methods or variables   So to call any property by a receiver the property must be the member of that receiver s class object   class Mahadi    val haveCar String   quot BMW quot              class Johnny       val haveTv String    quot Sony quot              val mahadi   new Mahadi       mahadi haveTv    Error happening  Here mahadi haveTv will produce an error  Because scala compiler will first look for the haveTv property to mahadi receiver  It will not find  Second it will look for a method in scope having implicit keyword which take Mahadi object as argument and returns Johnny object  But it does not have here  So it will create error  But the following is okay  class Mahadi   val haveCar String   quot BMW quot       class Johnny   val haveTv String    quot Sony quot       val mahadi   new Mahadi  implicit def z a Mahadi  Johnny   new Johnny  mahadi haveTv    compiler will use z here like new Johnny   haveTv  println mahadi haveTv    result Sony  amp  no error   Implicitly parameter injection  If we call a method and do not pass its parameter value  it will cause an error  The scala compiler works like this - first will try to pass value  but it will get no direct value for the parameter   def x a Int   a   x    ERROR happening    Second if the parameter has any implicit keyword it will look for any val in the scope which have the same type of value  If not get it will cause error  def x implicit a Int   a  x    error happening here  To slove this problem compiler will look for a implicit val having the type of Int because the parameter a has implicit keyword  def x implicit a Int  a  implicit val z Int  10  x    compiler will use implicit like this x z  println x     will result 10  amp  no error   Another example  def l implicit b Int   def x implicit a Int   l a   we can also write it like- def x implicit a Int   l  Because l has a implicit parameter and in scope of method x s body  there is an implicit local variable parameters are local variables  a which is the parameter of x  so in the body of x method the method-signature l s implicit argument value is filed by the x method s local implicit variable parameter  a implicitly  So  def x implicit a Int   l  will be in compiler like this def x implicit a Int   l a   Another example  def c implicit k Int  String   k toString  def x a Int   gt  String  String  a  x  x   gt  c    it will cause error  because c in x x  gt c  needs explicitly-value-passing in argument or implicit val in scope  So we can make the function literal s parameter explicitly implicit when we call the method x x  implicit x   gt  c    the compiler will set the parameter of c like this c x     This has been used in action method of Play-Framework in view folder of app the template is declared like     implicit requestHreader RequestHeader   in controller action is like  def index   Action  implicit request   gt   Ok views html formpage          if you do not mention request parameter as implicit explicitly then you must have been written- def index   Action  request   gt   Ok views html formpage   request          Extension Method  Think  we want to add new method with Integer object  The name of the method will be meterToCm   gt  1  meterToCm  res0 100   to do this we need to create an implicit class within a object class trait   This class can not be a case class  object Extensions           implicit class MeterToCm meter Int                    def  meterToCm                meter 100                      Note the implicit class will only take one constructor parameter  Now import the implicit class in the scope you are wanting to use import  Extensions    2 meterToCm    result 200

User · Answer

Also  in the above case there should be only one implicit function whose type is double   gt  Int  Otherwise  the compiler gets confused and won t compile properly     this won t compile  implicit def doubleToInt d  Double    d toInt implicit def doubleToIntSecond d  Double    d toInt val x  Int   42 0

User · Answer

I ll explain the main use cases of implicits below  but for more detail see the relevant chapter of Programming in Scala   Implicit parameters  The final parameter list on a method can be marked implicit  which means the values will be taken from the context in which they are called  If there is no implicit value of the right type in scope  it will not compile  Since the implicit value must resolve to a single value and to avoid clashes  it s a good idea to make the type specific to its purpose  e g  don t require your methods to find an implicit Int   example        probably in a library class Prefixer val prefix  String  def addPrefix s  String  implicit p  Prefixer    p prefix   s       then probably in your application implicit val myImplicitPrefixer   new Prefixer        addPrefix  abc       returns     abc    Implicit conversions  When the compiler finds an expression of the wrong type for the context  it will look for an implicit Function value of a type that will allow it to typecheck  So if an A is required and it finds a B  it will look for an implicit value of type B   gt  A in scope  it also checks some other places like in the B and A companion objects  if they exist   Since defs can be  eta-expanded  into Function objects  an implicit def xyz arg  B   A will do as well   So the difference between your methods is that the one marked implicit will be inserted for you by the compiler when a Double is found but an Int is required    implicit def doubleToInt d  Double    d toInt val x  Int   42 0   will work the same as  def doubleToInt d  Double    d toInt val x  Int   doubleToInt 42 0    In the second we ve inserted the conversion manually  in the first the compiler did the same automatically  The conversion is required because of the type annotation on the left hand side     Regarding your first snippet from Play   Actions are explained on this page from the Play documentation  see also API docs   You are using   apply block   Request AnyContent     Result   Action AnyContent    on the Action object  which is the companion to the trait of the same name    So we need to supply a Function as the argument  which can be written as a literal in the form   request   gt        In a function literal  the part before the   gt  is a value declaration  and can be marked implicit if you want  just like in any other val declaration  Here  request doesn t have to be marked implicit for this to type check  but by doing so it will be available as an implicit value for any methods that might need it within the function  and of course  it can be used explicitly as well   In this particular case  this has been done because the bindFromRequest method on the Form class requires an implicit Request argument

User · Answer

Why and when you should mark the request parameter as implicit    Some methods that you will make use of in the body of your action have an implicit parameter list like  for example  Form scala defines a method   def bindFromRequest   implicit request  play api mvc Request      Form T              You don t necessarily notice this as you would just call myForm bindFromRequest   You don t have to provide the implicit arguments explicitly  No  you leave the compiler to look for any valid candidate object to pass in every time it comes across a method call that requires an instance of the request  Since you do have a request available  all you need to do is to mark it as implicit   You explicitly mark it as available for implicit use   You hint the compiler that it s  OK  to use the request object sent in by the Play framework  that we gave the name  request  but could have used just  r  or  req   wherever required   on the sly     myForm bindFromRequest     see it  it s not there  but it is there   It just happens without your having to slot it in manually in every place it s needed  but you can pass it explicitly  if you so wish  no matter if it s marked implicit or not    myForm bindFromRequest   request    Without marking it as implicit  you would have to do the above  Marking it as implicit you don t have to   When should you mark the request as implicit  You only really need to if you are making use of methods that declare an implicit parameter list expecting an instance of the Request  But to keep it simple  you could just get into the habit of marking the request implicit always  That way you can just write beautiful terse code

[scala] Understanding implicit in Scala

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