Suppose I want to explicitly pass an ExecutionContext as a parameter to a method, but want to use that context implicitly in the body of that method. I can achieve this with the following simple code snippet:
def run(foo: Unit => Unit,
ex1: ScheduledExecutorService) {
import scala.concurrent.duration._
implicit val ex2 = ex1
scheduleAtFixedRate(foo, 1.seconds, 3.seconds) // pass ex2 implicitly, not explicitly
}
Is there a way to do this without reassigning ex1 to ex2, but still require the parameters to be passed explicitly?
No. If you want to require that a parameter be passed in explicitly, the only way to use it implicitly within the body of the function is to reassign it to an implicit variable. Why you would ever want to do that defies my reasoning, however.
Resolution of implicit parameters is type-based. The only way I can think of (especially since you want to receive the parameter from the caller) is to have run accept the executor service implicitly, as in:
def run(foo: Unit => Unit)(implicit ex1: ScheduledExecutorService) {
// no need for the below; ex1 already used for implicit resolution
// implicit val ex2 = ex1
}
Of course, that means that the caller for the run method must define the implicit value to used for the executor service, or pass it explicitly as in:
run(someFoo)(someScheduledExecutorService)
Related
(I am fairly new to Scala, hope this isn't a stupid question.)
From what I can see, declaring a parameter to a function implicit has two (related, but quite different) uses:
It makes explicitly passing a corresponding argument when calling the given function optional when the compiler can find a unique suitable value to pass (in the calling scope).
It makes the parameter itself a suitable value to pass to other functions with implicit parameters (when calling them from within the given function).
In code:
def someFunction(implicit someParameter: SomeClass) = { // Note `implicit`
...
// Note no argument supplied in following call;
// possible thanks to the combination of
// `implicit` in `someOtherFunction` (1) and
// `implicit` in line 1 above (2)
someOtherFunction
...
}
def someOtherFunction(implicit someOtherParameter: SomeClass) = {
...
}
implicit val someValue = new SomeClass(...)
// Note no argument supplied in following call;
// possible thanks to `implicit` (1)
someFunction
This seems somewhat strange, doesn't it? Removing implicit from line 1 would make both calls (to someFunction from some other place, and to someOtherFunction from within someFunction) not compile.
What is the rationale behind this? (Edit: I mean what is the official rationale, in case any can be found in some official Scala resource.)
And is there a way to achieve one without the other (that is to allow passing an argument to a function implicitly without allowing it to be used implicitly within that function when calling other functions, and/or to use a non-implicit parameter implicitly when calling other functions)? (Edit: I changed the question a bit. Also, to clarify, I mean whether there is a language construct to allow this - not achieving the effect by manual shadowing or similar.)
For the first question
What is the rationale behind this?
answers are likely to be opinion-based.
And is there a way to achieve one without the other?
Yes, though it's a bit trickier than I thought initially if you want to actually use the parameter:
def someFunction(implicit someParameter: SomeClass) = {
val _someParameter = someParameter // rename to make it accessible in the inner block
{
val someParameter = 0 // shadow someParameter by a non-implicit
someOtherFunction // doesn't compile
someOtherFunction(_someParameter) // passed explicitly
}
}
The rationale is simple:
What has been passed as explicit, stays explicit
What has been marked as implicit, stays implicit
I don't think that any other combination (e.g. implicit -> explicit, let alone explicit -> implicit) would be easier to understand. The basic idea was, I think, that one can establish some common implicit context, and then define whole bunch of methods that expect same implicit variables that describe the established context.
