CODEWITHSUNDEEP
scala
blue-sky
blue-sky

Reputation: 53916

How is this type parameter 'typed'?

In the method signature for groupBy : 

def groupBy[K](f : (A) => K) : Map[K, This]

How is the type parameter of [K] on groupBy typed ? What is the significance of [k] in this context : groupBy[K] . Why is the type parameter required here, is this a requirement of Scala that cannot use instead : def groupBy(f : (A) => K) : Map[K, This] ?

Upvotes: 0

Views: 111

Answers (4)

Aluan Haddad
Aluan Haddad

Reputation: 31873

K is specified first because the type of the KeySelector function, (A) => K, and the type of the result :Map[K, this] depend on its definition. K is the type parameter that allows the groupBy implementation to abstract over grouping by a Key of any type.

Upvotes: 0

Daniel C. Sobral
Daniel C. Sobral

Reputation: 297265

How is the type parameter of [K] on groupBy typed ?

From the type of the function passed to groupBy, unless passed explicitly.

What is the significance of [k] in this context : groupBy[K] .

It is a type parameter: a type that is not fixed at the definition site of groupBy, but, instead, varies and depends on the context at each usage site of this method.

Why is the type parameter required here, is this a requirement of Scala that cannot use instead : `def groupBy(f : (A) => K) : Map[K, This]` ?

It is a requirement. In the definition you provide, the function passed to groupBy must be from A (a type parameter of the class) to K, a fixed type known at the definition of groupBy. Since there's no type K in Scala, that would automatically an error.

Or, to put in another way, it would be like defining groupBy as def groupBy(f: (A) => String): Map[String, This].

Upvotes: 2

radumanolescu
radumanolescu

Reputation: 4161

The List is defined in the scaladoc as sealed abstract class List[+A] - so A is the type of elements in the list. You can group the list's elements by any function f: (A) ⇒ K, which is to say that K can be any type. If you are going to notify the compiler that you can group by "a function f: (A) ⇒ K, where K is some type", you need to declare the symbol K. This is done at the method level, because K is only used at the method level. This is accomplished by declaring def groupBy[K](f: (A) ⇒ K): Map[K, List[A]]. Hope this is clear.

Upvotes: 0

aepurniet
aepurniet

Reputation: 1727

K is a method level type parameter, it does not exist for the class. so what you propose would not make sense. K needs to be declared as a type parameter also, because im assuming you do not want to reference type K, but rather the type variable K.

Upvotes: 1

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