Scala: Case class with generic function argument

Question

I have the following case class.

case class CustomAttributeInfo[T,Y](
          attribute:MyAttribute[_],
          fieldName:String, 
          valueParser:T => Y){}

The case class takes three values.

The last argument is a function that will parse an input of any type and return the part of the input we wish to keep.

(Imagine, for just one example, I pass in a jsonstring, convert to json object, and extract an Int).

The companion object will supply a range of functions that we can pass to the case class. The one shown here, simply takes the input as a string and returns it as a string (the most simple possible example).

object CustomAttributeInfo {

  val simpleString = (s:String) => s
}

I create the case class as follows:

CustomAttributeInfo(MyAttribute(var1, var2), name, CustomAttributeInfo.simpleString)

Later, I call the function 'valueParser'

customAttributeInfo.valueParser(k)

Compilation error

Error:(366, 69) type mismatch; found : k.type (with underlying type String) required: _$13 case Some(info) => Some((info.attribute, info.valueParser(k)))

I am not a generics expert (obviously). I have done some reading, but I have not seen a discussion about a case like this. Any advice and explanation would be most welcome

Dmytro Mitin · Accepted Answer · 2019-11-12 09:48:46Z

3

You haven't provide enough information to answer your question.

The following code compiles.

If you still have compile error provide MCVE.

  case class MyAttribute[_](var1: Any, var2: Any)

  case class CustomAttributeInfo[T,Y](attribute:MyAttribute[_], fieldName:String, valueParser:T => Y) {}

  object CustomAttributeInfo {

    val simpleString = (s:String) => s
  }

  val var1: Any = ???
  val var2: Any = ???
  val name: String = ???
  val customAttributeInfo = CustomAttributeInfo(MyAttribute(var1, var2), name, CustomAttributeInfo.simpleString)

  val k = "abc"
  customAttributeInfo.valueParser(k)

edited Nov 12, 2019 at 9:48

answered Nov 12, 2019 at 9:29

Dmytro Mitin

53.1k3 gold badges34 silver badges76 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Jake Over a year ago

thanks. I just realized that an MCVE does compile and runs. I will debug a little more and then try to update my question. But to be clear, k should be a string, that is the argument for valueParser(), shouldn't it?

Jake · Accepted Answer · 2019-11-12 16:32:51Z

0

@Dmytro was right that a simple example compiled. In my actual codebase code, however, we needed to be specific about the type.

This worked:

object CustomAttributeInfo {

  type T = Any

  val simpleString = (s:T) => s.toString
}

edited Nov 12, 2019 at 16:32

answered Nov 12, 2019 at 11:20

Jake

4,68810 gold badges44 silver badges89 bronze badges

2 Comments

Alexey Romanov Over a year ago

1. You can't extend an object in standard Scala (there is a compiler option to allow it); 2. Even if it was a class/trait, you can only add more requirements when overriding a type member, when it's already exactly known it can't be changed.

Jake Over a year ago

Fair enough. I will investigate. If you are correct, I will update

Collectives™ on Stack Overflow

Scala: Case class with generic function argument

2 Answers 2

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related