CODEWITHSUNDEEP
scalascala-cats
Knows Not Much
Knows Not Much

Reputation: 31526

Not able to compose the Cats Reader Monad

I have written the following code

case class Foo(i: Int, j:Int)
def do1() : Reader[Foo, String] = Reader(f => s"doing ${f.i}")
def do2() : Reader[Foo, String] = Reader(f => s"doing ${f.j}")

now if I try to compose these into another function

def func() : Reader[Foo, List[String]] = Reader(f =>
  for {
    m1 <- do1()
    m2 <- do2()
  } yield List(m1, m2)
 }

What I was hoping was that I will do func().run(Foo(10, 20)) and this will give me a list of List("doing 10", "doing 20")

but instead I get a compiler error

type mismatch;
 found   : cats.data.Kleisli[cats.Id,$sess.cmd1.Foo,List[String]]
 required: List[String]
m1 <- do1()
   ^
Compilation Failed

Upvotes: 1

Views: 611

Answers (1)

Matt Fowler
Matt Fowler

Reputation: 2723

If you look at what the definition for Reader is in the Cats library, you'll find Reader is just a type alias for Kleisli: 

type ReaderT[F[_], A, B] = Kleisli[F, A, B]
val ReaderT = Kleisli

type Reader[A, B] = ReaderT[Id, A, B]

So your for comprehension evaluates to:

Kleisli[Id, Foo, List[String]]

Which is the same type as Reader! Just remove wrapping your comprehension in a reader and it works:

def func(): Reader[Foo, List[String]] = {
    for {
      m1 <- do1()
      m2 <- do2()
    } yield List(m1, m2)
}

Upvotes: 5

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