I want to define a default value for a variable argument parameter in a Scala method. Is it possible?
def aMethod(variableArguments: String* = ???){}
I have tried using variableArguments:String* = Seq("a","b","c"):_* but it doesn't work.
The compiler error makes it pretty clear-cut:
:10: error: a parameter section with a `*'-parameter is not allowed to have default arguments
You can work around this with overloading, if you wish:
def test(a: String*): String = a.mkString
def test(): String = test("a", "b", "c")
Related
scala> val names = List("Peter", "Paul", "Mary")
names: List[String] = List(Peter, Paul, Mary)
scala> names.map(_.toUpperCase)
res12: List[String] = List(PETER, PAUL, MARY)
In this case, the underscore represents the only input argument, which is the element of names. This string is implicitly converted to StringOps and toUpperCase is invoked.
However, this does not work:
scala> names.map(StringOps.toUpperCase _)
<console>:14: error: value toUpperCase is not a member of object scala.collection.immutable.StringOps
names.map(StringOps.toUpperCase _)
I thought that this syntax is how I would get a reference to the function from the toUpperCase method.
First of all, there is no implicit conversion for _.toUppercase that convert String to StringOps, Since toUppercase is belong to String type, it's unnecessary convert to StringOps.
so for _.toUppercase is actual expand to high order function: val a: String => String = (str: String) => str.toUpperCase.
and StringOps implicit conversion defined in Predef.scala: augmentString, and this conversion only will occur when used StringOps's method, like: slice, stripSuffix, stripPrefix etc, for Example:
"name".slice(0, 2) // convert "name" to new StringOps("name").slice(0,2)
"name".stringPrefix
StringOps is a class, hence to use its method you'll need to instantiate an instance:
names.map( x => (new StringOps(x)).toUpperCase )
Or
names.map( new StringOps(_) toUpperCase )
StringOps is a wrapper class of the String class which means that it "enriches" this class with additional methods like the one you are using.
When you want to call a method on a string instance, you simply do:
instanceName.methodName
This is exactly what you are doing in your first example.
However, in your second example you are doing the following:
methodName(instanceName)
toUpperCase does not take an argument, it is called on the instance of the string itself.
I have a method that with the implicit parameter. i get a error when i convert it to function in 2 case :
1:
def action(implicit i:Int) = i + " in action"
val f = action _
then i get a StackOverflowError.
2：
def action(implicit i:Int) = i + " in action"
val f = action(_)
then i get a error: missing parameter type
I must write like this :
val f = (i:Int) => action(i)
that's ok. And if the parameter of 'action' is not the implicit , all case are right. So how to explain , and what i miss ?
If you specify a parameter to a function to be implicit, you are inviting the compiler to supply the value of that parameter for you. So how does the compiler find those values? It looks for values of the same type (Int in your case) that have been declared as implicit values in a variety of scopes.
(For simplicity, I'll just use a local scope in this example, but you might want to read up on this topic. Programming in Scala, 3rd Ed is a good first step.)
Note that the names of the implicit values are ignored and have no bearing on proceedings, the compiler only looks at the types of implicit values. If multiple implicit values with the required type are found in the same scope, then the compiler will complain about ambiguous implicit values.
For example, the following provides a function with an implicit parameter and a default value for that parameter within the current scope:
def greetPerson(name: String)(implicit greeting: String) = s"$greeting $name!"
implicit val defaultGreeting = "Hello" // Implicit value to be used for greeting argument.
val y = greetPerson("Bob") // Equivalent to greetPerson("Bob")(defaultGreeting).
val z = greetPerson("Fred")("Hi")
Note that y is just a String value of "Hello Bob!", and z is a string with the value "Hi Fred!"; neither of them are functions.
Also note that greetPerson is a curried function. This is because implicit parameters cannot be mixed with regular, non-implicit parameters in the same parameter list.
