How to use scalaz's `Tagged Type` to replace my type alias? - scala

Question

I want to define objects as functions and depend it in other functions:
type FetchPage = String => String
type FindImages = String => List[String]
object WillFetchPage extends FetchPage {
def apply(url:String):String = /* get content of the url */
}
class WillFindImages(fetchPage: FetchPage) extends FindImages {
def apply(url:String):List[String] = {
val html = fetchPage(url)
// get image srcs from the <img> tags
}
}
Then I can inject WillFetchPage to WillFindImages:
val findImages = new WillFindImages(WillFetchPage)
Also test WillFindImages easily by injecting a mock function:
val testFindImages = new WillFindImages(_ => "html-have-3-images")
val images = testFindImages("any-url")
// assertion
You can see the type alias FetchPage is just a type alias, so I can pass other String => String functions to WillFindImages by mistake, so I'm looking for a type-safe solution.
Then I heard of Tagged type from scalaz: http://eed3si9n.com/learning-scalaz/Tagged+type.html
The example is exciting:
sealed trait KiloGram
def KiloGram[A](a: A): A ## KiloGram = Tag[A, KiloGram](a)
val mass = KiloGram(20.0)
2 * mass
You can see the mass here is actually a double 20.0, but it has some unique type.
I want to use it to improve my code, but sadly I can't find a way to do it. I tried:
object FetchPage extends ((String => Try[String]) ## FetchType)
But it provides an compilation error:
Error:(18, 51) class type required but String => scala.util.Try[String] with
scalaz.Tagged[object_as_func.FetchType] found
object FetchPage extends ((String => Try[String]) ## FetchType) {
^
How to fix it?

Answer 1

What scalaz version do you use? It's a bit different in 7.0.6 and 7.1.0. Here is example for 7.1.0
import scalaz.##
object PageFetcher
type FetchPage = String => String
type FindImages = String => List[String]
val WillFetchPage: FetchPage ## PageFetcher.type =
scalaz.Tag[FetchPage, PageFetcher.type](url => "Content")
class WillFindImages(fetchPage: FetchPage ## PageFetcher.type) extends FindImages {
def apply(url: String): List[String] = scalaz.Tag.unwrap(fetchPage)(url).grouped(1).toList
}
val images = new WillFindImages(WillFetchPage)("url")
println(images)
val testFetcher: FetchPage ## PageFetcher.type =
scalaz.Tag[FetchPage, PageFetcher.type](url => "TestContent")
val testImages = new WillFindImages(testFetcher)("url")
println(testImages)