How to read Scala code with lots of implicits?
Posted
by
Petr Pudlák
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Published on 2012-09-27T10:01:03Z
Indexed on
2012/09/27
15:50 UTC
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Consider the following code fragment (adapted from http://stackoverflow.com/a/12265946/1333025):
// Using scalaz 6
import scalaz._, Scalaz._
object Example extends App {
case class Container(i: Int)
def compute(s: String): State[Container, Int] = state {
case Container(i) => (Container(i + 1), s.toInt + i)
}
val d = List("1", "2", "3")
type ContainerState[X] = State[Container, X]
println( d.traverse[ContainerState, Int](compute) ! Container(0) )
}
I understand what it does on high level. But I wanted to trace what exactly happens during the call to d.traverse
at the end. Clearly, List
doesn't have traverse
, so it must be implicit
ly converted to another type that does. Even though I spent a considerable amount of time trying to find out, I wasn't very successful. First I found that there is a method in scalaz.Traversable
traverse[F[_], A, B] (f: (A) => F[B], t: T[A])(implicit arg0: Applicative[F]): F[T[B]]
but clearly this is not it (although it's most likely that "my" traverse
is implemented using this one). After a lot of searching, I grepped scalaz source codes and I found scalaz.MA
's method
traverse[F[_], B] (f: (A) => F[B])(implicit a: Applicative[F], t: Traverse[M]): F[M[B]]
which seems to be very close. Still I'm missing to what List
is converted in my example and if it uses MA.traverse
or something else.
The question is: What procedure should I follow to find out what exactly is called at d.traverse
?
Having even such a simple code that is so hard analyze seems to me like a big problem. Am I missing something very simple?
How should I proceed when I want to understand such code that uses a lot of imported implicit
s? Is there some way to ask the compiler what implicit
s it used? Or is there something like Hoogle for Scala so that I can search for a method just by its name?
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