Read case class object from string in Scala (something like Haskell's “read” typeclass)

Question

I\'d like to read a string as an instance of a case class. For example, if the function were named \"read\" it would let me do the following:

case class Pers         


        

Answer 1

Scala does not have type classes, and in this case, you cannot even simulate the type class with a trait that is inherited from, because traits only express methods on an object, meaning that they have to be "owned" by a class, so you cannot put the definition of a "constructor that takes a string as the only argument" (which is what "read" might be called in OOP languages) in a trait.

Instead, you have to simulate type classes yourself. This is done like so (equivalent Haskell code in comments):

// class Read a where read :: String -> a
trait Read[A] { def read(s: String): A }

// instance Read Person where read = ... parser for Person ...
implicit object ReadPerson extends Read[Person] {
  def read(s: String): Person = ... parser for Person ...
}

Then, when you have a method that depends on the type class, you have to specify it as an implicit context:

// readList :: Read a => [String] -> [a]
// readList ss = map read ss
def readList[A: Read] (ss: List[String]): List[A] = {
  val r = implicitly[Read[A]] // Get the class instance of Read for type A
  ss.map(r.read _)
}

The user would probably like a polymorphic method like this for ease of use:

object read {
  def apply[A: Read](s: String): A = implicitly[Read[A]].read(s)
}

Then one can just write:

val person: Person = read[Person]("Person(Bob,42)")

I am not aware of any standard implementation(s) for this type class, in particular.

Also, a disclaimer: I don't have a Scala compiler and haven't used the language for years, so I can't guarantee that this code compiles.