Why does this Free Monad Interpreter not resolve String to Id[A]

Question

I've been playing with Free Monads of Cats. I have written a DSL to process CSV Records. The primitive operation is to process CSV Record and I have written a helper sequence and map2 functions for processCSVRecords operation myself. I want the return type of the case class to be generic type R. Below is the code that I'm using.

import cats.data.Coproduct
import cats.free.Free.inject
import cats.free.{Free, Inject}
import cats.{Id, ~>}
import org.apache.commons.csv.CSVRecord

object ProcessCSVActions {

  sealed trait ProcessCSV[A]

  case class ProcessCSVRecord[R](csvRecord: CSVRecord) extends ProcessCSV[R]

  class ProcessCSVs[F[_]](implicit I: Inject[ProcessCSV, F]) {

    private def sequence[S[_], A](fs: Stream[Free[S, A]]): Free[S, Stream[A]] =
      fs.reverse.foldLeft[Free[S, Stream[A]]](Free.pure[S, Stream[A]](Stream()))((b, a) => map2(a, b)(_ #:: _))

    private def map2[S[_], A, B, C](ra: Free[S, A], rb: Free[S, B])(f: (A, B) => C): Free[S, C] = for {
      a <- ra
      b <- rb
    } yield f(a, b)

    def processCSVRecord[R](csvRecord: CSVRecord): Free[F, R] =
      inject[ProcessCSV, F](ProcessCSVRecord[R](csvRecord))

    def processCSVRecords[R](csvRecords: Stream[CSVRecord]): Free[F, Stream[R]] = {
      val res: Stream[Free[F, R]] = for {
        csvRecord <- csvRecords
      } yield processCSVRecord[R](csvRecord)
      sequence[F, R](res)
    }


  }

  object ProcessCSVs {
    def apply[F[_]](implicit I: Inject[ProcessCSV, F]): ProcessCSVs[F] = new ProcessCSVs[F]
  }

  object StringInterpreterOfCSV extends (ProcessCSV ~> Id) {
    override def apply[A](fa: ProcessCSV[A]): Id[A] = fa match {
      case ProcessCSVRecord(csvRecord) => csvRecord.get(2)
    }
  }
}

Now when I try to compile the above code, I get the below error for my interpreter:

[scalac-2.11]  found   : String
[scalac-2.11]  required: cats.Id[A]
[scalac-2.11]     (which expands to)  A
[scalac-2.11]       case ProcessCSVRecord(csvRecord) => csvRecord.get(2)
[scalac-2.11]                                                        ^
[scalac-2.11] one error found

What is the proper way to handle Stream with Free?

Edit:

I've found a hack. I'm taking a param of type R in the case class.

  case class ProcessCSVRecord[R](csvRecord: CSVRecord, a:Option[R]) extends ProcessCSV[R]
  def processCSVRecord[R](csvRecord: CSVRecord): Free[F, R] =
      inject[ProcessCSV, F](ProcessCSVRecord[R](csvRecord, None))

In the interpreter, I'm explicitly giving the type that matches the result.

  object StringInterpreterOfCSV extends (ProcessCSV ~> Id) {
    override def apply[A](fa: ProcessCSV[A]): Id[A] = fa match {
      case ProcessCSVRecord(csvRecord, _: Option[String]) => csvRecord.get(2)
    }
  }

The above works, but I wished there was a better solution instead of this hack.

Are you actually using R for the case class? Because in your example it seems redundant. — Yuval Itzchakov
Yes. I am using it in my program (for comprehension). I write CSV parsing Scripts and usually the pattern is same. So, I wanted to reuse my case class and write only the Programs and Interpreters for my scripts. — arjunswaj

Ziyang Liu Ziyang Liu · Accepted Answer · 2017-08-26T17:00:48

Since CSVRecord has nothing to do with A, the compiler has no evidence that String <:< A. If this compiles, you'll be able to create a ProcessCSV[Int] and pass it to StringInterpreterOfCSV.apply, which doesn't make sense.

If CSVRecord had a type parameter R, and get(2) returned R, then it would work:

case class ProcessCSVRecord[R](csvRecord: CSVRecord[R]) extends ProcessCSV[R]

Otherwise you can csvRecord.get(2).asInstanceOf[A].

Why does this Free Monad Interpreter not resolve String to Id[A]

1 Answers