Understanding implicit in Scala

问题:

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 =>
taskForm.bindFromRequest.fold(
        errors => BadRequest(views.html.index(Task.all(), errors)),
        label => {
          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?

回答1:

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 => 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 => ...

In a function literal, the part before the => 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.

回答2:

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")
}

回答3:

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.

回答4:

In scala implicit works as:

Converter

Parameter value injector

There are 3 types of use of Implicit

1. Implicitly type conversion : It converts the error producing assignment into intended type

   val x :String = "1"

   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 = "1"

val y:Int = x // compiler will use z here like val y:Int=z(x)

println(y) // result 2  & no error!

2. 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 ="BMW"
   
   }
   

---

    class Johnny{

    val haveTv:String = "Sony"

    }

---

   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 ="BMW"

}

---

class Johnny{

val haveTv:String = "Sony"

}

---

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 & no error

3. 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 & 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 => String):String =a

x{
x => c
}

it will cause error, because c in x{x=>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 => 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 =>

Ok(views.html.formpage())  

}

if you do not mention request parameter as implicit explicitly then you must have been written-

def index = Action{
request =>

Ok(views.html.formpage()(request))  

}

回答5:

Also, in the above case there should be only one implicit function whose type is double => 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

回答6:

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.