Here is how you can go from implicit to explicit and back:
Implicit -> implicit (default)
def foo(implicit x: Int): Unit = {
bar
}
def bar(implicit x: Int): Unit = {}
Explicit -> implicit:
def foo(x: Int): Unit = {
implicit val implicitX = x
bar
}
def bar(implicit x: Int): Unit = {}
Implicit -> explicit: I would just use Alexey Romanov's solution, but one could imagine that if we had the following method in Predef:
def shadowing[A](f: Unit => A): A = f(())
then we could write something like this:
def foo(implicit x: Int): Unit = {
val explicitX = x
shadowing { x =>
// bar // doesn't compile
bar(explicitX) // ok
}
}
def bar(implicit x: Int): Unit = {}
Essentially, it's the same as Alexey Romanov's solution: we introduce a dummy variable that shadows the implicit argument, and then write the body of the method in the scope where only the dummy variable is visible. The only difference is that a ()-value is passed inside the shadowing implementation, so we don't have to assign a 0 explicitly. It doesn't make the code much shorter, but maybe it expresses the intent a little bit clearer.
I wonder if it's possible to modify an implicit in a context with a function?
With a syntax like this
def modifyImplicit(implicit myImplicit: ImplicitType) : implicit ImplicitType {
myImplicit.setSomthing(something)
myImplicit
}
Because now I must return a type and after the function transform this in a new implicit
if I need to use the function more than once it's became quickly painful.
That's would introduce side-effect (automagically alter the environment without much notice), with it's not "very good".
Instead you can allow some operation to be executed within a managed context, in which you explicitly provide a replacement for the implicit.
implicit def TheDefaultTypeClass: ImplicitType
def withMyContext[T](f: (ImplicitType) => T): T = f(anotherTypeClass)
Then it can be used as following:
val s: String = withMyContext { i =>
val x: ImplicitType = i // Dumb statement just to check type of `i`
// some operations ...
"OK" // result
}
No, it isn't possible. You could write
implicit def modifyImplicit(implicit myImplicit: ImplicitType): ImplicitType = ...
but this won't work the way you want (because for it to ever be called, an implicit of this type must already be available, so either the compiler won't continue looking for an implicit or it will and report conflicting implicits).
Also, having a mutable implicit value seems very likely to lead to bugs.
One possible workaround (in addition to the method proposed by applicius): extract your code into a method and call it with a modified implicit value.
def myMethod(args: ...)(implicit i: ImplicitType) = ...
myMethod(args)(modifyImplicit(implicitly[ImplicitType]))
Yes I now but implicit are mutable because :
```
def modifyImplicit(implicit myImplicit: ImplicitType) {
implicit val myNewImplicit = myImplicit.setSomthing(something)
imASweetMethodWitchUseImplicit
....
}
```
imASweetMethodWitchUseImplicit will use the last implicit setted in the context so we can't "stuck the imutability of the implicit"
I's actually the way i use to made what I whan but I thinks it's a little bit ugly.
I do that for "preparing" the context for other's function so I'm confident because it's just the variable whitch are hide not the call of my function ( witch modify the variables ) you know?
so Alexey I use the same option than you,but I take directly un implicit.
If I call more than one function it's become ugly
```
val result = modifyImplicit()
val result2 = modifyImplicit(result)
implicit val result3 = modifyImplicit(result2)
```
So maybe the solution of applicius can be more beautiful ?
I try to feel the advantage of implicit parameters in Scala. (EDITED: special case when anonymous function is used. Please look at the links in this question)
I try to make simple emulation based on this post. Where explained how Action works in PlayFramework. This also related to that.
The following code is for that purpose:
object ImplicitArguments extends App {
implicit val intValue = 1 // this is exiting value to be passed implicitly to everyone who might use it
def fun(block: Int=>String): String = { // we do not use _implicit_ here !
block(2) // ?? how to avoid passing '2' but make use of that would be passed implicitly ?
}
// here we use _implicit_ keyword to make it know that the value should be passed !
val result = fun{ implicit intValue => { // this is my 'block'
intValue.toString // (which is anonymous function)
}
}
println(result) // prints 2
}
I want to get "1" printed.
How to avoid passing magic "2" but use "1" that was defined implicitly?
Also see the case where we do not use implicit in definition, but it is there, because of anonymous function passing with implicit.
