Here is one: scala> implicit def intToString(x : Int) = x.toString intToString: (x: Int) scala> showMe(47) 47 An implicit conversion can make this succeed. The call to showMe using the value 47 fails, as there’s a type mismatch. The showMe method is defined to take a String and print it. Let’s take an example scala> def showMe(msg : String) = println(msg) foo: (showMe: String)Unit scala> showMe(47) :9: error: type mismatch found : Int(5) required: String showMe(5) Instead we would provide a conversion method that can allows existing types to be converted into types into which the expression can be called. We would like to call expression on existing classes without changing the existing class. Implicits can be used to enhance existing classes. It uses the type parameter T to allow us to reuse it for every type we’re looking for. The implicitly function is defined as def implicitly(implicit arg : T) = arg. We can use this function to look up a type using the current implicit scope. Next line is call to Scala’s implicitly function. helperobject also defines an implicit method that creates an instance of trait B. scala> object helper defined module helper scala> implicitly res0: helper.B = My B The implicit scope of Foo would consist of the companion object to Foo. If no value of type Foo is found using the first rule, then the compiler will use the implicit scope of Foo. This means that if the compiler is looking for a parameter to the method def findAFoo(implicit x : Foo), that parameter will need to conform to the type Foo. The implicit scope of a type is defined as all companion modules that are associated with that type. In this case, the compiler will look for implicits defined within any object in the implicit scope of the type it’s looking for. The second rule for implicit lookup is used when the compiler can’t find any available implicits using the first rule. When we write this method call, findAFoo, the compiler will rewrite it as findAFoo(test) If we were to write test in the REPL, it would return a value of type Foo. This makes the identifier, test, available on the local scope with no prefix. The next line defines a val test with the implicit marker. scala> class Foo scala> def findAFoo( implicit x : Foo) = x findAFoo: ( implicit x: Foo)Foo scala> implicit val test = new Foo test: Foo = findAFoo method is declared with an implicit parameter list of a single Foo. The first rule is that the Scala compiler will search for an identifier in the local scope with no prefix. If there are no available entities from this rule, then all implicit members on objects belong to the implicit scope of an implicit parameter’s type.The implicit entity binding is available at the lookup site with no prefix - that is, not as foo.x but only x.Because the method call is complete, the compiler doesn’t need to look up a value using implicits.įollowing are the rules to look up entities marked as implicit This method call passes the second parameter y with a value of 2. We can still provide the parameter if desired scala> addNumber(2)(3) res3: Int = 5 The compiler was able to successfully complete the function call. The call to addNumber succeeds and returns the sum. Since this is in the REPL, the value will be available in the implicit scope for the rest of the REPL session. This marks it as available for implicit resolution. The couldBeY value is defined with the implicit keyword. We’ll provide one, as follows: scala> implicit val couldBeY = 2 couldBeY: Int = 2 The compiler complains that it can’t find an implicit value for the y parameter. The addNumber method is called without specifying any argument for the second parameter. Let’s look at the following example: scala> addNumber(2) error: could not find implicit value for parameter y: Int If it’s left off, the compiler will look for a variable of type Int in the implicit scope. The parameter y is marked with implicit, which means that we don’t need to use it. ![]() This function will add these two int numbers. The addNumber method declares a non-implicit parameter named x of type int and a single implicit parameter y of type Int. ![]() Lets look at implicit resolution in work : scala> def addNumber(x: Int)(implicit y: Int) = x + y This tells the compiler that the parameter list might be missing, in which case the compiler should resolve the parameters via implicit resolution. ![]() The implicit keyword can also be used at the beginning of a method parameter list. Implicit resolution is when the compiler determines that a piece of information is missing in code, and it must be looked up. When this keyword is seen on a method or variable it is basically a signal to compiler to use this methods or variables during an implicit resolution. Scala provides an implicit keyword to be used on methods or variable and on method parameter lists.
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