In general, it's bad practice to use common types (Int, Boolean, String, etc.) as values for implicit parameters. In a big program, there might be a lot of different implicit values in your scope, and you might pick up an unexpected value. For that reason, it's standard practice to wrap such values in a case class instead.
If you're trying to create a value that supplies some of the arguments of another function (that is, a partially applied function), then that would look something like this:
def greetPerson(greeting: String, name: String) = s"$greeting $name!"
val sayHello = greetPerson("Hello", _: String)
val y = sayHello("Bob") // "Hello Bob!"
val sayHi = greetPerson("Hi", _: String)
val z = sayHi("Fred") // "Hi Fred!"
In both cases, we're creating partially applied functions (sayHi and sayHello) that call greetPerson with the greeting parameter specified, but which allow us to specify the name parameter. Both sayHello and sayHi are still only values, but their values are partially applied functions rather than constants.
Depending upon your circumstances, I think the latter case may suit you better...
I would also read up on how the underscore character (_) is used in Scala. In a partially applied function declaration, it corresponds to the arguments that will be provided later. But it has a lot of other uses too. I think there's no alternative to reading up on Scala and learning how and when to use them.
I'd like to call a function returned by a function with an implicit parameter, simply and elegantly. This doesn't work:
def resolveA(implicit a: A): String => String = { prefix =>
s"$prefix a=$a"
}
case class A(n: Int)
implicit val a = A(1)
println(resolveA("-->")) // won't compile
I've figured out what's going on: Scala sees the ("-->") and thinks it's an attempt to explicitly fill in the implicit parameter list. I want to pass that as the prefix argument, but Scala sees it as the a argument.
I've tried some alternatives, like putting an empty parameter list () before the implicit one, but so far I've always been stopped by the fact that Scala thinks the argument to the returned function is an attempt to fill in the implicit parameter list of resolveA.
What's a nice way to do what I'm trying to do here, even if it's not as nice as the syntax I tried above?
Another option would be to use the apply method of the String => String function returned by resolveA. This way the compiler won't confuse the parameter lists, and is a little shorter than writing implicltly[A].
scala> resolveA[A].apply("-->")
res3: String = --> a=A(1)
Say I have a function
def f(a:Int = 0, b:String = "", c:Float=0.0, foos: Foo*) { ... }
Notice the use of default arguments for some parameters. Typically, to make use of default values, you invoke a function with named parameters like so:
val foo = Foo("Foo!")
f(foos = foo)
This syntax works because I'm only invoking the method with one foo. However, if I have two or more
val foo1 = Foo("Foo!")
val foo2 = Foo("Bar!")
f(foos = ...)
it's not so obvious what should be fed in here. Seq(foo1,foo2) and Seq(foo1,foo2):_* do not type check.
What's more, how can I call it with no foos?
// All out of foos!
f(foos = ...)
For this case, calling the function with empty parentheses (f()) does not work.
Thanks!
For the default parameters, see my comment to your question. For how to call the variadic part with a named argument, see below (scala 2.9.2):
scala> case class Foo(a:Int)
defined class Foo
scala> def f(a:Int, foos: Foo*) = foos.length
f: (a: Int, foos: Foo*)Int
scala> f(a=2, foos=Foo(2))
res0: Int = 1
// I'm not sure if this is cheating...
// am I just naming the first of the two variadic arguments?
scala> f(a=2, foos=Foo(2),Foo(3))
res1: Int = 2
//anyway you can do ....
scala> f(a=2, foos=Seq(Foo(1), Foo(2)):_*)
res2: Int = 2
// with no foos...
scala> f(a=2, foos=Nil:_*)
res3: Int = 0
Given the Scala 2.9 limitations that Paolo mentioned, you can still use currying to divide the parameters in different sections, one which uses named parameters with default arguments, and one for varargs (or multiple curried argument sections if you want more than one vararg parameter). Regarding readability the result is imho almost better than using named arguments only:
f(b="something")()
f(c=3.14)(foo1, foo2)
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.