EDITED:
Just in case, I'm posting another example - simple emulation of how Play' Action works:
object ImplicitArguments extends App {
case class Request(msg:String)
implicit val request = Request("my request")
case class Result(msg:String)
case class Action(result:Result)
object Action {
def apply(block:Request => Result):Action = {
val result = block(...) // what should be here ??
new Action(result)
}
}
val action = Action { implicit request =>
Result("Got request [" + request + "]")
}
println(action)
}
Implicits don't work like this. There is no magic. They are just (usually) hidden parameters and are therefore resolved when invoking the function.
There are two ways to make your code work.
you can fix the implicit value for all invocations of fun
def fun(block: Int=>String): String = {
block(implicitly[Int])
}
implicitly is a function defined in Predef. Again no magic. Here's it's definition
def implicitly[A](implicit value: A) = value
But this means it will resolve the implicit value when declaring the fun and not for each invocation.
If you want to use different values for different invocations you will need to add the implicit paramter
def fun(block: Int=>String)(implicit value: Int): String = {
block(value)
}
This will now depend on the implicit scope at the call site. And you can easily override it like this
val result = fun{ _.toString }(3)
and result will be "3" because of the explicit 3 at the end. There is, however, no way to magically change the fun from your declaration to fetch values from implicit scope.
I hope you understand implicits better now, they can be a bit tricky to wrap your head around at first.
It seems that for that particular case I asked, the answer might be like this:
That this is not really a good idea to use implicit intValue or implicit request along with implicitly() using only one parameter for the function that accept (anonymous) function.
Why not, because:
Say, if in block(...) in apply() I would use implicitly[Request], then
it does not matter whether I use "implicit request" or not - it will use
request that is defined implicitly somewhere. Even if I would pass my
own request to Action { myOwnRequest =Result }.
For that particular case is better to use currying and two arguments and.. in the second argument - (first)(second) to use implicit
Like this:
def apply(block:Request => Result)(implicit request:Request):Action2
See my little effort around this example/use case here.
But, I don't see any good example so far in regards to how to use implicit by passing the (anonymous) function as argument (my initial question):
fun{ implicit intValue => {intValue.toString}
or that one (updated version):
val action = Action { implicit request =>
Result("Got request [" + request + "]")
}
I would like to have a higher order function that takes in parameter a function that accepts a specific implicit parameter.
To be more precise, I am trying to make a function that takes a Future creation method that depends on an implicit context and returns a method that doesn't depend on the context.
To be more concrete, let's say that I have something like this:
def foo(a: Int)(implicit ctx: ExecutionContext): Future[Float] = future { somelongBar... }
I would like to do have a method like this:
def provideCtx[A](func: ExecutionContext => A): A = {
val ctx = setupCtx
func(ctx)
}
but if I call provideCtx(foo), the compiler complains about the implicit execution context missing.
The fact that I am dealing with an ExecutionContext is not very important. What I would like to find is how to write the parameter type to accept a function with an implicit argument of a specific type. I understand that the implicit part is a curryed argument, so that in fact I have a function like so: ExecutionContext => Int => Future[Float], and I am pretty sure that at runtime, the jvm doesn't know that that ExecutionContext is implicit, but I can't make the compiler understand that.
The problem is that foo is a method, not a function, and eta-expansion (which converts methods to functions) is not attempted until after implicit application. See section 6.26.2 of the language specification for the details, and this issue for additional discussion.
One workaround would be to write something like this:
provideCtx((ctx: ExecutionContext) => (a: Int) => foo(a)(ctx))
I'm not sure a more generic solution is possible (at least without some kind of reflection, etc.), since we can't even refer to foo (except in a method call, of course) without an implicit in scope.
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?
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.
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")
}
In scala implicit works as:
Converter
Parameter value injector
Extension method
There are some uses of Implicit
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!
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
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))
}
Extension Method
Think, we want to add new method with Integer object. The name of the method will be meterToCm,
> 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
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.
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
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 != "" => s"$name, $words"
case _ => "$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 -> val num: String = 10 -- I am a String now!
Hope this can help.
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.